Symptoms: After some random amount of printing, the carriage would get stuck and just twitch instead of moving across the platen.
Testing: Using its internal test function, I let it print out until the problem ocurred.
Once the problem ocurred, the carriage would just kind of twitch back and forth. Grabbing it, it was obvious that the stepper motor was providing no effective power. Since this is usually indicative of a missing phase to the stepper motor, I immediately suspected either:
I then removed the cover to gain access to the circuit board and motor connections. It was necessary to defeat the cover interlock to get it to come one. Now, to get it to screw up again.
I left it printing out the ASCII character set and got a byte to eat. When I came back it was busy gouging a hole in the paper. So now for the critical test: Will pressing on the stepper motor connector cause a change? The answer is --- Yes! The carriage started moving again meaning that it is likely a bad connection.
Of course, to gain access to the underside of the circuit board required removing a zillion screws and the entire mechanical assembly. But once this was accomplished - immediate gratification. There were obvious bad solder joints around several pins of the stepper motor connector. I resoldered these and few others that were suspicious and inspected the rest of the board. If only I could remember which screws went where! Apparently, the continuous vibration of the assembly eventually caused the connections to fail. This is not likely a heat related problem though it could be just plain bad quality control.
Once reassembled, I left it happily printing out page after page of ACSII characters. Then, just to be sure, I connected an old laptop and printed a few pages of Repair Notes.
Comments: This is one of those dream problems since their solution is so obvious and so definitive. There is no doubt that the cure will last. Unfortunately, the tough 'dogs' are the ones you lose hair over.
Symptoms: The power supply had been identified as being bad by the seller.
Testing: I plugged it in - nothing as expected. This was to verify that all functions and display were dead.
I removed cover and found that the main fuse had been removed. Hmmm, this usually means catastrophic power supply problem. This is the typical Panasonic switching power supply. Well, I have completely rebuilt these, so no biggie. (Of course, there was one I blew up, but that is another story).
The hardest part was removing the power supply. It is buried underneath the bottom circuit board necessitating the removal of this board and the front panel. Do you think they design these things this way to discourage tampering? There was plastic in specific places to prevent removal from the top even though it would have been trivial to design for easy removal. I disconnected power supply from VCR. Fortunately, this was just a connector.
I tested across switchmode power transistor with ohmmeter - dead short C-B-E.
I looked in ECG for 2SC3890 - ECG379 with that infamous # indicating an electrical but not mechanical match.. Since I had some BU406s, I looked this number up in ECG guessing that the BU406 would be have similar ratings and be usable in a small switcher - guess what, ECG379. The difference is that the 2SC3890 is totally plastic while the BU406 is a metal tab TO220. OK, so I cut out a bit of mica to serve as an insulator and used a nylon screw. This is temporary as I intend to get the proper replacement (2SC3890 - $2.15 from MCM Electronics).
I also checked continuity from the main filter cap to the C and E leads of the transistor to rule out a blown fusable emitter resistor. I checked other semiconductors as well - all fine as far as my VOM was concerned. Fortunately, the only casualty seems to have been the transistor and the fuse was fast enough to prevent any damage due to its shorting.
To power up the supply, I initially used a Variac with a 25 W light bulb in series with the line. Note that since I do not have any 1.6 A fuses, the fuse is shorted. The light bulb will provide the current limiting for now. I use my dual outlet widget box plugging in the supply to one outlet and a lamp with the 25W light bulb into the other (the outlets are wired in series for exactly this sort of application). This whole rig was plugged into an isolation trasformer for safety.
I then identified the primary output and connected my VOM to this. It would be in the range 5-15 V, probably 12 V based on the filter capacitors (16 V).
All set? Crank up power. Output comes up to about 13 V at around 50 VAC in. Light Bulb hardly flickered. If it had not stopped in the 12 V range, this would either indicate that there was a problem in the regulation or that a load was needed. Had this happened, I would have put a 22 ohm 5W resistor on this output and retested. Cranking up the Variac to full voltage causes no noticeable change to output.
OK, connect VCR. Lights, VCR, power! Nothing. Light bulb is now glowing but there is no indication of life from front panel and no action from motors.
Hmmmm.
Examining the nameplate, the expected power consumption is 29 W which is way more then one can get through a 25 W light bulb. (Expect to be able to draw maybe half of the bulb's ratings). So I go hunting for a 60 W light bulb.
This time the motors twitch and the front panel comes alive. Inserting a tape and holding my breath - tape starts to load but then aborts with poweroff. Go for it! The Variac had been set at about 90 VAC, so I crank it up to 120. Now everything works. I go dig up a TV and verify that the basic functions are ok. Doesn't appear to even need much cleaning. Even the idler tire appears to be in good condition.
Add a 2SC3890 and box of 1.6 A fuses to my next MCM order.
Comments: this is the sort of repair that might not pay for a professional shop to undertake. The time to disassemble the VCR, identify the problem, replace the transistor and fuse, and verify correct operation could be excessive, or at least has the potential to be excessive. If it turned out to be more than a transistor and fuse or was beyond repair, they might have to eat the cost in time and materials. In addition, there is no real way to guarantee that other marginal components won't cause the problem to repeat a week or month in the future. Upgrade/repair kits are available for these supplies and would probably represent the lowest risk investment for a permenant repair. No doubt, the previous owner had taken it in for repair and been quoted a rediculous price to replace the entire power supply module.
Symptoms: Modem works in most respects but when it goes to dial, the tones are superimposed on the dialtone which never goes away.
Testing: I plugged it into an old laptop kept for the specific purpose of testing of random external peripherals. Indeed, the AT commands worked just fine but tone dialing did not work. Interestingly, pulse dialing would work 90% of the time but a connection was never completely established.
At this point, I put a scope on each side of the 600 ohm coupling transformer. A normal 2 or 3 volt p-p signal was present on the logic side of the transformer during dialing. However, a much attenuated signal was present on the phone line side - probably .2 V p-p or so. I probably could have used a set of crystal headphones to just listen to the relative amplitudes of the logic and phone line side signals instead of a scope. (Magnetic headphones would have too low an input impedance.)
On a hunch, I did try replacing the transformer with one from my junkbox but as expected, this produced no change.
The phone line is a nasty place for electronic components - 90 V ringing signal, lightning strikes, pickup of EMPs (from nuclear bombs), etc. The first place to look for fried components is therefore the phone line circuitry. For a modem and no schematics, with the possible exception of the power supply, it is also probably the only place where there is any real chance of finding a problem.
Since there are a manageable number of discrete parts that connect the phone line to the transformer, an ohmmeter check was in order. Unfortunately, many of them were really itty bitty surface mount parts. After a couple of go-arounds, this proved to bear fruit as an SO23 device marked on the circuit board as a diode turned out to be shorted though I seem to have missed it on my first pass. Carefully unsoldering the almost microscopic part confirmed that it had turned into a dead short.
Note: markings for these devices were not always complete. Nonetheless, basic ohmmeter checks could be made with enough confidence to tentatively eliminate all except the single shorted diode.
Removing the diode and retesting proved that dialout was possible and that normal communications at 14.4 K baud was normal. The markings on the diode did not permit me to identify whether it was a simple diode or a zener. I still do not know what function the diode actually served.
I replaced it with a 1N4148. The modem has been tested to confirm that it is not damaged by the ringing voltage. Since the application does not require dial-in access, I do not know if there is still a problem with this mode.
Comments: Where a problem can be narrowed down to a small section of circuitry, ohmmeter tests can prove successful in identifying parts that have failed shorted or open. This may be the only option when confronted with a device for which obtaining a schematic would be difficult or impossible - or not worth the effort. While we might consider a modem to be a throwaway item these days, the time need to do basic testing of the phone line side components is minimal and, as in this case, may be all that is needed.
Symptoms: Dead. Located fuse - blown due to short (not overload). Blackened inside glass. Note: appearance of blown fuses is very significant.
Testing: Replaced fuse. This was in hindsight a mistake without using a series light bulb to limit the current. The new fuse did not blow but the lights dimmed momentarily. Apparently a fusable resistor had sacrificed itself to protect the fuse.
Without schematics, I decided to trace the AC line circuits. These were partially buried inside a metal box with some big fat resistors on a separate board.
Measuring across the power resistors revealed that one 2.7 ohm resistor was now open. That explained the new failure. Some further searching located a TO3 power transistor inside the metal shield. Measuring C-E came up 0 ohms. This was not the horizontal output transistor but was rather in the power supply - a flyback switching supply separate from the deflection circuits.
The transistor was marked with an RCA part number. My handy dandy ECG Semiconductor Master Substitution guide listed it as a typical 350/800 volt switching transistor. I did not have an exact replacement but figured that a horizontal output transistor would probably work at least temporarily in this application, so I used my favorite BU208A in its place.
As a temporary substitute for the fusable 2.7 ohm resistor, I put in a regular power resistor with the understanding that a safe replacement would be installed before the TV was buttoned up.
Now for the test. This would be classic use for a series light bulb and/or variac for the initial power on. However, for whatever reasons, I did not bother - and lucked out in this case. The TV came on just fine. The only adjustments needed were to focus and horizontal position - totally unrelated to the original problem.
The recommended ECG replacement for the chopper transistor was an ECG385, a 350 V 10 A switching transistor. I elected to put in an ECG386 to give myself a little extra margin. The ECG386 is rated at 500 V and 20 A. Other specifications were similar or superior.
Comments: Since most modern consumer electronics are powered all the time even if the power switch is off, it is always a good idea to unplug everything during a lightning storm or if a blackout occurs. A nearby lightning strike can easily impose huge transients on the AC line. When power is restored following a failure, the initial power-on may not be clean including mini-brownouts, spikes, multiple cycles, etc. These are all hard on switching supply based equipment. Now, I fully realize that few of ua actually follow this advise. (Of course, other screwups can result in similar damage. I once was given a bag of dead stuff from a friend of a friend who had been doing a little wiring in his house. Somehow he managed to connect 240 V to a 120 V circuit - only for a second....)
This is one example where a failure with the most catastrophic impact on performance (it was dead, after all) has among a very simple solution (transistor, resistor, fuse). I much prefer these to the 'color noise on channels in the UHF band' variety.
Symptoms: Disc is loaded from cassette, disc spins at various speeds for about 5 seconds, then it gives up, unloads this disc and loads next until there are no more.
Testing: Testing consisted of removing cover and observing behavior. Everything appears to happen normally except that disc directory is never read.
This is one of those problems that has an obvious cause and solution once experienced, diagnosed, and repaired the first time. I will outline my approach the first time I came across this one describing this particular case history from the perspective of the novice wannabe repair expert.
Since the disc is loaded and spins, it is likely that the laser and focus servo are functioning (though perhaps no guarantee, but I hope to get to that in another episode).
At first, I suspected (incorrectly) that an adjustment was needed, so I did what I always now warn against - turning any of the internal controls. I thought that they had been returned to the original settings but was not positive.
Pioneer CD players usually include a test function - a button on the main circuit board marked 'TEST'. Normally, test functions are invoked either by simply depressing the button or by holding it depressed when power is turned on. In this case I discovered that if this button were depressed when the unit was switched on, the display would change and certain front panel buttons would now function controlling the servo circuits directly.
(Note that the approximate duration of the previous paragraph was about 1 year as I put the thing aside unable to make any headway during the first go-around.)
After some experimentation with the front panel controls, I came to the conclusion that:
All this was leading nowhere until I accidentally happened to engage all servos in the middle of a CD - and the track display suddenly appeared. I reached for my headphones and confirmed that the CD was playing, just fine.
A little further investigation allowed me to determine that the CD would play fine from about the middle to the end but would get progressively noisier when moving toward the starting track and would be totally unplayable at the very start.
Now, what could depend so fundamentally on track position? Well, there are two possibilities: spindle motor speed and PLL frequency. A little careful tweaking of PLL center frequency had little effect.
One thing I noticed was that one of the servo driver ICs was running quite hot. This should have raised a red flag but again, this was the first time I had seen this problem. I also observed that putting a heatsink on the IC and blowing on this would permit the disc to play error free much closer to its start.
So now, you are saying, "what a moron, everyone knows that Pioneer CD spindle motors are crap". The confusing thing here was that the spindle motor was not dead, just marginal. So, all basic observations came up negative.
Anyway, back to the saga. Suspecting the driver IC, I obtained a replacement from MCM Electronics and swapped it. No change. Measuring motor voltage showed a maximum of 1.7 V or so at any time including startup. Since the driver is known now to be good and power was confirmed to be stable, I started to suspect the motor.
Disconnecting the spindle motor and cycling the player revealed that the driver was putting almost 10 V on the motor terminals but the motor was no doubt partially shorted and dropping this to less than 2 V with the consequential high power dissipation in the driver.
Now for the long shot. While the player was attempting to spin up and read the disc directory, I gave the motor a squirt of degreaser through its ventilation holes. The motor took off - went totally overspeed. Power off. Wait for degreaser to evaporate. Try again. Now, the directory came up the first time even though my internal controls were still no doubt not perfectly tweaked. All functions worked perfectly. For the first time is about 2 years, the player was producing music (without the help of TEST mode!
I then performed a normal alignment of the internal controls (but this is another story).
Measuring the motor voltage now showed greater than 5 V at spinup and a range of 2-.5 V between start and end of disc.
Comment: At this point the proper course of action would be to replace the spindle motor. However, since this is my CD player and replacing spindle motors is sometimes a pain, I will just keep an eye on performance. A pretty good indication of the motor's state is the time to spinup. If this deteriorates again I will be forced to replace the motor. For now, it continues to be satisfactory.
The initial confusion here was due to the fact that the motor was not totally dead, just weak enough to cause a problem with the inner tracks and more importantly, the directory. This is one of those cases where the old style turntable with a bad weak motor would have been much easier to troubleshoot,
Symptoms: Totally dead. No schematics available.
Testing: Applying power produces no output. Only observation is that lights flicker indicating the input filter capacitors are charging so this eliminates a blown line fuse as a possibility.
Unfortunately, this unit is not what I would call a 'simple switcher'. In addition to the main switchmode transistor, there are 2 other power transistors, a uA723 IC regulator, countless discrete transistors, resistors, capacitors, some components I cannot identify. This is all on the primary side of the transformer. Tracing the circuit is virtually out of the question due to its complexity. The only good news is that I have several identical units so I can compare readings between the bad one and a working unit. Powering with a Variac produces similar lack of any output. This is not a case of the outputs shutting down - there is simply no startup.
First check: power to main filter caps, continuity of thermal protector, power to uA723, power to switchmode power transistor. These all are fine.
Next, checked components around input including power transistors, large resistors (suspecting a startup problem), capacitors, etc. All ok.
Finally, about to give up, I decide to just test resistance across a more or less random selection of components. Everything is identical until I put my meter across a 6.8 M ohm resistor. On the good unit, this measures above 1 M ohm. On the bad unit, it measures about 40 K ohms. I unsolder components around this resistor until I located a 2N4124 transistor that makes a difference when removed. Testing on the x1 range of my VOM it tests fine but on the x1K scale, there is significant leakage in the reverse biased junctions. Comparing with a nearby 2N4124, this is definitely not the normal characteristics of a 2N4124. The 2N4124 is a general purpose transistor so I replaced it with a handy 2N3904.
Powering with a Variac, the supply now comes up fine. I have no idea of the function of the bad transistor.
Comments: This might be called a blind repair. Like bad connections, the failure mechanism and function of the bad part will probably never be known. It is not known whether the transistor was marginal to begin with and ts characteristics just drifted over time, or whether it went bad.
The basic assumptions which permit this technique to work at all are that for a sudden change in behavior in a system with mostly discrete parts, one of these parts has changed its resistance enough that an ohmmeter check has a chance to find it and that the circuitry is interconnected enough that checking a relatively small subset of node combinations has a good chance of locating the bad part.
Symptoms: Basically operational except absolutely no color - not even any color noise between channels. Color and Tint knobs have no effect. In addition, brightness control has very limited range - brightness slightly too high. Several other minor problems including cracked tuner knob, dirty tuner, broken antenna wires.
Testing: This involved tuning local strong channels, adjusting fine tuning, tweaking AGC, etc. Under no circumstances was there any hint of color in the picture or between channels.
Fortunately, such symptoms narrow down the possible area of investigation to the chroma decoder circuitry. Searching the circuit board for a likely subsystem, I found that the set uses a TA7608P chroma chip. Fortunately, this chip is listed in my ECG Semiconductor Master Replacement Guide with a cross to ECG1532. Naturally, I suspect the IC at this point but know better than to just go out and find a replacement - I have been burned in this way with an RCA TV color problem - maybe I will say more on that in another Repair Brief. While the ECG does have a pinout, this does not really provide enough information to probe the circuit.
Off to the library to obtain the Sam's Photofact for this set. $0.75 poorer, I copied the complete schematic and another interesting page - a chart showing the resistances to ground for all pins on all of the intergrated circuits used in the TV. One item I forgot to look for was the block diagram that may have been included of the TA7608P chip. Oh well.
One thing I did try once armed with the Sam's was to attempt to tweak the subbrightness control. Even this had very little effect. OK, on to the fun stuff.
First test: confirm that the resistances of the circuit match those printed on the resistance chart. The chart specifies a DMM that applies less than .1 V on the ohms scales (to prevent forward biasing of any semiconductor junctions). Hopefully, my DMM satisfies this requirement. First step: make sure the main filter capacitor is discharged before making any resistance measurements. Done. Unfortunately. these tests do not reveal anything amiss.
Tests of this type are not guaranteed to find any problems. However, there is a fair chance that a shorted or open part would show up as a bad reading in the near that part. In my case, if there were any bad parts, the circuit topology prevented this simple resistance test from detecting them.
Now for the live tests. I don't have a color bar generator so I just have to hope that a broadcast channel will provide a signal that is close enough.
In the interest of safety, all the following tests are made with the set powered off of my isolation transformer.
Voltage measurements were inconclusive. Although some where off by 20%, this could be due to my non standard input signal. There did not appear to be any particular pattern.
Next, I used a scope to look at the testpoints for which Sam's supplied waveforms. Again, these will be different than the ones using a standard colorbar signal but the overall appearance should enable me to determine if a particular output is dead or the amplitude is way off.
All the waveforms looked reasonable except one - the output of what I deduce to be a gated chroma amplifier. Its output is almost dead. The color reference oscillator (3.58... MHz) looks fine as do the chroma input and color burst gating pulses. All the supply voltages and decoupling pins look fine as well. This doesn't look good for the chip. The signal seems to be getting in but the chroma amp would seem to be dead. Fortunately, I could not locate an inexpensive replacement from my usual sources. The TA7608P is probably obsolete. Even the ECG1532 is not available from MCM Electronics or Dalbani. I did not get to the point of trying ECG directly. You will see why I say 'fortunately' in a moment.
When confronted with a situation of this type, I usually try some experiments.
What would happen if I apply the chroma signal directly to the point in the circuit that is the output of that dead chroma amp? I take a 10K resistor and jump the input of the chip to the chroma amp output (and input to the chroma demodulator.) Now, I have colored stripes on the screen indicating that the chroma demodulator circuit is probably functioning. (Without the color burst to phase lock, I could not hope for anything more). So, it still looks like that bad amplifier. Well, one last desperate effort....
I use my Sam's patented magic spit. This has served in on numerous occasions mostly when locating clock noise, marginal timing, or glitches in high speed digital systems but hey, the world is really analog anyhow. I am not joking. Those who have done serious debugging know exactly what I am talking about. Moisten your finger. Run it up and down the pins on a suspect device. If something changes you have either (1) found a particularly critical or high impedance but normally behaving circuit (for example the frequency determining LC network of an oscillator) or (2) something that is open or on the edge.
Magic spit to the rescue: running my finger (one hand in my pocket, isolation transformer, etc.) over the two rows of pins on the chroma chip proved to yield immediate results. Around the low number pins, the color suddenly appeared grossly overloaded but with some indication of correlation with the picture. This is the first time I have seen any indication of picture related color. Hum.... OK, now to narrow it down. I take an insulated wire, and strip both ends. Holding one bare end with my fingers, I touch each of the pins in turn of the chip. Touching pin 6 has the most dramatic effect producing the distorted colors. What is connected to pin 6? According to the schematic, it is a 2.2 M ohm resistor to pin 3 and a decoupling capacitor to the power supply. It is unlikely that the capacitor would fail in such a way as to cause this behavior. So it must be the resistor. Rummaging around in my resistor cabinet I come up with a 2.2 M ohm resistor. and tack it across pins 3 and 6 (with power off!).
Now I get distorted color but this looks a lot more like a color TV than what I had before. Time for a better antenna. That helped a little but the picture is way too dark. Well, I did fiddle with the sub-brightness control back when it had minimal effect. Locating the sub-brightness control again, it now functions as expected having more than a sufficient range. When adjusted to produce a picture of normal brightness (with the user control mid-range), the color appears normal. Some additional fiddling with the Color and Tint controls yields a fine looking color picture.
Removing and testing the original 2.2 M ohm resistor confirms an open circuit. Soldering in my replacement completes the repair.
Now to clean the tuner, wipe down the case, repair the knob and antenna....
Total cost $1.77 including original purchase price (I threw in a hypothetical $.02 for the resistor).
As I am writing this, I am watching the Goldstar TV - my trusty RCA has just died after 14 years with no picture (raster and sound ok - that will be another Repair Brief).
Comments: It is always tempting to suspect the expensive or unavailable part first. Very often, as in this case, this proves to be erroneous. Had the TA7608P been readily available at reasonable cost, I would have probably replaced it only to find no change in behavior. This would, however, have saved time.
Be warned that the "Sam's Magic Spit(tm)" approach must be used with caution. You must understand the safety implications of touching *any* live circuit especially with moistened fingers. I use an isolation transformer for debugging. However, even with this precaution, I would think twice before doing this on a live chassis (the Goldstar signal circuits are isolated from the power line).
Symptoms: Band of what looked like tracking noise would come and go depending on tape being played, speed of tape, whether at start or end, etc. The noise was confined to the top 1/3 of the picture. Its height could vary from just a couple of video lines to a band occupying 20 % of the screen.
Testing: Several tapes were played initially. Problem would be nearly absent with some but severe on others. It was generally worse with EP recorded tapes compared to SP tapes.
I generally do not like problems of this type because one of the more likely possibilities is of a worn video head. This is one of the classic symptoms yet it could have a number of other causes. The approach must be to eliminate as best as possible the alternative causes until the risk of purchasing a new video is minimized.
Alternate #1: dirty heads. Head cleaning with a wet cleaning tape followed by a manual cleaning had little effect.
Alternate #2: tape path alignment. Visual inspection of the tape movement showed nothing out of the ordinary. Tape motion was very smooth and uniform with no wiggling, wavering, or wondering. All tape guides were properly positioned, perfectly vertical (where appropriate) and the tape appeared to be riding at the correct height on the video head cylinder.
Alternate #3: backtension. Insufficient backtension could result in similar problems. Inspection seemed to indicate that backtension was normal. Manually increasing backtension by gently pressing the backtension level to the left made a slight improvement. Increasing the spring tension did the same. However, these were not dramatic effects and backtension is not a critical setting to obtain a clean picture (though it is important to be accurate to minimize head wear and clogging).
Alternate #4: roller guide height. Although visual inspection of the of the tape path alignment proved negative I decided to confirm roller guide height by careful adjustment of the supply side roller guide - carefully noting its original position. (Problems at the top of the picture would be related to the supply side roller guide.) Optimal position for both EP and SP was at the original setting.
This left the video head as a likely candidate and at this point based on the age of the machine, a new video head cylinder (MCM Electronics, ERH433) was ordered and installed. Success! There was no doubt about the improvement. The noise bards completely disappeared and the normal backtension provided more then adequate head-tape contact.
Comments: Subtle problems that eventually point to the video heads are among the more difficult to diagnose with enough confidence to risk ordering an expensive video head and find out that the problem was elsewhere. This was a case of video head wear (as opposed to a mechanical or electronic failure of the heads). The chance of having an identical video head available to swap - which is the best test - is quite small, especially for a 3 head type. If this were a 4 head machine, some meaningful comparisons could be made during playback since a different set of heads is often used depending on tape speed and mode.
Thus, unless there is visible damage to the video heads or something like an open winding that could be revealed by simple testing, it comes down to eliminating as best as possible the alternatives until only the head remains a likely possibility.
Symptoms: Everything worked fine except dialing. For some buttons, dial tone would not go away. For others, tones would be accepted but would be erratic and incorrect digits. Certain tones sounded weak or single frequency.
Testing: All buttons were tested. It was found that the problem was not even consistent as some buttons would not work all the time.
While the internal wiring of one of these old phones is intimidating, the basic tone dialing circuitry is an amazing example of simplicity. About the only things that fail yet still permit some tone generation are the pot core coils that determine tone frequency. Therefore, this is the first thing to check.
Sure enough, the core that deals with rows has split where the two halfs are joined. This seems to be a common problem due to both the age and brittle cement used on some revs of this model phone, and probably, as a result of rough treatment when hanging up the handset.
These cores must be aligned before being glued back together. In addition, there is an adjustment plug which may need to be tweaked. I align by ear as follows: Put a known good tone dialing phone and the bad phone on the same phone line. Depending on which core is bad, depress either an entire (same) row or column of buttons on both phones. (Adhesive tape is handy to hold down the buttons unless you have four hands.) By depressing the entire set of buttons, you are disabling the other tone generator so you hear a pure tone. Without turning the fine adjustment plug (assuming it was not disturbed; if it was, set it mid-range or the same as the one in the other core), rotate the loose core top until a zero beat is obtained. As your rotate the core, you will hear the tone change. As it approaches the correct setting, you will hear the tones beat against each other. When you are set correctly, the pitches will be equal and the beat frequency will go to zero. Mark the position of the core with a pen or pencil and then glue with Epoxy or other general purpose adhesive (around the outside - not on the mating surfaces as this will affect the tone frequencies). After the glue sets, confirm and adjust the plug core if needed. These cores use a strange triangular core tool - I made mine by filing down an aluminum roofing nail (do not use a ferrous material).
Comments: Those classic ATT touch tone phones are virtually indestructible. However, broken cores (or actually, just broken joints on the cores) are common but easily repaired once you know what to look for. Setting the tones by referencing a known good phone seems to be a very reliable technique as the zero beat permits an adjustment to better than .1%. Note that if the reference phone is a more modern (and flimsy digital one), then pushing multiple buttons may not work as it does with the old analog models. Setting the frequency using the normal dual tones will work - it is just not as easy.
Symptoms: Tray wasn't even on track, just sitting inside; Flapper ripped off of mountings, electronic condition unknown. It is obvious that the owner had attempted something - it would be generous to call it a repair - and was unable or did not bother to get it back together.
Testing: Not applicable at this point. With loose parts removed, power was applied to determine if there was any hope at all. At least the front panel came alive and pressing Eject resulted in the tray loading motor spinning.
In order to attempt to play a disc, the controller needs to think that the tray is closed. It will then go through its startup cycle. In the case of this player, there is a limit switch - somewhere. Rather than trying to locate it, I decide to put the tray back on its tracks. This is easy but there is still something wrong as it jams when the Eject button is pressed. So be it, leave that for later. At least the limit switch will be activated. Rummaging around in the pile of lonely parts removed from the carcass, I locate the clamper cover with the magnet. I pop in a garbage CD, put the clamper cover on top (make a mental not to press Eject under any circumstances as the tray, disc, cover, and anything else that is not screwed down would probably fly across the room) and press the power button. Some success - the disc spins and the directory is correctly displayed. The display came up rather quickly indicating that most of the optics and servos cannot be far out of alignment. This is quite remarkable!
With mounting anticipation, I connect the audio outputs to my amplifier and press play. The disc spins and makes repeated attempts to start playing at track 1 but it is obvious that something is terribly wrong. Attempting to play other tracks results in similar behavior.
The pickup appears to actually move to the general vicinity of the correct track but is unable to locate and lock onto the time/track that is selected. Pioneer CD players perform a very audible search to home in on the correct disc location; there was no evidence of this search.
I next attempt careful adjustment of the servo controls. Note that I do not expect this to help the problem based on how quickly the directory was displayed. However, the tracking could still be off and with care, there should be little risk of making things worse. Who knows what controls the owner touched in a misguided hope of performing a miracle. First, I marked the *exact* position of each control with a felt tip pen. This will get me back to the supposedly good positions no matter what. The only controls that would likely have an effect are those related to tracking. Careful tweaking of tracking balance, tracking offset, and tracking gain have no detectable effect. I put them back in their original position and verify that the player still recognizes the disc. So far so good?
At least the moron who butchered this thing does not seem to have touched the electronic adjustments.
At this point what do we know? Well, we know that all of the major components of the optical deck work including the laser, photodiode array, fine focus and tracking voice coil actuators, and spindle motor. These are all needed to read the disc index. The spindle motor, a common problem in Pioneer CD players is fine as its toughest task is at disc startup where the speed is greatest. Since the disc index is located at the very inner extent of the disc, we do not know if the sled servo (coarse tracking) is working correctly, only that it is doing something - it resets to the inner track if manually moved away and it does move to the approximate position of the selected track.
Well, Pioneer CDs have a TEST mode. Where is the button? I hunt all over for the little button and am about to give up when there it is! Hidden by the cables to the front panel.
OK, press TEST while switching on power. Now I have control of the servos. A little experimentation confirms that focus and spindle rotation seem to be functional: (of course, we knew that, right?) With no disc in place, the focus search routine is initiated by pressing TRACK FWD. The disc will only spin if focus lock is achieved and this is confirmed with a disc in place. So far nothing new. I am able to move the pickup back and forth on its tracks by pressing SEARCH FWD and REV.
However, when entering the correct sequence to play at an arbitrary point on the disc, weird things happen. If I use the SEARCH FWD and REV buttons to move the pickup to a particular spot on the disc, press TRACK FWD to close the focus servo, PLAY to start spindle rotation, and then PAUSE to actually start playing, the track and time info is only displayed for an instant. Then, the pickup seems to move toward and bump against the inner limit. Sometimes, a couple of times are displayed in rapid succession which are not sequential as they should be. In fact, they nearly always are far apart and the second is usually a lower time than the first. Then the display is blank.
Hum, I don't have a schematic so this could be the end of the line. But, I do see one chip on the circuit board that is getting unusually hot and I know from past experience that it is a servo driver - TA8410K. I have absolutely no idea if it is related in any way to the problem or really, for that matter, what the problem is. I only know that (1) it has only 10 pins and is easy to replace, (2) I have a replacement in my parts box, and (3) it is getting hot (which may or may not be a fault since I know these type of chips to run at least warm).
Getting to the bottom of the circuit board proves to be a bit harder than anticipated requiring removal of most of the snap type connectors. I guess these are cheaper than real connectors for Pioneer but a pain for servicing (cables are terminated in tinned wires and placed in the connector housing, then a cover is pressed down to lock them in place). I manage to only mangle one of these (cosmetic damage only).
Replacement goes smoothly. Getting all the connectors back in place is loads of fun but the effort is worthwhile! Now, the disc plays on the first attempt. There are still some tracking problems but this is a distinct improvement. In all honesty, I am not sure that the chip made the difference - it could have been a bad connections at one of the connectors. The new chip runs warm, perhaps not quite as hot as the old one, I am really not positive. I put a heatsink on it in any case (as I always do with these chips - just for insurance.
Next I tackle the mechanical restoration. First step: get the tray to move smoothly. Without going into terrible detail, the tray consists of two parts whose relative motion raises and lowers the disc. There appears to be something missing which controls when this raising and lowering takes place as the disc is lowered even before the tray moves into the machine. Sometimes there is a ball that controls this and a little examination reveals a grease trail where such a ball could have been. A corresponding hole in the tray bottom confirms this. I didn't notice any such ball in the parts pile but it could have easily been lost (I later found it near a corner of my workbench) but for now, I located a similar sized steel ball in my steel ball collection. With the ball in place, the tray now moves smoothly in and out and the disc is raised and lowered at the proper time.
Now for the clamper.
This is a much sorrier affair as the clamper is mounted to the deck sheetmetal with a couple of plastic standoffs that have been totally snapped off at their bases. First I try simply glueing them but this does not appear to be solid enough. In addition to the glue, I am able to clamp one down with a metal scrap that I carefully shape and screw down. For the other, I made a splint using a screw through a drilled hole into a neighboring strut. Now the clamper moves up and down at the proper time but the cover disk with the magnet seems to hit the tray. The part that seems to help out has totally disappeared so I take a brass rod and mount it in its place. Even without the rest of the mechanism, this seems to work fine. This rod, wrapped with electrical tape to prevent damage to the disc, prevents the disc from flopping around too much. Disc loads; disc unloads; all is well.
I then went through the electrical servo adjustment procedure as outlined in the CD Player Notes, final tweaking by maximizing the amplitude and stability of the 'eye' pattern. I made the mistake of attempting to touch the 'tangential adjustment' (at least that is what I think it is - without a proper alignment disc, this appears to be very difficult) and spent some frantic minutes until I was able to restore it to its original position. Beethoven's Ninth Symphony comes in handy as it runs almost to the edge of the disc (74 minutes) necessary to access the tangential adjustment. I even risked careful adjustments of the LD - laser power just to determine that it was not at the limit of its power. It was not. I am fairly confident at this point that the adjustments are pretty much where they should be - and they are very close to their original position.
Now the CD player works fairly well though it does not seem to have as much disc defect tolerance as I would expect. I do not know if there is still a fault either optical, mechanical, or electronic as all tests that I can perform without service info seem satisfactory. Considering what the player went through, this has still been a rewarding experience.
Comments: I consider this to be more of a learning experience than a
repair. At the outset, I did not expect to be able to get nearly as far as I
did. It was fun as such things go.
While I am in favor of home repairs, this is an example of a situation where whoever attempted the repair of a problem due most likely to the bad servo driver IC, totally destroyed any possibility of a professional even going beyond looking at the unit and stating: "Yup, that was a CD player once upon a time long long ago. To whom should I send the flowers?"
Symptoms: Fm reception is totally dead. Station numbers change erratically, not possible to save presets. Some AM stations work but most do not. This happened without warning - turned it on one day and it was sick.
Testing: This involved methodically checking to see what functions are operational. Incrementing and decrementing of FM station frequencies is not operational in seek mode, only in manual. There is not reception on any FM station frequencies. Incrementing or decrementing AM station numbers across certain boundaries (I forget the exact locations) causes a sudden jump of 800 kHz and may actually jump to an illegal station frequency. Various other modes are non functional including saving of memory presets. Even the hard reset does not store the factory presets.
I purchased the service manual for this unit - a nice piece of documentation and very reasonable - about $12. However, this is an example of modern technology where even schematics, pin descriptions of the various LSI chips, parts lists, etc. are not really adequate when so much depends on firmware (in 3 microcontrollers) which is not provided. It turned out to be difficult to even determine where each function is centered.
Some electrical tests that were performed:
Power supply voltages were verified.
Waveforms were checked on frequency synthesizer chip (LC7210).
Function of PLL charge pump was verified in both AM and FM. Output (VCO control voltage) was consistent with frequency display when reception was possible but not at other times. However, this could not be a problem with the charge pump, only the digital control.
Intermediate 4 bit busses were checked for stuck-at faults - there were none.
The first real clue is that since even some manual tuning functions are faulty, this is probably a digital fault. Presumably in manual, the station display is driven by the microcontroller that drives the synthesizer chip rather than being returned by that chip after a station search. Even in this mode (for AM), there is the issue of the 800 kHz jump. This is not approximate but exact and probably due to a stuck bit representing the 800 place value. The question then became: where was the bad bit? It is not on one of the intermediate busses as these were tested.
Could it be in the tuning microcontroller? Maybe, but then I would expect other functions controlled by this chip to be faulty (like mode setting, etc.) This is not the case. Could it be in the frequency synthesizer chip? Probably as only station tuning functions are defective. Could it be elsewhere? There do not appear to be any other busses or digital control lines that could cause the set of problems that are present.
However, not confident enough of the diagnosis of the faulty LC7210 synthesizer to spend the $25 or so that Yamaha would probably charge and not finding this part in any of my normal mail order sources, I set the receiver aside for a while. I dig out my garage sale NAD for use in the meantime.
A couple of MCM catalog editions later - what's this? LC7210 - $6. I will spring for that. Next MCM order arrives, solder in a socket as I always do where possible. Replacement chip cures all problems!
With 20/20 hindsight, it is almost possible to identify the place inside the LC7210 where the 800s bit bus fault occurs based on the symptoms and the rudimentary block diagram provided in the service manual.
Comments: Although not evident from the description above, this was a frustrating experience even with the service manual because there really was not enough information present to make the logical inferences needed to come to a definitive conclusion as to the defective part. Modern consumer electronics include more and more microcontrollers where the intelligence is buried in firmware and not the hardware itself. Without firmware listings, a microprocessor is just a black box even with pins listings and internal block diagrams. It would be nice if the service manual would at least provide better indications of which functions is located where - identifying the functions of each of the components. (It would also be nice if they were written or at least edited by Americans (in the case of a manual destined for the U.S. market). Some of the translations are, well, a bit strange.
Symptoms: Suddenly, the picture lost *all* horizontal hold. There was no evidence of any kind of attempt at lock in. I do not know whether this happened at power-on or while in use.
Testing: With strong signal, it was determined that horizontal hold had no effect. It is as though the H sync is not making it to the lock circuitry. Adjusting horizontal hold makes picture move across screen. Angle of sides of picture changes but there is no lock - even incorrect - at any setting.
Using my isolation transformer, I prepare to scope the relevant signals. I obtain the Sam's for the set. I check for the sync signal at the input to IC400 (I think). It is there. This should be a snap - bad IC! Well, that is exactly what happens - a careless slip of the scope probe and not only a snap, but a crackle and a pop - and now I have no video, no HV, no deflection - nothing.
OK, so what started out as a simple signal problem is now a major (at least cost and pride wise) power supply problem.
Checking the first TO3 transistor I can locate - short - one dead transistor. This is the power supply series chopper.
Checking the horizontal output transistor (HOT) with an ohmmeter - short - second dead transistor.
After removing transistor, I check for rectified line voltage at the input to the chopper - nothing. Tracing this back I soon locate an open fusable resistor.
So, whatever I touched probably caused the HOT to fail (forced on for too great a time can blow the HOT as a single shot event). The shorted HOT probably then took out the chopper transistor.
This is not fun. It is not likely to be inexpensive either. It does seem that no other parts have been sacrificed. Fusable resistors and driver transistors seem ok in so far as my meter is concerned. I still assume that the original problem was caused by a faulty IC400 but this point it is impossible to confirm this since the set id dead-dead.
Damage:
Chopper transistor - $10, horizontal output transistor - $6. IC400 - $15, fusable resistor - $1.
After replacing the components, making sure to use mica insulators and silicone heat sink compound for the transistors, the set comes back alive. Sync is fine. A little touchup of the video background and gain controls (unrelated to the sync problem) and we are done. Ouch.
Comments: The lessons learned here came at a cost - but mostly to my pride. Cascade failures are all too easy to induce through carelessness. Power supply circuits are not forgiving. One would think that probing the sync signal would not be able to kill anything. However, the design of power supply and deflection systems share some common characteristics. One of these is that a single instance of an improper drive waveform can blow the switching transistor as a single shot event - excessive current or excessive flyback voltage. This is a matter of exceeding the safe operating area of the transistor.
What you learn: if possible, make all connections to your test equipment with power off. Insulate all but the last mm of your probe so that any slip cannot cause a short. Work methodically, think things through, don't be over-eager, don't take shortcuts.
Symptoms: Totally dead - no front panel display or anything else.
Testing: Plugged unit into live outlet confirms description of problem.
First step: remove cover.
Second step: confirm that HV capacitor is discharged. Although the unit has been unplugged for several days, it never hurts to be careful. Discharge with high value high wattage resistor (well insulated) and confirm with HV voltmeter.
WARNING WARNING WARNING etc. Microwave ovens are probably the most dangerous piece of consumer electronic equipment in terms of potential for electrocution while being repaired. Much more so than TVs, for example.
Third step: test fuse. Open. Since these have ceramic bodies, it is not easy to determine if the fuse died due to an overload or a short by visual examination.
A microwave oven can blow a fuse for several possible reasons. Some of these are:
Some quick checks reveal that the capacitor is a dead short.
When replacing a microwave oven capacitor, it is important to get a fairly close match for the capacitance. The uF rating of the capacitor affects the microwave power output. Note that the 'working volts' rating on a microwave oven capacitor is not the same as on common capacitors found in other electronic equipment. It is not the maximum voltage permitted across the capacitor but closer to the VRMS rating of the HV transformer. And of course, before you start pulling wires off (1) mark down where they go and (2) discharge/check for voltage on the cap one more time.
Replacing the capacitor with one from MCM Electronics brings the oven back to life.
Comments: It is highly likely that the capacitor failed due to a defect in manufacture rather than some other underlying problem in the circuitry. When one thinks about how a capacitor is constructed - rolled up layers of foil and dielectric - it is amazing that capacitors do not fail that often. Any nick, thin spot, etc. represents a point of excess stress and can fail as in this case after considerable use - resulting in a short circuit, dead oven, and unhappy chef.
Symptoms: When first turned on, TV appears to function normally. Why was it tossed? Well, after 30 seconds or so, a pair of hum bars begins to appear in the picture gradually getting worse until horizontal width and sync are affected.
Testing: Using a Variac, there is a point below normal line voltage where set operates perfectly. OK, so I will keep a Variac attached to the unit!
After removing the cover, the first thing to suspect is the main filter capacitor. If this should dry up and lose some of its value, these would be the exact symptoms. Jumpering (with power off) of a known good capacitor I keep for this purpose doesn't change anything. But what is this? A discolored resistor catches my eye. Maybe it is changing value as it heats and causing these symptoms. I wait a reasonable time for the set to cool and measure the resistor - 360 ohms. OK, replace with new one. Expecting this to cure the problem I am disappointed when there is absolutely no change.
Off to the library for the Sams' Photofact. Darn - Sams' does not have a service folder for this model. Nor for any similar models that I can determine (the librarian was very cooperative).
Well, the problem seems to be heat related. I get out my trusty can of cold spray. After going through nearly the entier can, it would seem that there is only one part that has an effect on the hum bars when it is chilled. It is the SCR that is part of the power supply regulator. Rather than simply obtaining a replacement, I decide to trace the circuit to determine, if possible, the possible cause of the problem figuring at this point that the SCR is simply sensitive to heat.
During normal operation, an IC drives the gate of the SCR but what is this?? Until the secondary supplies kick in and provide power to the IC, the SCR is driven by - you guessed it - the mysterious resistor. The other end of the resistor goes to the raw DC on the main filter capacitor. Now that is odd....Since I do not believe much in coincidences, I now start rethinking the significance of this. Maybe that resistor is not quite what it appears to be.
First, I remove it and see what happens: nothing. Power on, power off, nothing.
Next, I momentarily touch the resistor to the circuit pad - the set comes alive. Then I remove it. The set remains alive. And, after several minutes, no hum bars. Hum....
I then try increasingly larger values of resistance until turn on is not reliable - 15 K seems marginal, so I will go with 8.2 K ohms = that is over 25 times what I measured! No wonder there were hum bars indicating regulation problems - that low value resistor was totally overwhelming the poor IC in driving the SCR.
The set is used daily and has been operating without further problems for over 5 years since reviving it. It works great with a $10 universal remote control.
How did the resistor get damaged in the first place? I have no idea - maybe its wattage was slightly underrated and it just finally decided to poop out. I have no way of knowing what the original value was supposed to be or even, for that matter, the wattage.
Comments: resistors can and do change value, sometimes, as in this case, quite dramatically. Without a schematic, there is no easy way to determine when and if this has happened - and what the original value should have been. However, any discoloration, burn, or scorch marks should arouse suspicion. With 20/20 hindsight, these signs may indicate the presence of carbon - a fairly low resistance substance and thus reduced resistance is likely.
And yet again, a $.02 part brings a complex renders a complex piece of equipment inoperative.
Symptoms: Player will shut off at totally random times or sometimes will not recognize the disc. There is a clicking associated with the problem - probably focus search failing.
Testing: Attempting to play various CDs to completion provided no indication that the particular CD or power adapter made the slightest difference. There were some false leads with respect to the latter but these turned out to be strictly coincidence. The lens was inspected and cleaned anyhow with no change.
The first hint of the source of the fault came as a result of an observation that pressing on the cover would sometimes either cause the player to stop in the middle of a disc or allow it to recognize and begin playing a disc when it would not otherwise cooperate.
Perhaps, the interlock switch was not being pressed in far enough. So, rather than open the unit (I really don't like messing with the insides of portable CDs if I can help it - you will see why in a few moments), I glued a bit of plastic to the post that pokes the switch.
This seemed to help. For a few weeks, the problems had for the most part gone away and the owner was a happy camper. Not surprisingly, this fix was only temporary.
Since the quick fix had some effect, it is very likely that I am on the right track. I will have to open it and deal with the switch face-to-face.
This is not too bad except that it is necessary to remove the main circuit board to access the switch which is mounted on a little board of its own. Four screws (large enough to actually see without a microscope) to get the bottom off, another couple to remove the main board. One more and I can remove or at least extend the switch circuit board far enough to inspect its solder connections and get at the switch.
The solder doesn't look too bad but there might still be hairline cracks that are not readily visible. A little reflow and they should be fine. (Problems with solder joints here are not related to heat as in a TV or monitor but rather due to the mechanical stress that is applied to the switch every time the lid is closed.)
Now for the switch. It appears that the cover of the switch can be snapped off relatively easily. The contacts appear somewhat gummy so I clean these and pop the cover back on.
Tests with an ohmmeter now show the switch action to be solid. Wiggling the switch lever and/or the entire switch has no effect.
Great, put it back together and I am done.
After replacing the switch board, main board, and bottom cover - the test.
Fanfare please!
Nothing. The player is dead as a door nail. It now will not even focus and gives up almost immediately.
Off come the screws. Almost immediately, it is obvious what has happened. In replacing the main board, I accidentally squashed one of the printed cables linking the optical pickup and main board, partially severing the cable. In fact, 2 of the 4 conductors are cut. This is the focus and tracking drive cable so it is pretty important. What a pain!
Fortunately, luck is on my side with respect to the location of the break - it is at a non-flexing part of the cable. Therefore, repairing the cable should not be that difficult since once the conductors are connected electrically, they can be coated with a sealer and flexing will not be a problem.
To repair a cable of this type, I have two options: I can attempt to jumper the break with some fine strands of wire or I can go point-to-point from the circuit board to the destination on the optical pickup. However, the latter connections are nearly hidden and would be difficult to solder.
I opt for the first. Using an Xacto knife, I carefully scrape the orange mylar coating from both sides of the break. Then with #30 wire, I carefully solder across the break for each of the conductors. A spring clothespin holds the wire in place during the soldering. The entire affair is then coated with some clear sealer to reinforce it mechanically and provide insulation. It isn't pretty, but it will work fine. For added protection, I add a layer of plastic electrical tape.
Now, finally, reassembling the unit keeping cable routing firmly in mind, there should be no problem.
And, as expected, the player comes back to life and is rock solid with respect to playing and recognizing discs. The oops should have no effect on the expected longevity of the player.
Comments: we all can point to those minor disasters where we have overlooked something where we should have been more careful. Whenever reassembling anything, it is imperative that lead dress (ok, fancy term for how the cables are routed) is kept firmly in the forefront of your mind. It seems that with more and more miniaturization, this is an increasingly important and at times, frustrating consideration. First of all, it is very tempting to say when disassembling the unit 'this is obvious, no need to write it down'. Bad move. Often, it appears much less obvious when putting everything back in its place. I have never quite figured out how they do it during manufacturing - correctly most of the time.
Ignoring cable routing can lead, as in this case, to severed wires. It can also result in shorting between wires or between wires and sharp metal brackets or shields. Broken wires can usually be repaired if they can be located. Shorted signals can result in additional hard-to-locate collateral damage which can really turn your hair gray.
What is even scarier is that with line connected electronics or appliances like vacuum cleaners and even toasters - this can lead to electrically live parts accessible to the user. Sometimes, the plastic insulation on typical internal wiring will not fail immediately but will cold flow and cause problems later. So, one should always make every effort to assure that no wiring is being pinched and for metal cased appliances, check that the case is not electrically live - has a high resistance (usually infinite, but at least a few M ohms to both wires of the AC line with any on/off switches in both positions) after the repair is completed. For non-heating appliances or electronics, a little electrical tape goes a long way. For heating appliances you really need to make sure that bare wires are routed far from any exposed metal of the case taking into account as well any motion that may occur during normal operation or due to being knocked about or dropped.
Symptoms: Drive behaves the same as a similar working drive until it is accessed. Then, there is no response by any DOS or Windows software. No CDs are recognized, always get the message: Abort, Fail, Retry?
Testing: I keep an old (well, what other type are there?) 286 PC clone system around for the primary purpose of testing peripherals. Installing the drive and software confirms the reported behavior. I was given two similar drives. The other one was reported as being intermittent but seems to work fine in my test system. This one was indeed dead.
Since it is impossible to observe the behavior of the pickup and, in particular, the lens with the cover on, the first step is to get at the guts.
Fortunately, the CDU33A is quite simple to disassemble.
There are only two major components: the Printed Wiring Board (PWB) where all the active electronics are located and the Optical Deck including laser, optics, and pickup worm drive mechanism.
The other parts include the upper plastic casting and metal shroud, solenoid latch assembly, right and left guide rails, drawer assembly, and front bezel, two springs, bottom plate, 6 screws.
There are only two electrical connectors inside: one flat printed cable linking the PWB and optical deck and a two pin connector supplying power to the eject solenoid. This is in pleasant contrast to some other CDROM drives I have seen with a half dozen or more small connectors spread all over the PWB making removal and testing very difficult and risky.
After about 10 minutes, I have the drive apart and can now reassemble the major components on the bench outside the case to observe behavior.
I prop up the circuit board and reconnect the flexible cable - noting the orientation marks. I can now run the drive with full visibility of the mechanism and optics. With a CD in place, there is no danger from the laser beam. I make sure the PWB cannot short to anything and that the whole affair cannot tip over.
Having set up this contraption (you would have to see it to appreciate appreciate this terminology), I am ready to continue testing.
Naturally, it now works perfectly.
No amount of abuse seems to phase it - wiggling cables, flexing the circuit board, trying multiple CDs, all fail to reproduce the original problems. Could it be the case? I can think of no reason why it should make a difference? Is there anything else different? I don't think so. Perhaps the sled was jammed somehow and disassembling the drive fixed it. Who knows.
After reassembly, the drive continues to function perfectly.
Comments: How many times has someone brought you a 'broken' device which has magically started working again on your bench. It certainly cannot hurt your reputation. Admittedly, here, I had to actually do an exploratory before rejuvenation to convince it that I meant business.
It has now been almost a year and the drive continues to function. I can only guess that the cable may have been poorly seated or had some dirt stuck in the contacts. Until it fails again, there isn't much more to try. Unfortunately, the saying: "if it ain't broke, don't fix it" now applies. I have no idea if the drive will ever again fail within its normal life expectancy, but in the meantime, where did I put my Win95 CD? (No comments, please, about choice of OS).
Symptoms: Width slightly reduced. Slight evidence of 60 Hz hum bar, brightness pulsating, raster shaking, somewhat channel dependent.
Testing: All of these symptoms were easily reproduced on the bench. The 60 Hz hum bar is the giveaway indicating a low voltage power supply problem.
Rather than operating the TV off a Variac to confirm lack of regulation, I decide to just try the most likely solution - a replacement main filter capacitor. With power off and making sure the main filter capacitor is discharged, I use a pair of clip leads to jumper my test cap across its terminals.
The set now works perfectly.
Removing the old capacitor (not easy as the rivlets really do make nice heat sinks), testing with my trusty Radio Shack DMM on its capacitance scale reveals that the value has dropped by over 85% - pretty amazing that the set worked at all!
One highly overpriced replacement filter capacitor (I used a local distributor instead of my favorite mail order sources) and the deed is done.
No disasters on this one!
Comments: This capacitor was mounted next to a large heat sink - possibly the power regulator. When replacing electrolytics, we often ignore one very important specification - the temperature rating. Either the original capacitor was defective or it was not rated for the thermal conditions inside a compact TV. The TV was not that old - maybe 3 or 4 years at most. We all can point to equipment we own that is still working after 20 or 30 years going strong on the original filter capacitors.
Symptoms: Dead as a door nail. Only evidence that it is connected to the line is a momentary flicker of lights when TV is turned on indicating that the main filter capacitor is being charged.
Testing: This set has a pull-type on-off switch. There were no blown fuses. Checking with a voltmeter shows 150 V on the main filter capacitor with the switch in the on position. Ditto for the collector of the HOT.
This would seem to indicate that there is a problem with the startup drive to the Horizontal Output Transistor (HOT).
Off to the library for the SAMs....
Many Zenith TVs use a simple multivibrator to generate a startup signal to the horizontal driver transistor until the flyback can generate the secondary voltages needed to operate the deflection ICs. Once these voltages are present, the startup circuit is disabled. Indeed, such a design is used for this TV.
Checking with a scope (powering the TV through my isolation transformer) at the base of the HOT shows no drive signal.
Tracing back, there is no signal at the driver transistor or from the output of the startup circuit. One of the two transistors in the startup multivibrator is bad.
I do not have a suitable replacement - it is a high voltage low current Zenith part similar to an MPSA43 - 200V. I will need to obtain one, or better yet, two to replace both transistors in the multivibrator.
To confirm that the rest of the TV is operational, I use a common technique to 'jump start' a TV where the startup circuit is defective. This is to inject a signal of around 15-16 kHz directly into the base of the HOT to substitute for the startup circuit.
With the TV turned on, momentarily touching the output of a pulse generator set for 15 kHz and a couple of volts amplitude to the HOT base brings the TV to life. Everything appears normal except that the TV does not start on its own. Somehow, I don't think my neighbor would approve of this solution. (Also, I am not giving up my pulse generator!).
Caution: jump starting a TV like this is risky. In addition to the dangers of mucking with a live TV, injecting a signal with improper characteristics into the HOT can destroy it and possible a lot of other circuitry - instantly. For example, a single cycle with too long an ON time can blow the HOT from overcurrent while driven on or overvoltage during flyback. Use this approach with care.
Replacement of the multivibrator transistors with the exact Zenith parts completes the repair successfully.
Comments: Examining the schematic of the startup circuit reveals that it appears to be designed to fail - especially with kids about. While the transistors are rated at 200 V (they are running on the 150 B+ from the line power supply), the transistor power rating is only .6 W. Even though they are running in a switching mode, I believe that repeated on/off cycles can stress these to the breaking point. Something was mentioned about my neighbor's kids turning the TV on and off repeatedly. I have not duplicated this experiment but suspect that such treatment at least may contribute to premature failure. Fortunately, in this case, it was only in the startup circuit.
Power-on is a stressful time for many types of equipment due to inrush current, transient voltage, so many things changing quickly, etc. In addition, designers may not study and characterize the behavior during startup with the same amount of care that they presumably (we hope) do for steady-state operation.
Symptoms: Inserting a tape works fine - it plays, it records, it FFs, it REWs. However, attempting to eject a cassette results in an infinite loop - the VCR grabs the tape back just before it pokes out of the slot. Sometimes, the tape can be grabbed in time but usually the cassette does not exit far enough.
Testing: Symptoms confirmed. With the top off, it is easy to catch the tape but I don't suppose this would be an acceptable solution. In addition, the cassette carriages seems king of sloppy - loose for want of a better term. This would indicate a mechanical problem with the cassette basket - the mechanism which moves the cassette into position inside the VCR.
First step: a close examination of the basket mechanism. Nothing obvious - no broken parts visible.
Next step: attempt to remove the basket. With most VCRs, this is a simple matter of 4 screws and perhaps a connector. Not here. There are 4 screws, but once the screws are removed, only one side wants to come loose. The left side, with most of the gears and whatsits, is firmly fixed to the base of the tape deck. No doubt, there are critical timing relationships that might be disturbed once removed. It stays for now.
Perhaps, removing the bottom cover will reveal something. 8 screws later, bottom cover off. What's this? A spring!! So now, we know that something is indeed broken and most likely in the basket somewhere. This sort of spring is not the type to have just popped off - it is a close wound coil spring with hooks at each end. And, guess what, there is also a tiny bit of white nylon which was probably the tab onto which the spring was hooked at one end.
A close examination of the visible portions of the basket above and below deck finally turns up something now that I know generally what I am looking for. Thankfully, it is accessible and I hopefully don't need to pursue removing the basket which almost certainly would not be a fun thing to do.
The spring is supposed to be connecting two gear-type wheels in the EJECT mechanism. With the spring sprung, these were free to rotate when they should not have and their free play was sufficient to cause the EJECT operation to screw up.
So, how to repair? There is no good way to glue nylon and even if there was, the tab is so small that it would be impossible to provide a strong enough bond to withstand the spring force. Replacement of the part with the broken tab is a possibility though again not a pleasent one - it would require removing the basket. Of course, replacing the entire basket is another unpleasent and expensive options. Installing a metal post in place of the tab is also a possibility - one that I do not really want to contemplate.
Well, it appears as though there is nothing particularly critical about the spring placement. Is there an alternative location to connect the end with the broken tab? Yes, it would appear that it will be sufficient to hook it around another large wire spring. However, then it is probably stretched too long, so I make a link out of a piece of a paper clip and this seems to be about right. (Paper clips, bailing wire, scotch tape and chewing gum (well maybe not chewing gum) are among my favorite things). Getting all this in place under spring tension between the edge of the case and the basket plastic frame proves a bit of challenge - requring a dental picks, needlenose pliers, patience, and few carefully chosen four letter words - but I prevail. The EJECT operation now works perfectly.
While not pretty, I believe the newly designed spring attachment will be much more robust than the original. I should write to Samsung!
Comments: This is a another case of poor design - there can be no other way of describing it. The spring is rather large (you can visualize it, can't you?) and the tab much too small. Another .0001 cent of plastic and it would outlast the rest of the VCR. There was absolutely no excuse as there is plenty of space to enlarge and reinforce the tab.
Symptoms: Power light blinks indicating that it is not able to run the program contained in the game cartridge.
Testing: Tried multiple cartridges without success.
The most common problem with these units is a worn or dirty system unit game cartridge connector. In this case, the red power/status light will continue to flash even after the RESET button is pressed with a game cartridge in place. Replacements are available for about $9 from the usual sources (MCM Electronics, etc.)
First, I try another game cartridge - the one that is not working may just have dirty contacts or may be defective. This does not work.
So I need to get inside. Fortunately, unlike some other consumer stuff, this is quite easy. Six screws underneath followed by about a dozen to remove the metal shield and circuit board so the connector can be removed and inspected.
Before removing the connector from the circuit board edge, I give the system another chance to redeem itself. With the latching mechanism removed, it is possible to press the cartridge down somewhat lower than normal increasing the chances for good contact. Indeed when this is done, it is possible to occasionally get a good reset and game startup on the TV. This certainly confirms the original suspicion.
Now, can I revive the original connector or must it be replaced? There are three kinds of problems that generally occur with these connectors:
Don't neglect the game cartridge connectors. These generally do not wear but may collect all kinds of strange stuff. Rather than fight with the security screws that you may find holding the case together, I usually simply use a Qtip with water, contact cleaner, or alcohol - or one after the other - to clean these contacts. Again, very fine sandpaper may be needed in extreme cases.
Even if these procedures only make a slight improvement - you can press down on the cartridge and the machine will respond to the RESET button - you have confirmed that the connector is indeed the problem. In many cases, just cleaning will result in reliable operation for a long time to come.
In the case of this particular system, all three problems were present. However, for the time being at least, the system has responded well to treatment.
Comments: While the original Nintendo game machine is a couple of generations out of date, many are still in use. And, hey, young kids usually don't care. OK, you don't have to admit to being the one who cannot resist just a couple rounds of 'Super Mario III'!
Old Nintendos can usually be picked up for $5-20 at garage sales sometimes complete with a selection of games, sometimes bare. The games go for $1-$5 depending on the barganing skills of the kid selling the stuff.
However, a bad connector is almost a sure bet with a secondhand system. Consider that most electronic connectors are typically rated in terms of hundreds of insertion/removal cycles. A Nintendo machine must endure thousands of not necessarily gentle cycles over its lifetime. The connector was not designed for that. Furthermore, you are likely to find all kinds of muck inside, mostly unidentified, and often difficult to remove. Nonetheless, these things are remarkably robust, electronic failures are infrequent, and they can usually be revived without much difficulty.
Symptoms: Former owner complained about difficulty in ejecting.
Testing: Tried playing multiple cassettes (not all at once!). For the most part, the VCR behaved normally. Maybe just a bit sluggish loading but no other obvious problems. Why did he dump it?
I did my usual cleaning - rubber parts did not look to bad, leave them for now. Even the idler tire appears to be in decent condition. I will use the VCR and see if any problems appear.
The first sign of trouble appears once when attempting to use REVIEW mode - the VCR abruptly stopped and attempted to unload the tape. The loading motor was spinning but nothing was happening (I think it was turning in the wrong direction and the belt was slipping - I am not sure). Oh boy, time to leave the cover off. Manually giving the motor shaft (fortunately it is accessible from above the deck) a couple of turns convinces the VCR to complete a correct unload cycle.
Well, this sounds like the classic 'if it is an erratic Sharp VCR, the mode switch must be dirty or bad' problem.
(2 years pass as I am in no mood to bother with this repair at the moment.)
OK, now I have a need for a reasonably decent VCR to replace my cousin's Mitsubishi HS328U which is finally dying. So, I dig the Sharp out of the closet and see about its condition. Now, it doesn't even want to play a tape at all. Well, I know I have to deal with the mode switch, so first things first.
The mode switch on this model is sandwiched between the loading gears and a mounting plate - all parts of what I will call the 'loading gear assembly'. To access the mode switch, this entire unit needs to be removed and partially disassembled. The gears operate the roller guide loading mechanism, and a couple of cam operated levers which are conveniently hidden when it is removed or reinstalled. It is driven by the loading motor via a couple of idler gears.
Timing marks: In the unloaded position, there is a hole in one gear that appears to line up with a slot. So, with the roller guides retracted (and the gears which operate this linkage have timing marks which also line up), this hole should be centered in the slot. Fine. This appears to be the only critical relationship with respect to removing the loading gear assembly.
I unsolder the 4 connections to the mode switch, remove 3 screws, and - sproing! What was that? OK, one or both cams still had a lever with spring pressure applied. Hopefully, it will be possible to extend these these when replacement time comes along.
With the loading gear assembly removed, it is still not possible to access the mode switch. Now to disassemble it. There are two fancy cam gears which obviously must be timed correctly - in one position there appear to be an arrow and triangular hole which line up. I add a couple of marks of my own for good measure with a felt tip pen. A simple split washer holds the gear I need to remove onto its shaft. (Note: these split washers are not designed to be reused but with care in removal, they can usually be replaced without any long term problems. Of course, a professional would have an assortment of replacement sizes handy.) Removing the gear carefully, there don't appear to be any flat washers or spacers to worry about.
Once the gear is removed - making a note as to which side is up though this is pretty obvious - the mode switch is exposed. Squeezing the center of the split shaft enables the cover to be popped off and the interior appears. I almost lost the springy wiper as it is not fixed to the plastic cover but popped free when first removed. A frantic search was needed to locate it on the floor. The wiper fingers and encoder contact traces seem to be in good condition but whatever was used as a lubricant is a little gummy and might be the problem. A simple cleaning seems to take care of that. I also bend the wiper fingers a bit to increase the contact force very slightly.
Now, to get everything back together. First, the wiper is replaced and the mode switch cover is snapped back in place. Free rotation is confirmed. Then, the gear that was removed is returned to its shaft along with a cam follower lever that was under it. The split washer is replaced.
To install the entire loading gear assembly means that the original gear timing relationships must be re-established. In addition, care must be taken to make sure those two cam follower levers I mentioned previously are properly positioned. This takes a bit of work but eventually, I am convinced that everything works as it should. The screws are tightened and then free movement of all the parts is confirmed by manually cycling the loading mechanism. The 4 mode switch connections are then resoldered.
Now for the test. Since this was not a hard failure to begin with, there is no guarantee that any problems will be detected.
The tape seems to load correctly but then the VCR unloads and shuts down. What is wrong? It would appear that the takeup reel is not turning. Hum, probably that rubber wasn't as great as I had assumed a couple of years back. I now do a more complete cleaning and, in particular, remove the idler tire and inspect it. It appears to be ok but as a test, I turn it inside-out.
Now, everything works as expected. Testing with a cassette cheater (shell), there appears to be adequate takeup torque. I clean the idler tire again and reinstall it in the normal configuration. All modes appear functional even when testing with a full takeup reel - requiring the most takeup torque. I will order new rubber anyhow and replace it at a convenient time or if problems reappear.
This VCR now appears to operate reliably and consistently. I have seen no evidence of the original erratic behavior. Only time will tell for sure.
Comments: I cannot overemphasize the importance of making careful notes as well as adding timing marks of your own when removing any parts of a VCR which could conceivably have critical timing relationships. Not doing this can really mess up your day. Err on the side of excess - it won't cost you anything.
Sharp VCRs seem to be particularly prone to mode switch problems: Of the 3 Sharp VCRs under my control, 3 of them have developed dirty mode switches resulting in a variety of erratic symptoms including, as noted, going into the wrong mode as well as aborting the tape loading operation for no good reason.
It would seem that a VCR design using an optical mode switch instead of one with sliding contacts would be much more reliable at only modest additional cost. After all, VCRs already use a number of optical sensors and cheap computer mice use optical encoders not very different in design from a mode switch. At least, it would be nice if mode switches were readily accessible. Some are visible as soon as the bottom cover is removed. Others require substantial disassembly with associated risks of incorrect reassembly resulting in mechanical timing problems or even damage when the unit is cycled.
Symptoms: Horizontal deflection jittery, possible vertical collapse, arcing flyback - all in one set! This info from Dave whose friend owns the set. Dave is a tech at work who is now doing more software than hardware (not necessarily by choice).
Testing: I did not actually see the original problems, nor did I have access to the entire set as Dave came in one morning with the guts of this set under his arm (more like both arms). We actually attempted to power it without the yoke or CRT but there was absolutely no evidence of anything. Surprise surprise.
Since the original description of the problems is somewhat incomplete, a visual inspection is made and the HOT is tested for shorts just to be sure. There were none. However, the visual inspection did confirm that the flyback had a narrow but rather long (maybe a couple of inches) crack in its housing, There was no conclusive evidence of arcing but this is one area where the original symptoms were fairly definitive as the owner stated that there was arcing around the flyback. (He probably knew just enough to be dangerous, but hopefully has not done anything we will regret.)
This would explain the jittery horizontal but what about the vertical problems? Were there really vertical problems. I never did get a good answer to this question - at least not until later.
While it is likely that the flyback could be patched up at least temporarily, it was decided to order a new one. The owner was willing to spend up to $150 to repair the set - I have no idea why. No match from places like MCM Electronics - must go directly to Magnavox (Philips, actually). $71, ouch. Admittedly, this is one of the spiffiest flybacks I have seen lately (at least since that A-line Zenith with the cool ribbed plastic coil form). It has a detachable CRT anode wire - wire and suction cup sold separately! Well, for $71, you cannot expect everything.
Although we have agreed to order the flyback, I decide to test the old one anyhow, so next day I bring in my flyback testing widget (12 V chopper, see document on flyback testing). This is the first time Dave has seen this tester and Ed (our chief digital design engineer) is also curious but stands at a safe distance, having a great deal of respect for a few puny kV. Ed always stands at a safe distance when anything higher than 5 V is involved!
First step - locate the HV return. In this case, it is obvious because (1) a separate bare wire is brought out to a pin and (2) this wire is connected to no other pins on the flyback. (With a built in HV rectifier, it is not possible to use a normal DMM to locate this wire.)
Next step - wrap a ten turn coil around the core of the flyback and connect this to the chopper.
Apply power - a nice healthy arc can be drawn from the HV lead of the flyback to the return connection, current draw on power supply is low. Flyback is quite functional. This does not test for breakdown at full voltage but does rule out hard shorted turns. (Ed can be overheard mumbling something about sticking with 5 V logic.)
Result is fine by me, owner wants new flyback and this one will make a great HV supply for a plasma globe or something - someday.
So, we pile the chassis and all its attachments onto a table in the corner of the testing lab awaiting our shiny new flyback (minus the red wire, some assembly required). It looks kind of pathetic there but no one else dares go anywhere near let alone touch it after an off-hand comment about charged capacitors!
Approximately 5 days later, our new flyback arrives and is soldered into place.
Next morning: I see Dave pulling up in his Chevy wagon. Guess what is in the back? The entire huge, heavy TV, belly down. Oops. We quickly find a place for it somewhat out of the way in a back room.
Apparently, there is no arcing and the horizontal deflection is stable. But, there is absolutely no vertical at all - flat lined. OK, so the rumors about vertical collapse were not exaggerated.
A little more visual inspection reveals a couple of interesting observations. First, all the deflection circuitry - both horizontal and vertical - is clustered in a small area near the flyback. In addition, the crack in the crack in the original flyback is adjacent to some of the *vertical* output circuitry. So, perhaps, the arcing was making its way to something in the vertical deflection. What kind of output chip is it? Ah, my favorite - a TDA3654. Fortunately, I have a bunch of them to keep one of my tough dogs fed. So, I am well prepared if need be.
A quick measurement of power to the TDA3654 reveals that there is none. Maybe this won't be so bad after all. Tracing back with an ohmmeter and what do I find? An open fusable resistor! And, in exactly the right place to be killing power to the chip. Could it be this easy? Actually - yes in this case. I install a normal 1 ohm 1/4 watt resistor (only for testing). I also use the ohmmeter to confirm that the rectifiers in the vicinity are healthy. We are set!
Power! At first there is nothing on the screen but then snow gradually appears - and it is full screen. There is no antenna. Of course, reception inside our building is nearly non-existent due to all the computer RF interference and steel beam construction. However, we quickly locate a pair of rabbit ears (or maybe it was just a couple of feet of hookup wire) and tune one of the few channels that is viewable at all - which happens to be broadcasting the morning cartoons. But that is just fine. Everything appears normal and I remind Dave to replace that resistor with a proper flameproof variety.
Dave cannot believe it.
Ed is nowhere to be found.
Everyone loves the cartoons.
Comments: The mechanism for the vertical failure still remains obscure. Apparently, the arc caused a momentary but not fatal short circuit in some part in the vertical output circuitry which blew the resistor. We always hear how sensitive ICs are to static - here we have 25 kV of raw power discharging nearby with apparently no permanent damage except to a 25 cent resistor.
Symptoms: Horizontal position shifted almost off the screen; delayed sweep and B timebase inoperative. Alternate triggering erratic at low intensities(??).
Testing: How does one test a scope? Well, put it through its paces with reasonable input signals - a 10 MHz clock oscillator provides a nice test signal. Maybe another scope would be handy?
Prologue: (You can tell right off that this will be a feature length saga.) I bought this scope at a garage sale. Now, understand, garage and tag sales around the Philadelphia area where I live are usually of the "Aunt Minnie's old silver plate" variety. Electronic equipment is usually limited to comatose VCRs and color TVs that play in B/W (not that I complain about this sort of stuff for the right price - as little as possible). However, one little ad catches my eye: one item amongst all the bric-a-brac is 'test equipment'.
BTW, I never go to flea markets with any serious intention of buying anything. It is clear where their stuff generally originates. All the junk I turn down at garage sales ends up with hugely inflated prices at flea markets!
I get to THE sale relatively early (I am not quite the garage sale addict type who gets up at 5 AM to be first in line). All that is visible are a couple of pathetic old signal generators - one audio, the other RF. Well, $10 for an RF signal generator isn't too bad. I could probably have bargained him down to $5 but first the all important question: Anything else? (Not that I expected any sort of affirmative response given the assortment of hat boxes, deflated basketballs, and old Christmas decorations.) However, surprise surprise! "There is one other item." So he crawls under a table and drags out an HP AN/USM281A - a real oscilloscope! "Well, I have this, um, oscilloscope. It is solid state, dual channel, 50 Mhz, etc." Now, I am paying really close attention (but of course, not wanting to show it). The only oscilloscopes I had seen at garage sales until this time (beside my $3 Tek 321, but that is another story) are usually the really beaten up Eico variety). He is actually doing a pretty fair sell job. So, how much are you asking? "I would like to get $100 for it." Very interesting. Can I try it? "Sure." So, he props 'my' scope on top of a rickety old bar stool (I would have been quite upset if the thing had gone crashing to the floor but still didn't want to act interested enough to suggest he find a more stable spot.) I figure that if it appears to work at all, $100 is a good price even if I have do some repair and calibration. I fiddle with the controls, also noting that it comes with two nice looking 1X/10X probes. Suddenly, the scope really cooperates - it must really want a new home being so lonely stuck in the back of that garage. The trace scoots off to the right of the screen. None of the front panel controls have enough range to bring it back. I mumble: I cannot get the trace back. He says "Oh, um, uh..." Before he can get too far, how about $50. "Sure, ok." I didn't do enough testing to find out that the delayed sweep was also dead. For that matter, even with the 10 turn delay time pot in plain view, the existence of a delayed sweep mode did not register. I only found that out later. No amount of fiddling would produce any difference between the A timebase and Mixed A+B. The B timebase was totally dead.
So, how to go about tackling this? I have no service manual, no schematics, and looking at circuit boards, the semiconductors are HP house numbers. I didn't have an ECG manual. Fortunately, the component side of the circuit boards are readily accessible. However, tracing the wiring is a real treat with HP's love affair with multicolored striped bundled wiring harnesses. I could try to buy a manual (this was somewhat before the days of sci.electronics.repair). Nah, that would be cheating (and probably expensive). I did try our local HP sales rep when we were looking into their logic analyzers but he did the usual salesperson thing and lost interest once we signed on the dotted line.
So, I had to repair it the old fashioned way - ohmmeter, circuit tracing, seat of the pants, etc.
Objective #1: find the horizontal position problem. Even if the delayed sweep remains broken, a dual trace 50 MHz scope is very useful. Fortunately, this problem appears solid now (and not intermittent as was the case previously) so ohmmeter tests of the horizontal sweep board components should be possible. Tracing back from the deflection plate connections to the CRT finally results in a difference between apparently symmetric sections of the output stage. An 11 K ohm power resistor tests open! Well, that wasn't too bad. It doesn't appear as though there is any cause other than age. Replacing the resistor restores full range control of horizontal position. About 1/2 hour to find. Not too shabby.
At this point, I considered the operation a success and put the other problems aside. Partially, this was because what I had was usable and partially because I did not expect the delayed sweep/B timebase problem to be nearly as easy to solve.
(Actual elapsed time: about 2 years. Well you know how I suggest 'sleeping on a problem' when you cannot solve it immediately!).
Objective #2: Fix the B timebase. The only help I have with this is the fact that the A and B timebase circuitry is quite similar so some comparisons of resistance measurements will be possible. However, for some reason, my first pass over the components with an ohmmeter does not turn up anything obvious.
Using another scope (a Tek 564), I poke around the B timebase circuitry a bit to see if I can locate any interesting signals. I suppose my definition of an 'interesting signal' is one that is doing anything - not flatlined.
What's this? A ramp waveform - it must be derived from the A timebase as it maintains a fixed relationship with the A timebase output but goes into the B timebase circuitry. My guess would be that this was be used to provide a gating pulse of variable width controlled by the delayed time (10 turn) pot. Maybe following this signal will lead to something. Sure enough, it goes to what must be a comparator circuit since the pot also connects nearby. And - what is that? A charred resistor! Now, why didn't I see that before? Fortunately, there is another identical circuit across the board and that resistor is readable - 100 ohms. Now we are cooking (hopefully not literally). With great expectations - switch on. Still no action from the Mixed A+B or B timebase. As far as I can tell, not a thing has changed. The B timebase is still as dead as a door nail. The good news is that the resistor is not getting hot, so that is encouraging - probably.
Next, I continue on with my search for shorted or open parts with an ohmmeter. This time, I will be more systematic hopefully not missing anything. Finally, results! Another blown part. This time, it is a transistor. From the remaining good junction I can at least tell it is supposed to be PNP. Measuring the voltages across C-E, it appears as though a 2N2907 will be of adequate ratings (at least voltage wise). Hopefully. Soldering in the new transistor from the top of the board is not fun - but more fun than attempting to remove the board entirely. Power: Darn, still no action.
Continuing with the ohmmeter finally turns up a shorted diode but what type? Again, duplication comes to the rescue. It is a 54 V zener. I use a pair of 25 V or so zeners temporarily to substitute for this unusual diode. Now, finally, the B timebase is responding to treatment! Having done what I always caution against - turning adjustments without marking them - the behavior is a bit - strange, but the two relevant controls (don't ask me at this point what they did) could be set for proper operation of the Mixed A+B as well as B timebase.
How could 3 parts in apparently not directly connected circuits be blown? I have no idea except to suspect that the previous owner may have attempted to repair the B timebase and never stumbled upon the actual cause which was most likely the bad zener but managed to blow other parts in the process.
Objective #3: Determine why turning the intensity way down causes the alternate sweep mode to get stuck on one trace. This one is a minor annoyance only and really doesn't affect the utility of the scope in any way. It is just not quite perfect. I finally locate a couple of adjustments in the vertical plug-in that seem to have an effect on the rise time of the start of the sweep or something - they seem to modify the strange behavior as well as affecting the linearity of the left 1 cm or so of the screen for high sweep speeds. I finally find a compromise position that seems to be satisfactory.
Wow, a whole bunch of simple problems but now it appears to be in pretty good condition. A couple of drops of oil for the delay time control, clean the switches and controls, and a piece of Plexiglas to protect the CRT and it is done.
Comments: What do they say? "Real engineers don't need no f***ing manuals". Or is that programmers? Well, whatever. A service manual would sure have made the task a lot easier. But where is the sport in repairing something with actual accurate documentation?!
The AN/USM281A is still a good solid easy to use oscilloscope for general design and debugging. I don't know if this one ever saw duty in the Navy but I have seen this model going for about $250-300 from test equipment outfits like Tucker. In my tests, the scope will display a viewable waveform and lock at frequencies greater than 80 MHz even though the specs are for 50 MHz vertical bandwidth. The mainframe itself, I think, is rated for 100 MHz. It takes standard HP180 series plug-ins so if I ever come across any of those at another garage sale.... (about the same time pigs learn to fly!)
Symptoms: Platter turns but 'lock' light flickers and speed is slow and uneven (even to the unaided eye and without listening, it is obviously struggling).
Testing: Both speeds (33-1/3 and 45) have similar problems. The selector switches appear to be clean and solid. By gently touching the spinning platter, it is found that there is also essentially no torque - it stops very easily.
It was quite obvious that the servo system was having difficulty reliably locking - as evidenced by the flickering 'lock' indicator and lack of torque. In addition, the platter did not appear to want to start up reliably at some rotational positions.
So, what does it use as a reference? Remove the platter! This requires popping a cosmetic cover and large E-clip. Aha, what is this? A little pickup near the edge that looks sort of like a tape head. However, unlike an audio or digital tape head, it has a series of offset laminations about 1 mm in thickness. Not visible but inferred is a magnetic stripe pattern on the inner surface of the platter which is in close proximity with the pickup when the platter is installed. Better keep magnets far away. That pattern was put on with a special jig at the factory - there would be no way to reconstruct it if some, say, accident were to take place.
As a long shot, I attempt to adjust the pickup closer to the platter surface. Perhaps the magnetic pattern has weakened or something else has drifted. Ouch, now it is rubbing. A little further back. Ah, now it is clear.
Not too surprisingly, there is no change. Symptoms are identical.
Now what? Unfortunately, there is no way to get at the circuitry - under the platter - when the platter is installed. There is no way to excite the magnetic pickup with the platter removed. Not true! After storing the platter at a safe distance, a simple magnet will generate a 20 mV signal out of the pickup. This is probably greater than the normal signal level.
The circuit is pretty simple - couple of transistors and other stuff being fed from the pickup for feedback and hall effect sensors for motor control. The motor is a brushless DC 4 pole type with the commutation control external and on the same board as the servo lock PLL.
Maybe a little signal tracing is in order. Using my magnet, I can see the feedback signal making its way through to what I assume is part of the PLL - probably the phase detector.
In retrospect, even suspecting the PLL's feedback signal was probably not valid. The key is the dependence on rotational position during startup.
So, what about the motor signals? There appear to be two outputs from the motor (in addition to the coil driving signals). What do these look like? The first (from H1 - Hall sensor 1) flips between +5 V or so and ground as the motor rotor is rotated through a complete revolution (Wow, Peter Piper picked a pack of...., sorry,