Contents:
If you suspect shutter speed problems, there are several easy ways to measure this for your camera. The most accurate require some test equipment but you can get a pretty good idea with little or no equipment beyond a stopwatch (for slow shutter speeds - above 1/2 to 1 second and a TV (for fast shutter speeds - below about 1/60 of a second (NTSC 525/60). Some of these approaches assume that you have access to the film plane of the camera - this may be tough with many highly automated compact cameras which will be unhappy unless a roll of film is properly loaded with the back door closed. Note that the behavior of focal plane and leaf (in-the-lens) shutters is significantly different at high shutter speeds and this affects the the interpretation of measurements. Some simple homemade equipment will enable testing of the intermediate shutter speeds. 1. Testing slow to medium shutter speeds - the use of a stopwatch is self evident for really long times (greater than .5 second or so). However, viewing or photographing the sweep hand of a mechanical stop watch or a homemade motor driven rotating white spot or LED can provide quite accurate results. Accurate timing motors are inexpensive and readily available. Mount a black disk with a single small white spot at its edge on the motor shaft and mark some graduations around its perimeter on a stationary back board. For a high tech look, use an LED instead. Use your creativity. Making measurements from the photographic images of the arcs formed by the spot as it rotates while the shutter is open should result in accuracies better than 1 or 2% for shutter speeds comparable to or slower than the rotation frequency of the motor. In other words, shutter speeds down to 1/10th of a second for a 600 rpm (10 rps) motor or down to 1/60th of a second for a 3600 rpm (60 rps) motor. At these speeds, focal plane and leaf shutters should result in similar results since the open and close times are small compared to the total exposure time. 2. Testing fast shutter speeds - view a TV (B/W is fine) screen on a piece of ground glass at the focal plane or take a series of snapshots of a TV screen (a well adjusted B/W TV is best as the individual scan lines will be visible). Note: If your camera has a focal plane shutter (e.g., 35 mm SLRs), orient the camera so that the shutter curtain travels across - horizontally (rather than up or down). If you are photographing the screen, take a few shots at each speed in case the timing of your trigger finger is not quite precise and you cross the vertical blanking period with some of them. This will also allow you to identify and quantify any variations in shutter speed that may be present from shot-to-shot. For a focal plane shutter, you will see a bright diagonal bar. (The angle of the bar can be used to estimate the speed of the shutter curtain's traversal.) For a leaf (in-the-lens) shutter, you will see a bright horizontal bar. but the start and end of the exposure (top and bottom of the bar) will be somewhat fuzzy due to the non-zero time it takes to open and close the shutter leaves. You will have to estimate the locations of the 'full width half maximum' for each speed. In both cases, there will some additional smearing at the bottom of the bar due to the persistence of the CRT phosphors. The effective exposure time can then be calculated by multiplying the number of scan lines in the bar at any given horizontal position by 63.5 uS (the NTSC horizontal scan time). If you cannot resolve individual scan lines, figure that a typical over- scanned (NTSC) TV screen has 420-440 visible lines. If you can adjust your TV (remember this can be an old B/W set when knobs were knobs!) for underscan, about 488 or so active video lines will be visible. If you have an oscilloscope or electronic counter/timer, fairly accurate measurements can be made at all shutter speeds using a bright light and a photodetector circuit. 3. Using an electronic counter/timer or oscilloscope. A gated 24 bit counter clocked at 1 MHz would permit (ideally) testing shutter speeds from 1/2000th second to 16 seconds with an accuracy of better than .2 percent. Of course in practice, the finite size of any photodiode and/or the finite open/close time of any shutter will limit this at high shutter speeds. Any resonably well calibrated oscilloscope will be accurate enough for shutter speed determination. Construct the IR detector circuit described in the document: "Notes on the Troubleshooting and Repair of Hand Held Remote Controls". (Note that the fact that it is called an IR detector is irrelevant since the typical photodiode is sensitive to visible wavelengths of light as well.) Connect its output to the minus gate of your counter or the vertical input of your scope. Put a diffuse light source (i.e., light bulb) close to the lens so that it is not in focus. Position the detector photodiode in the center of the focal plane - mount it on a little piece of cardboard that fits on the film guide rails. Using this setup, it should be a simple matter to measure the shutter timing. Take multiple 'exposures' to identify and quantify any variations in shutter speed that may be present from shot-to-shot. For a focal plane shutter, the time response will be the convolution of the photodetector area and the slit in the shutter curtain. The smaller the aperture of the photodiode, the less this will be a factor. Masking it with black tape may be desirable when testing fast shutter speeds. In simple terms, make the photodiode aperture narrow. For between-the-lens shutters, the finite open and close times of the leaves will show up on the oscilloscope in the rise and fall times of the trace. The measurement on the electronic timer will be affected by its trigger level setting for this reason. However, since this photodetector is not linearly calibrated, the open and close times cannot be accurately determined from the waveform.
All modern electronic flash units (often called photographic strobes) are based on the same principles of operation whether of the subminiature variety in a disposable pocket camers or high quality 35 mm camera, compact separate hot shoe mounted unit, or the high power high performance unit found in a photo studio 'speed light'. All of these use the triggered discharge of an energy storage capacitor through a special flash tube filled with Xenon gas at low pressure to produce a very short burst of high intensity white light. The typical electronic flash consists of four parts: (1) power supply, (2) energy storage capacitor, (3) trigger circuit, and (4) flash tube. An electronic flash works as follows: 1. The energy storage capacitor connected across the flash tube is charged from a 300V (typical) power supply. This is either a battery or AC adapter operated inverter (pocket cameras and compact strobes) or an AC line operated supply using a power transformer or voltage doubler or tripler (high performance studio 'speed' lights). These are large electrolytic capacitors (200-1000+ uF at 300+ V) designed specifically for the rapid discharge needs of photoflash applications. 2. A 'ready light' indicates when the capacitor is fully charged. Most monitor the voltage on the energy storage capacitor. However, some detect that the inverter or power supply load has decreased indicating full charge. 3. Normally, the flash tube remains non-conductive even when the capacitor is fully charged. 4. A separate small capacitor (e.g., .1 uF) is charged from the same power supply to generate a trigger pulse. 5. Contacts on the camera's shutter close at the instant the shutter is fully open. These cause the charge on the trigger capacitor to be dumped into the primary of a pulse transformer whose secondary is connected to a wire, strip, or the metal reflector in close proximity to the flash tube. 6. The pulse generated by this trigger (typically around 10 KV) is enough to ionize the Xenon gas inside the flash tube. 7. The Xenon gas suddenly becomes a low resistance and the energy storage capacitor discharges through the flash tube resulting in a short duration brilliant white light. The energy of each flash is roughly equal to 1/2*C*V^^2 in watt-seconds (W-s) where V is the value of the energy storage capacitor's voltage and C is its capacitance in. Not quite all of the energy in the capacitor is used but it is very close. This energy storage capacitor for pocket cameras is typically 200-300 uF at 330 V (charged to 300 V) with a typical flash energy of 10 W-s. For high power strobes, 1000s of uF at higher voltages are common with maximum flash energies of 100 W-s or more. Another important difference is in the cycle time. For pocket cameras it may be several seconds - or much longer as the batteries run down. For a studio 'speed light', fractional second cycle times are common. Typical flash duration is much less than a millisecond resulting in crystal clear stop action photographs of almost any moving subject. On cheap cameras (and probably some expensive ones as well) physical contacts on the shutter close the trigger circuit precisely when the shutter is wide open. Better designs use an SCR or other electronic switch so that no high voltage appears at the shutter contacts (or hot shoe connector of the flash unit) and contact deterioration due to high voltage sparking is avoided. Note that for cameras with focal plane shutters, the maximum shutter speed setting that can be used is typically limited to 1/60-1/120 of a second. The reason is that for higher shutter speeds, the entire picture is not exposed simultaneously by the moving curtains of the focal plane mechanism. Rather, a slit with a width determined the by the effective shutter speed moves in front of the film plane. For example, with a shutter speed setting of 1/1000 of a second, a horizontally moving slit would need to be about 1/10 of an inch wide for a total travel time of 1/60 of a second to cover the entire 1.5 inch wide 35 mm frame. Since the flash duration is extremely short and much much less than the focal plane curtain travel time, only the film behind the slit would be exposed by an electronic flash. For shutter speed settings longer than the travel time, the entire frame is uncovered when the flash is triggered. See the section: "Photoflash circuit from pocket camera" for the schematic of a typical small battery powered strobe. Red-eye reduction provides a means of providing a flash twice in rapid succession. The idea is that the pupils of the subjects' eyes close somewhat due to the first flash resulting in less red-eye - imaging of the inside of the eyeball - in the actual photograph. This may be done by using the main flash but many cameras use a small, bright incandescent bulb to 'blind' the eyes when the shutter is pressed to meter, then it goes off and the flash preserves the 'closed' pupils. This approach works. Using the main flash would require sub-second recycle time which is not a problem if an energy conserving flash is used (see the document: "Notes on the Troubleshooting and Repair of Electronic Flash Units and Strobe Lights". However, it would add significant additional expense otherwise (as is the case with most cameras with built in electronic flash). A separate little bulb is effective and much cheaper. Automatic electronic flashes provide an optical feedback mechanism to sense the amount of light actually reaching the subject. The flash is then aborted in mid stride once the proper exposure has been made. Inexpensive units just short across the flash tube with an SCR or even a gas discharge tube that is triggered by a photosensor once the proper amount of light has been detected. With these units, the same amount of energy is used regardless of how far away the subject is and thus low and high intensity flashes drain the battery by the same amount and require the same cycle time. The excess energy is wasted as heat. More sophisticated units use something like a gate turnoff thyristor to actually interrupt the flash discharge at the proper instant. These use only as much energy as needed and the batteries last much longer since most flash photographs do not require maximum power. Failure of red-eye reduction or the automatic exposure control circuits will probably require a schematic to troubleshoot unless tests for bad connections or shorted or open components identify specific problems. It is also possible for that extra red-eye incandescent light bulb to be burnt out but good luck replacing it! Remotely triggered 'fill flashes' use a photocell or photodiode to trigger an SCR - or a light activated SCR (LASCR) - which emulates the camera shutter switch closure for the flash unit being controlled. There is little to go wrong with these devices.
A variety of failures are possible with electronic flash units. Much of the circuitry is similar for battery/AC adapter and line powered units but the power supplies in particular do differ substantially. Most common problems are likely to be failures of the power supply, bad connections, dried of or deformed energy storage or other electrolytic capacitor(s) and physical damage to the to the flashtube.
* Power source - dead or weak batteries or defective charging circuit, incorrect or bad AC adapter, worn power switch, or bad connections. Symptoms: unit is totally dead, intermittent, or excessively long cycle time. Test and/or replace batteries. Determine if batteries are being charged. Check continuity of power switch or interlock and inspect for corroded battery contacts and bad connections or cold solder joints on the circuit board. * Power inverter - blown chopper transistor, bad transformer, other defective components. Symptoms: unit is totally dead or loads down power source when switched on (or at all times with some compact cameras). No high pitched audible whine when charging the capacitor. Regulator failure may result in excess voltage on the flash tube and spontaneous triggering or failure of the energy storage capacitor or other components. Test main chopper transistor for shorts and opens. This is the most likely failure. There is no easy way to test the transformer and the other components rarely fail. Check for bad connections.
WARNING: Line powered units often do not include a power transformer. Therefore, none of the circuitry is isolated from the AC line. Read, understand, and follow the safety guidelines for working on line powered equipment. Use an isolation transformer while troubleshooting. However, realize that this will NOT protect you from the charge on the large high voltage power supply and energy storage capacitors. Take all appropriate precautions. * Power source - dead outlet or incorrect line voltage. Symptoms: unit is totally dead, operates poorly, catches fire, or blows up. Spontaneous triggering may be the result of a regulator failure or running on a too high line voltage (if the unit survives). Test outlet with a lamp or circuit tester. Check line voltage setting on flash unit (if it is not too late!). * Power supply - bad line cord or power switch, blown fuse, defective rectifiers or capacitors in voltage doubler, defective components, or bad connections. Symptoms: unit is totally dead or fuse blows. Excessive cycle time. Test fuse. If blown check for shorted components like rectifiers and capacitors in the power supply. If fuse is ok, test continuity of line cord, power switch, and other input components and wiring. Check rectifiers for opens and the capacitors for opens or reduced value.
WARNING: the amount of charge contained in the energy storage capacitor may be enough to kill - especially with larger AC line powered flash units and high power studio equipment. Read and follow all safety guidelines with respect to high voltage high power equipment. Discharge the energy storage capacitors fully (see the document: "Capacitors: Testing with a Multimeter and Safe Discharging") and then measure to double check that they are totally flat before touching anything. Don't assume that triggering a flash does this for you! For added insurance, clip a wire across the capacitor terminals while doing any work inside the unit. * Energy storage capacitor - dried up or shorted, leaky or needs to be 'formed'. Symptoms: reduced light output and unusually short cycle time may indicate a dried up capacitor. Heavy loading of power source with low frequency or weak audible whine may indicate a shorted capacitor. Excessively long cycle time may mean that the capacitor has too much leakage or needs to be formed. Test for shorts and value. Substitute another capacitor of similar or smaller uF rating and at least equal voltage rating if available. Cycling the unit at full power several times should reform a capacitor that has deteriorated due to lack of use. If the flash intensity and cycle time do not return to normal after a dozen or so full intensity flashes, the capacitor may need to be replaced or there may be some other problem with the power supply. * Trigger circuit - bad trigger capacitor, trigger transformer, SCR (if used), or other components. Symptoms: energy storage capacitor charges as indicated by the audible inverter whine changing frequency increasing in pitch until ready light comes on (if it does) but pressing shutter release or manual test button has no effect. Spontaneous triggering may be a result of a component breaking down or an intermittent short circuit. Test for voltage on the trigger capacitor and continuity of the trigger transformer windings. Confirm that the energy storage capacitor is indeed fully charged with a voltmeter. * Ready light - bad LED or neon bulb, resistor, zener, or bad connections. Symptoms: flash works normally but no indication from ready light. Or, ready light on all the time or prematurely. Test for voltage on the LED or neon bulb and work backwards to its voltage supply - either the trigger or energy storage capacitor or inverter trans- former. In the latter case (where load detection is used instead of simple voltage monitoring) there may be AC across the lamp so a DC measurement may be deceptive.) * Trigger initiator - shutter contacts or cable. Symptoms: manual test button will fire flash but shutter release has no effect. Test for shutter contact closure, clean hot shoe contacts (if relevant), inspect and test for bad connections, test or swap cable, clean shutter contacts (right, good luck). Try an alternate way of triggering the flash like a cable instead of a the hot shoe. * Xenon tube - broken or leaky. Symptoms: energy storage and trigger capacitors charges to proper voltage but the manual test button does not fire the flash even though you can hear the tick that indicates that the trigger circuit is discharging. Inspect the flash tube for physical damage. Substitute another similar or somewhat larger (but not smaller) flash tube. A neon bulb can be put across the trigger transformer output and ground to see if it flashes when you press the manual test button shutter release. This won't determine if the trigger voltage high enough but will provide an indication that most of the trigger circuitry is operating.
The unit may be totally dead or take so long to charge that you give up. For rechargeable units, try charging for the recommended time (24 hours if you don't know what it is). Then, check the battery voltage. If it does not indicate full charge (roughly 1.2 x n for NiCds, 2 x n for lead-acid where n is the number of cells), then the battery is likely expired and will need to be replaced. Even for testing, don't just remove the bad rechargeable batteries - replace them. They may be required to provide filtering for the power supply even when running off the AC line or adapter. For units with disposable batteries, of course try a fresh set but first thoroughly clean the battery contacts. See the Chapter: "Batteries". The energy storage capacitor will tend to 'deform' resulting in high leakage and reduced capacity after long non-use. However, I would still expect to be able to hear sound of the inverter while it is attempting to charge. Where the unit shows no sign of life on batteries or AC, check for dirty switch contacts and bad internal connections. Electrolytic capacitors in the power supply and inverter may have deteriorated as well. If the unit simply takes a long time to charge, cycling it a dozen times should restore an energy storage capacitor that is has deformed but is salvageable. This is probably safe for the energy storage capacitor as the power source is current limited. However, there is no way of telling if continuous operation with the excessive load of the leaky energy storage capacitor will overheat power supply or inverter components.
This schematic was traced from an electronic flash unit removed from an
inexpensive pocket camera, a Keystone model XR308. Errors in transcription
are possible.
Note that the ready light is not in the usual place monitoring the energy
storage capacitor voltage. It operates on the principle that once nearly
full charge is reached and the inverter is not being heavily loaded, enough
drive voltage is available from an auxiliary winding on the inverter
transformer to light the LED. It is also interesting that the trigger
circuit dumps charge into the trigger capacitor instead of the other way
around but the effect is the same.
Inverter Flashtube
+------------------------------+---------------------+--+--------+---+
| 1 K Ready LED | S1 Power | | | |
| +--/\/\-----+--|<|-----+ | ______ On | +-+ T2 +-+ |
BT1 _ | R1 | IL1 | | | \___| )||( |
3 V ___ | || +------|--/\/\/---+ | C1 | __ Off )||( +|FL1
2-AA _ | ||(2 .4 | R2 10 | Energy | | )||( _|_
___ | || +-------------+ | Storage | +-------+---+ ||( | | |
| | ||(5 .2 | | +| 280 uF | | ||( || |
+---+ || +------+ | __|__ 330 V | S2 Fire -| ||( || |
| ||(1 | | _____ | (Shutter) | +--|| |
+---+ ||( | C3 | | | +-----+ Trigger || |
3)||( 142 -|47 uF | -| | | | || _ |
<.1 )||( _|_ 6.3 | | | R1 \ _|_ C2 |_|_|
)||( ___ V | | | 1M / ___ .02 uF |
+-+ || +-+ | | | | \ | 400 V -|
C| 4 T1 6 | +| | | | / | |
B|/ | | | D1 | | | | |
+--| 2SD879 +--------------|<|--+----------------+-----+--------------+
| |\ Q1 | | HV Rect. |
| E| | | |
| +-------------+------|------------------+
| |
+-------------------------+
Operation:
1. The inverter boosts the battery voltage to about 300 V. This is rectified
by D1 and charges the energy storage capacitor, C1.
2. The LED, IL1, signals ready by once C1 is nearly fully charged.
3. Pressing the shutter closes S2 which charges C2 from C1 through T2
generating a high voltage pulse (8-10KV) which ionizes the Xenon
gas in the flashlamp, FL1.
4. The energy storage capacitor discharges through the flashlamp.
Notes:
1. The inverter transformer winding resistances measured with a Radio Shack
DMM. Primary resistance was below .1 ohms.
2.
| |
---+--- are connected; ---|--- and ------- are NOT connected.
| |
Developing timers only provide a display or clock face (possibly with an alarm) while enlarging timers include a pair of switched outlets - one for the enlarger and the other for the safe light. These are usually self resetting to permit multiple prints to be made at the same exposure time setting. Where the device plugged into a controlled outlet does not come on, first make sure these units are operational (i.e., the bulbs of the enlarger and/or safelight are not burned out and that their power switches are in the 'on' position. The problem could also be that one of these devices is defective as well. Two types of designs are common: 1. Electromechanical - using an AC timing motor and gear train with cam operated switches controlling the output circuits directly or via relays. If the hands fail to move or it does not reset properly, the timing motor or other mechanical parts may require cleaning and lubrication. The motor may be inoperative due to open or shorted windings. See the section: "Small motors in consumer electronic equipment". Where the timer appears to work but the controlled outlets (e.g., enlarger and safe light) do not go on oroff, check for a loose cam or bent linkages and dirty or worn switch or relay contacts. If the dial fails to reset after the cycle completes, it may be binding or require cleaning and lubrication or a spring may have come loose or broken. 2. Electronic - digital countdown circuits and logic controlling mechanical or solid state relays or triacs. Where the unit appears dead, test as with AC line powered digital clocks (see the section: "AC powered digital clock problems"). If the buttons have the proper effect and the digits count properly but the external circuits are not switching, then test for problems in the power control circuits. If the unit is erratic or does not properly count or reset, there could be power supply or logic problems.
Here is one for the photo album: "Ever since I bought the Mamiya 645 Pro 2 months ago, I've had exposure problems. I usually bring any new eqpt up to Twin Peaks (in SF) to test for lens sharpness, and overall function. Well my first shots from there were 2 stops overexposed, and the meter was reading wrong, so I returned the camera for repair, assuming it was broken out of the box. Mamiya went over it with a fine tooth comb, and could find nothing wrong with it. I got it back on Monday, and went up to Twin Peaks again. Same problem as before! The meter read 2 stops over! I cursed the techies at Mamiya, I cursed the product, I cursed MF, and then I decided to get scientific about it. So I took the camera off the tripod, and pointed it around at various things: all normal readings... I pointed the camera back at the scene I had just metered on the tripod...normal reading. I remounted the camera on the tripod ... 2 stops over. I removed the camera ... normal reading. I remounted the camera ... 2 stops over. Unbelievable. So that's when I started thinking about the RF and TV signals being transmitted from the big tower there, and how the tripod might act as an antenna, and cause a small current to enter through the ground socket and perhaps change the ground reference voltage. But it's a carbon fiber tripod! Still, I was on a quest. So I borrowed another 645 Pro from the store, and I took my 3 tripods up the hill. They were the Gitzo 1228, a Slik U212, and a Tiltall. All 3 tripods and both cameras exhibited this phenomenon, but to varying degrees. The Gitzo was off the most, anywhere from 1-3 stops. The other 2 did not affect the meter as much, at the most 1-2 stops. Funny thing is, the cameras did not even have to *touch* the tripod to have their readings affected! As I moved the camera closer, the meter would start overexposing by up to a stop, then jump even more once mounted. As a control, I then went halfway down the hill, and repeated the test. The effect was less, with the Gitzo giving 1-2 stops. I then went downtown, and tested again. No difference between on/off camera. I tested again when I got home. Again, no difference." What you have described could indeed be due to RF interference. Metal and carbon fiber are both conductors so the construction of the tripods may not make that much difference. How is it happening? This is anyone's guess but enough of a current could be induced in the sensitive electronic circuitry to throw off the meter. The ICs are full of diode junctions which can be rectifying (detecting) the relatively weak RF signal resulting in a DC offset. If this were the case and you happened to adjust the tripod height to be around 1/4 wavelength of one of the transmitters you *would* know it! :-)
It seems that the world now revolves around AC Adapters or 'Wall Warts' as they tend to be called. There are several basic types. Despite the fact that the plugs to the equipment may be identical THESE CAN GENERALLY NOT BE INTERCHANGED. The type (AC or DC), voltage, current capacity, and polarity are all critical to proper operation of the equipment. Use of an improper adapter or even just reverse polarity can permanently damage or destroy the device. Most equipment is protected against stupidity to a greater or lessor degree but don't count on it. The most common problems are due to failure of the output cable due to flexing at either the adapter or output plug end. See section: "AC adapter testing". 1. AC Transformer. All wall warts are often called transformers. However, only if the output is stated to be 'AC' is the device simply a transformer. These typically put out anywhere from 3 to 20 VAC or more at 50 mA to 3 A or more. The most common range from 6-15 VAC at less than an Amp. Typically, the regulation is very poor so that an adapter rated at 12 VAC will typically put out 14 VAC with no load and drop to less than 12 VAC at rated load. To gain agency approval, these need to be protected internally so that there is no fire hazard even if the output is shorted. There may be a fuse or thermal fuse internally located (and inaccessible). If the output tested inside the adapter (assuming that you can get it open without total destruction - it is secured with screws and is not glued or you are skilled with a hacksaw - measures 0 or very low with no load but plugged into a live outlet, either the transformer has failed or the internal fuse had blown. In either case, it is probably easier to just buy a new adapter but sometimes these can be repaired. Occasionally, it will be as simple as a bad connection inside the adapter. Check the fine wires connected to the AC plug as well as the output connections. There may be a thermal fuse buried under the outer layers of the transformer which may have blown. These can be replaced but locating one may prove quite a challenge. 2. DC Power Pack. In addition to a step down transformer, these include at the very least a rectifier and filter capacitor. There may be additional regulation but most often there is none. Thus, while the output is DC, the powered equipment will almost always include an electronic regulation. As above, you may find bad connections or a blown fuse or thermal fuse inside the adapter but the most common problems are with the cable. 3. Switching Power Supply. These are complete low power AC-DC converters using a high frequency inverter. Most common applications are laptop computers and camcorders. The output(s) will be fairly well regulated and these will often accept universal power - 90-250 V AC or DC. Again, cable problems predominate but failures of the switching power supply components are also possible. If the output is dead and you have eliminated the cable as a possible problem or the output is cycling on and off at approximately a 1 second rate, then some part of the switching power supply may be bad. In the first case, it could be a blown fuse, bad startup resistor, shorted/open semiconductors, bad controller, or other components. If the output is cycling, it could be a shorted diode or capacitor, or a bad controller. See the document: "Notes on the Troubleshooting and Repair of Small Switchmode Power Supplies" for more info, especially on safety while servicing these units. Also see the chaper on "Equipment Power Supplies".Go to [Next] segment
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