If you have the service manual and it provides a procedure not requiring a laser power meter (which you probably do not have), then by all means follow that procedure. As noted elsewhere, it is possible to destroy the laser diode by attempting to adjust its output power. However, if you suspect a weak laser as indicated by noisy playback or poor tracking performance (not a dead one as this will not help), and have exhausted all other possibilities such as the servo adjustments - and feel you have nothing to lose, you may attempt one of the procedures described below (with some risk) to determine if the laser diode is at fault. The following requires that you can play a disc - even if it has some problems with noise or tracking. This is best done with an oscilloscope. However, if you do not have one, you can still try the procedure. The risk is that without a visual indication of the signal amplitude, you will turn the control too far before you realize it and destroy the laser diode. * If you have an oscilloscope, put a probe on the RF test point. While the disc is playing, you should see the eye pattern. Mark the exact amplitude of the peaks. Also, note the 'playback quality' so you will recognize if it changes. Note: while the correct voltage for the eye pattern is not the same in all players, typical values are in the 1 to 2 V range. If you see a few hundred mV or less, there is likely a problem. Caution: a weak eye pattern can also be due to improper focus bias adjustment (check it) or an electronic problem. The laser power may be normal. It may be safer to turn the laser power adjustment with player power off to avoid the possibility of electrical noise causing current spikes. Your choice. Mark the exact position of the laser power adjustment so you can get back to it if there is no effect or it makes things worse. Turn the control the slightest amount clockwise. Caution: this control may be very sensistive - 'slightest' really does mean just a very small amount. Turn power back on and/or note the eye pattern amplitude. If the laser diode is not at the limit of its power and thus bad, you should see the amplitude change from what it was. If it has decreased, try the other direction. Note the playback quality. Has it changed any? If not, then laser power is probably not your problem. If the amplitude of the eye pattern is unchanged, you either are turning the wrong control or the laser is at its power limit - and probably near the end of its life. Try the same test in the counterclockwise direction if the amplitude decreased - not every designer knows left from right. If there is improvement, you can risk leaving the control at the new (most likely) higher power setting realizing that you may be shortening the ultimate life of the laser diode. Do not push your luck by continuing to turn up the power unless you have exhausted all other alternatives. * If you do not have an oscilloscope. you can still attempt the procedure above, using audio listening exclusively to determine if there is any change. It is just a little bit riskier. As noted, the laser power adjustment may be very sensitive you will have no direct way of knowing how much you have increased the setting.
If there is a question of whether the lens is focusing or tracking properly, perform the following. Again, if the unit is able to read the disc directory at all, then these tests are not needed. Note that if you have a CD player with a rotary positioner, there may be no separate tracking coil as coarse and fine tracking may be combined. Typical linkages between the lens/coil assembly and the body of the pickup are (1) a sliding shaft (focus) and rotation on the shaft (tracking) or (2) a hinged-hinge. With (1), the slide can get gummed up preventing reliable focus and tracking. With (2), one or both hinges can break - they are often made of thin flexible plastic. Repair is not really possible. First, identify the cable leading to the focus and tracking voice coil mechanism. This is usually a 4 conductor cable separate from the data and laser cable (at least at the pickup end). Disconnect it from the mainboard before testing. Using a DMM or VOM, you should be able to locate a pair of coils with very low resistance - a few ohms. One of these is focus coil and the other is the tracking coil. Construct one of the following test circuits: 1. Your 4-5 V DC wall wart plugged into a Variac with its output connected to a 22 ohm 1W resistor in series with a pair of 2 foot #24 insulated wires. 2. Your 5 V DC power supply connected in series with the 100 ohm variable resistor and 22 ohm 1W resistor with a pair of 2 foot #24 insulated wires. Gain access to the lens for visual inspection. This may mean ejecting a disc, opening the drawer, or in some cases, actually removing the clamper. In a portable or boombox, the lens will be readily accessible. Unplug the CD player from the wall or remove the batteries - you will not be using its internal power. Locate one pair of the two pairs of low resistance connections you identified above. With your power supply off or the Variac turned all the way down, connect the #24 leads to one of these pairs. Now, turn on the power and slowly adjust the Variac or reostat while watching the lens. If you are connected to the focus coil, you may see the lens moving up and down. If you are connected to tracking coil, you may see it moving from side to side. If there is no motion, turn off the power supply, reverse the polarity and try again. For a typical pickup, the 4-5 V power supply and minimum of 22 ohms should cause the lens to move through the entire range of motion up and down or side to side as appropriate. Once you have exercised the first coil, switch connections and repeat for the other. If the motion is jerky, the lens assembly may be dirty. Clean it carefully first with a bit of compressed air (not high pressure, a photographic air bulb is fine) and then with Q-tips and isopropyl alcohol. Do not lubricate. Repeat the tests after the cleaning. If both the tests are positive, you have confirmed operation of the focus and tracking actuators. If either you were unable to locate both pairs of coils or one or both actuators did not move, then you have located a problem. An open coil can be due to a cable problem or a break at the coil. If the break is right at the solder connections which are usually visible once the plastic protective shroud is popped off, then it may be possible to repair it. This will require a great deal of manual dexterity and patience - the wire is really really fine. It is still possible for there to be shorted turns in the fine coils or an intermittent that was not detected. * Shorted turns would reduce the frequency response of the servo, reduce the reliability of focus or tracking, and increase the needed servo driver power. A CD player that is overly sensitive to slight disc defects even after all the proper adjustments have been performed may conceivably be a result of this type of fault. An additional symptom may be an unusually hot servo driver IC. However, many of these ICs run hot normally so don't panic as the possibility of shorted turns is really quite remote. * An intermittent may only show up during dynamic operation or with certain particularly finicky CDs or other peculiar circumstances. The intermittent could be at the solder connections or the fine printed ribbon cable that connects the moving lens assembly to the remainder of the pickup.
The photodiode array in a optical pickup consists of an IC with typically 4 or 6 detector segments. Four segments may be used for the less common 'single-beam pickup' while 6 segments are used in the 'three-beam pickup'. These segments are usually designated A-F. A, B, C, and D are the main detector which is used for both focusing and data recovery. Segments E and F are used in a 'three-beam pickup' for fine tracking feedback. We will assume a three-beam pickup for the remainder of this discussion. All 6 photodiodes are connected to a common point which during operation has a DC bias voltage on it typically around 5 V. If they are connected common anode, it will be negative; if common cathode, it will be positive. The reason is that the photodiodes need to be reverse biased for normal operation. The outputs of the photodiodes feed several operational amplifiers which are set up to amplify the current from the photodiodes. The normal connections may be at virtual ground potential or they may feed into large value resistors. The connector to the photodiode array is usually separate and will typically have at least 8 wires - photodiodes A-F, ground, and bias voltage. You will need to identify the wiring. First locate the ground using the ohmmeter. Then locate the bias - it will probably go to a low value resistor and then to the supply. Another way to identify the bias wire is to turn on the player and measure each of the possibilities. The bias will be the highest or lowest and will be solid with no noise or ripple. It will probably be powered all the time. Now for the photodiode segments. Very often the connections or some of the connections are marked on the circuit board. For example, there may be several labeled test points designated A+C, B+D, E, and F. Since the A and C segments and B and D segments are usually shorted together on the circuit board, this provided all the info needed to identify the photodiode connections. It is not important to distinguish between A and C or B and D for the following tests though you will want to be able to separate them. With power off, there is essentially no light on the photodiode array. Unplug the photodiode connector from the main board. Using your ohmmeter, test each diode for opens and shorts as you would test any signal diode. There should be a junction drop in the forward direction and very high resistance in the reverse direction. If you are using a DMM with a diode test mode, the junction drop will typically measure .7-.8 V. There may be a very slight difference between the readings for segments A to D and those for E and F. An initial test of photodiode response can be made using an external light source - a flashlight or other incandescent bulb or IR remote control shining into the lens from directly above. With the multimeter connected to reverse bias each diode segment, shine the light into the lens. The resistance reading should drop somewhat - possibly dramatically. Segments A to D should show reasonably similar sensitivities but these may differ from segments E and F (which should be similar to each other). Similarly, with with the photodiode connections restored to normal, you can use an oscilloscope to monitor the RF test point. A source of IR directed into the lens from above may result in a detectable change in the signal - but only when the photodiode array is properly biased. This may be all the time that the CD player is turned on or only when it is trying to focus or perform some other operation. With an IR remote, you should actually see the pulsed signal for each key-code. On a typical Sony CD player, I was able to get about a .1 V signal at the RF test point using a VCR remote control as an IR source. However, even on a functional pickup, due to the nature of the optics, these responses may be very weak or undetectable. Thus, failure of either of the above tests is not strong evidence of a bad photodiode array. Any unusual readings such as a significantly lower resistance for one of the diodes, a short or open of a particular diode, a short between diodes, or variations in sensitivities is an indication of a problem. While it is possible for there to be a cable or soldering defect, this is somewhat unlikely though bad solder connections or breaks in the flexible cables are not out of the question. A defect found in the photodiode array will usually mean that the laser pickup is not salvageable with reasonable effort. Even if you could locate a replacement photodiode array, aligning and soldering the (most common) surface mount package would be quite a challenge without the factory jigs. Assuming these tests do not turn up anything, the next step will verify that the photodiodes are picking up an optical signal and will evaluate the relative strengths of each segment using the laser diode, optical system, and disc combination. Note that for these tests to confirm proper operation, the optical alignment must also be correct. For the tests using the internal laser diode, we will need to setup one of the following. Method (2) is more straightforward but requires the optional signal generator for best results. In each case the objective is to cause the lens-disc distance to sweep through perfect focus without requiring that the focus servo loop be closed. This will then result in a signal that will include the point of maximum signal amplitude on a periodic basis. Alternative methods may be used to accomplish the same purpose. Both techniques require the adjustable power supply previously used to test the focus coil. 1. Adjustable focus with continuously rotating spindle. For the spindle motor, you will need a 1.5 V battery or your power supply with a suitable series resistor to cause the spindle to turn at approximately 1-2 Hz (rps). Warning: disconnect the motor from the mainboard! The unavoidable wobble of any disc is essential in this case and will sweep the focus distance by more than enough to cover the entire focus range of interest. Note: this assumes that the spindle is driven by a conventional PM DC motor. If it is a brushless DC motor, then some of the control electronics may be external to the motor and you will not be able to just provide a DC voltage to get it to rotate. If this is the case, you must use method #2. 2. Stationary spindle but sweeping focus. This is the better method but requires a signal generator for easiest use. You can do this by hand using a Variac or reostat (this is easier if you have three functioning hands). A better method is to use a 1-10 Hz sinusoid or triangle wave from a low frequency signal generator with a low impedance output or feeding an emitter follower or audio amplifier to boost the current. This signal is then fed into the coil along with the focus offset derived from your power supply. Note: it may be possible to dispense with these test setups and just use the normal focus search of the CD player to provide the sweep. However, since we will be interfering with the proper feedback by removing selected sensors, there is no telling what the microcontroller will do. Therefore, breaking the feedback loop as we are doing is preferred. If the CD player appears to make many attempts at focus, this may be worth a shot, however. You will also need a disc - preferably one you do not care much about as sometimes it will get scratched due to opening the drawer accidentally or something equally idiotic while the disc is still rotating. Locate a 1 M ohm resistor and securely fasten it to a ground near the photodiode connector. Put your scope probe on the other end with its ground clipped to the same ground point as the resistor. Bend the free lead of the resistor completely over so that it will be able to hold the end of a wire like a mini-clip lead. Make sure you remember or mark down exactly how the connector is wired so that as you remove individual wires, you will be able to get them back in the proper spot. Presumably, you have already made a diagram of the photodiode connector wiring. Component players often have connectors with individually removable socket pins. A fine jeweler's screwdriver or paper clip may prove handy in removing these one at a time. Turn on your power supply and adjust the focus to about midrange. Start the spindle rotating or turn on the signal generator to provide a small sweep - about 1/10 V p-p as measured on the coil should be fine.
Remove the wire corresponding to the photodiode (say, A) to be tested from the connector but leave the connector itself plugged into the main board. Set the scope for 1 V/div. vertical on a slow free running sweep. Clip the A wire into the resistor. Now, turn on power to the CD player. While the player thinks it is focusing, slowly adjust the focus voltage while observing the scope. As you approach proper focus, you will see the signal increase (depending on polarity) greatly, pass through a maximum, and then decrease. Depending on the design of the CD player, you may need to turn it off and on several times before you locate the signal as the microcontroller may give up pretty quickly with no focus or tracking coil servos (since you disconnected the actuators). If you have the service manual it may tell you how to force the laser to be powered all the time. Leave the focus set near the middle of the region of high signal. If you are using the signal generator to perform the focus sweep, you may need to optimize the amplitude of the signal by adjusting the signal generator output and offset from your power supply. You probably should not need to touch the settings for the remaining photodiode segment tests. Repeat the above procedure for each of the photodiodes A-F. All should produce fairly similar signals, say within 20 % of one another in amplitude. If A,B,C,D or E,F differ from one another by more than say, 20 %, there may be a serious optical alignment problem in the pickup (the player may have been dropped or bounced around without securing the hold-down screws, if any). Alternatively, the photodiode array may be bad. It is also possible for there to be partially shorted photodiode segments in which case, the outputs will not be independent as they should be. Loading one segment's output with a resistor may affect the output of one or more other segments. In any of these situations, such a discrepancy in A-D will prevent the establishment of proper stable lens position at the optimal focal distance. This will prevent the formation of a proper 'eye pattern' and subsequent data recovery. A significant difference between E and F (beyond the adjustment range of the tracking or E-F balance control) will prevent proper tracking. Note, however, that the signal amplitude from A-D and E,F may differ as A-D operate off of the main beam and E,F operate off of the first order diffracted beams which are weaker. As with the basic photodiode tests above, a failure here usually will require the replacement of the entire optical assembly. As noted, if the pickup's optical alignment is way off, there could be significant differences in photodiode responses. On component type units, it is unlikely that the optical alignment would shift on its own. Portables that have been dropped or automotive units subject to constant bumps and vibration could have alignment problems, however. If this is your last hope, then some experimentation with adjustment of the optical alignment on a successive approximation basis might be worth the effort. Mark the original position of any adjustments and try small variations on either side to determine their effect. You might get lucky. If this eventually results in improved uniformity of photodiode response, alignment may be the problem. If you can more or less equalize the response, reconnect the servos and attempt to get an eye pattern. If you can, optimize the eye pattern stability and amplitude using the optical alignment adjustments and servo adjustments.
Note: For general information on optical pickups see the section: "CD optical pickup operating principles". These are probably the most common optical pickups in the universe. Many variations - many dozens if not hundreds - on the basic design have been produced from before 1988 until the present. In general, they are compact, simple, robust (despite what you may have heard), and no doubt dirt cheap to manufacture. Depending on the type of player and mechanical constraints, the specific optical arrangement and construction will differ. Many brands of CD players and CDROM drives actually use Sony pickups. While these are all recognizable for their octagonal black lens cover and parallelogram type lens suspension for focus and tracking (neither of which has changed noticeably in 10 years), the construction of the fixed optics has gone through quite an evolutionary process: * Early KSS pickups were quite complex with most of the components described in the section: "CD optical pickup operating principles" mounted as separate components. These had accessible optical alignment adjustments and were also quite large and bulky compared to today's pickups. An example of one of these is the Sony KSS110C Optical Pickup. * Most of the KSS pickups found in consumer CD players and older CDROM drives combine some optical elements and eliminate others. For example, types like the very common KSS361A do not have a separate collimating lens or cylindrical lens. All parts are totally glued at the factory so no possible optical alignment adjustments are possible. A diagram showing the organization of the Sony KSS361A optical pickup is available in both PDF and GIF format. * Get CDKSSP: cdkssp.pdf or cdkssp.gif. * The newest KSS series pickups appear to have combined the laser diode and photodiode into a single package. They are offset by a very small distance so the outgoing and return beams pass through the same optics and thus there is no longer a beam splitter - more cost reductions! By eliminating the optical components for redirecting the two beams, performance should also be better since this operation was not 100 percent efficient and additional optical surfaces can only degrade the beam quality. The small reduction in the clarity of the detected analog signal resulting from the very slight non-perpendicular (with respect to the disc 'pits' surface) beams should be more than made up for by these simplifications. While I do not yet have a sample of a Sony pickup of this design, the CMKS-81X Optical Pickup and Optical Pickup from Philips PCA80SC CDROM combine the laser diode and photodiode array into single package and eliminate all of the other optical components except the diffraction grating and turning mirror. I expect that Sony versons are similar. The description below is for pickups similar to the KSS361A and KSS210A. These are horizontally organized and less than 1/2 inch thick. The laser diode, grating, and beam splitter are mounted inside the casting of the optical block. The turning mirror is glued to its base plate, the photodiode array is glued to a port on its side and the objective lens and its focus and tracking actuators are mounted on a self contained removable unit. Please refer to the closeup views of the Sony KSS361A Optical Pickup. The following can be seen from the underside after removing a cover plate (1 screw). The descriptions are for the outgoing beam which originates at the laser diode, passes through the diffraction grating, then reflects from the dichroic beam splitter mirror on its way to the objective lens: * Laser diode. This is clamped and glued in place in a nicely finished brass barrel which is itself clamped and glued in place in the optical block. An adjustment for optical power sensitivity, is mounted on the flex cable next to the laser diode. This may mean that identical model pickups should be interchangeable without laser power adjustments - hopefully. Many players don't have a laser power adjustment pot on the electronics board. The front face of the laser diode package is angled so that the exit window (anti-reflection coated) is also mounted at what may be the Brewster angle, probably to further prevent stray reflections from the window's surfaces from feeding back into the laser diode's cavity or interfering with the detected signal. (At the Brewster angle, light polarized parallel to the window is totally reflected and light polarized perpendicular to it is totally transmitted. The output of these edge emitting laser diodes is polarized.) The Closeup of Laser Diode from Sony KSS361A Optical Pickup shows the angled front face and optical window. The reason it appears so HUGE is that the photo was scanned at 600 dpi - this is not a monster laser diode! It can be seen more like 'actual size' in the upper left corner of the group photo, A Variety of Small Laser Diodes. * Diffraction grating. Glued onto the end of the barrel in which the laser diode is mounted. The grating is at a 45 degree angle to produce the 3 spots for tracking in the appropriate orientation. (Once reflected through the lens the spots are in the direction tangential to the tracks). * Collimating lens. On some versions, there is an actual collimating lens. However, the most common models do not appear to have one. There is nothing really wrong with such a design, it is just unexpected. Their optical efficiency will be lower since some of the beam will be lost to the side walls but other than that, a shorter focal length objective lens should be able to compensate fully for a non-parallel beam. The optical path is so compact in these pickups that the losses are likely to be small. It is also not clear why otherwise very similar model pickups in very similar model CD players differ in this respect. A test of the laser diode barrel assembly removed from a KSS361A pickup shows that its output is an ellipsoidal beam with a divergence of at least 10 degrees on the narrow axis (across the grating) and somewhat greater than this in the orthogonal direction. These angles are consistent with a raw laser diode. If there were a collimating lens, the beam should be much less divergent. (My curiosity finally got the better of me and I ripped the laser diode from the barrel to confirm that there was indeed no collimating lens hiding inside!) * Polarizing dichroic beam splitter mirror. This thick mirror is mounted at a 45 degree angle and glued in place. The outgoing beam is reflected by the mirror toward the turning mirror and/or objective lens. The outgoing beam reflects off of the turning mirror and then passes through the objective lens: * Turning mirror (models with horizontally oriented optics only). This is implemented as a coated glass front surface mirror glued to a 45 degree angled support which is in turn glued to the casting. The coating is mostly transparent to visible wavelengths of light - it is not aluminized. * Lens assembly. This appears to be very similar for all models. Of course, there are probably variations in focal length and other optical properties which cannot be determined by inspection. - Objective lens uses a double convex plastic molded design glued into a plastic frame which mounts the focus and tracking coils and is attached to the lens' suspension. Both surfaces are coated and the top surface, at least, is aspheric. A raised guard ring protects the optical surface from damage should the lens come in contact with the spinning disc. - Focus actuator is a pair of rectangular formed coils surrounding a pair of vertical magnet pole pieces. - Focus suspension is a parallelogram molded plastic design. This assures that the lens remains parallel to the disc as it moves up and down. The four hinges appear to be just very thin portions of the molded 4 sided box structure. These hinges are susceptible to weakening or failure. - Tracking actuator is a set of 4 circular coils glued to the outside surfaces of the focus coils and moving with respect to the same magnetic fields. - Tracking suspension is a single vertical molded hinge of similar design to that of focus. A second vertical hinge is also present but is restricted from free movement by a resilient rubber material. This appears to protect against sideways shocks. These hinges are susceptible to failure. - The magnets appear to be of a rare-earth type - very strong for their size. - A short flex cable links the terminals of the coils to 4 solder pads where the flex cable would normally connect from the electronics board. - Optical alignment is achieved with a 3-point mounting arrangement for the lens assembly. One of 3 screws with a spring clamps the frame. The two other screws are used for adjustment. The entire affair is aligned and then glued in place at the factory so adjustment in the field is virtually impossible - and unneeded in any case. - The lens assembly can be removed by unsoldering the 4 solder pad flex cable and unscrewing two very small Torx type screws from the top (these will succumb to a roughly .7 mm hex wrench. It then lifts off. Optical alignment should not disturbed. Note: Just loosening the Torx screws permits lens assembly to be shifted slightly though some small amount of adhesive may need to be removed to free it. This should have an effect on optical alignment. I will do some experiments at some point to determine its precise effect. After passing back through the objective lens and reflecting off of the turning mirror, the return beam passes through the dichroic beam splitter mirror and hits the photodiode array. * Cylindrical lens. As far as I can tell there is none. Viewing an image using the entire return optical path (including the objective lens) through the photodiode port shows no indication of astigmatic behavior. There is nothing else that can be a cylindrical lens. Therefore, one must assume that the astigmatism present in the laser diode itself is used to advantage instead of a separate cylindrical lens. The effect should be similar. Another possibility is that the lens itself IS astigmatic but I could not detect it with my primitive (Mark-1 Eyeballs) instrumentation! * Photodetector array. Glued to a plate with the 8 pins (7 connected) poking out the back and soldered to the flex cable. Approximate dimensions of actual sensor area shown. |<---------- .6 mm ------------>| ---- +-------------------------------+ ^ | | | | | | | | | | | | | | A | B | | | | | | | | | | | | | | .3 mm | E |-------+-------| F | | | | | | | | | | | | | | | | C | D | | | | | | | | v | | | | | ---- +-------------------------------+ These are the 6 segment silicon photodiodes (for a three-beam pickup. (For a single-beam pickup, there will be 4 but as far as I know, all Sony pickups are all 3 beam types). Note that the entire active area is a fraction of a mm in each dimension. This emphasizes the likely critical nature of optical alignment. Nonetheless, with everything screwed and/or glued in place, the likelihood of this ever changing is small. * Flexible cables. In most cases, there are two - a 12 or so conductor cable for the laser power and photodiode return signals and a 4 conductor cable for the focus and tracking actuator drive. However, there are many many variations on the specific layout. These are either soldered to the electronics board or more commonly, terminate in clamp-type connectors.
If you have looked inside a variety of CD players, you probably have noticed (1) that many use Sony pickups (the characteristic octagonal black lens cover) and (2) that many of *these* appear similar even if their model numbers differ. A closer examination will reveal that many many different types use what would appear to be the identical optical block - the casting that mounts the lens and its actuators, the laser diode, and photodiode array. If you delve even deeper, you would find that the optical paths are identical as well. The only obvious difference in many cases are in the mounting and the way the sled is driven, and in the configuration of the flex cable and its connections. So, are the optical blocks themselves indeed interchangeable? The answer is a definite 'maybe' and servo adjustments may be needed in some cases (where none would possibly be necessary with an exact replacement). However, there could be cases where where differences are too great. I am not sure I believe the differences listed below since much of the pickup behavior in terms of bump immunity and drop-out performance is based in the servo loop electronics, not the pickup. So, while I do not know for sure, my guess is that the A and B versions would be totally interchangeable if the CD player electronics have enough adjustment range. (From: Lance Edmonds (email@example.com)). Sony KSS150A is compatible with KSS210A and KSS212A. However, due to signal levels KSS210A and KSS210B have differing specs. The rule here is that a KSS210B can be used in place of a KSS210A, but for optimal performance, an A should not be used in place of a B. * B versions designed for "ghetto-blasters" (lower drop-out performance and higher vibration resistance). * A versions for desk-top models (higher drop-out performance, lower vibration resistance). Source of info: Sony Japan Designer who visited me a few years ago. Yes they actually send their technical staff around the world to get an idea of what happens to the products after sale! Not often, but it does happen. Over the years I've met designers, technical managers, technicians, and a load of marketing folks from Japan and Singapore.
Some of the modern generation designs are about as simple as possible and still perform the needed functions of a single-beam or three-beam optical pickup. While the objective lens assembly with its focus and tracking actuators is of standard construction, there are few additional components. The CMKS-81X Optical Pickup and Optical Pickup from Philips PCA80SC CDROM are typical of such designs. Sony also manufactures such a pickup, apparently used in some revisions of its PlayStation PSX and elsewhere. The smallest ones such as the Optical Pickup from the Philips CR-206 CDROM are only about 1/2" x 5/8" x 3/4" overall - just about the size of the lens cover! A diagram showing the organization of these simplified three-beam optical pickups is available in both PDF and GIF format. * Get CDS3BP: cds3bp.pdf or cds3bp.gif. This diagram shows the three-beam type. The only difference for a single-beam pickup would be to eliminate the difraction grating (and its side beams) and segments E and F from the photodiode array (or simply not use them). * The laser diode and photodiode array (LD/PDA) are combined into a single package about the size of a larger LD by itself but with 10 pins - 3 for the LD and its monitor photodiode and 7 for the PDA (a single-beam pickup such as used in Philips/Magnavox products would only need an LD/PDA with 8 pins). * A glass block or plate roughly 3 mm on a side is glued to the front of this LD/PDA package. In the center is a spot about 1 mm in diameter etched on the surface which is the diffraction grating. This is directly over the emitting facet of the LD. The laser beam passes through this diffraction grating on its way out but the return beam is offset to hit the PDA and misses the spot entirely. (A single-beam pickup would not even require this diffraction grating!) * The LD/PDA is pointed at the objective lens (either directly or via a simple turning mirror depending on design). The pickups in the photos use a turning mirror but this is not needed if there is adequate space below deck since the turning mirror's only function is to redirect the beam to minimize physical height. By placing the LD and PDA very close together, the outgoing and return beams can follow almost the same path forward and in reverse through the optics. This eliminates all parts associated with separating these beams including the polarizer, polarizing beam splitter, and quarter wave plate. There may be a very slight reduction in signal quality since the optical 'stylus' does not strike the disc at a precisely perpendicular angle but this is probably very minimal and more than overcome by the reduction in losses due to the multiple surfaces and less than perfect performance of the redirection optics. Thus, performance is probably better overall, robustness and reliability are improved, and manufacturing cost is greatly reduced. Everyone wins!Go to [Next] segment
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