I've discovered the two common failure points that kill every Philips CDM12 pickup I've had in for service and I'd like to share because I know this is a hot topic for folks with these pickups and their variants (CD12 series including popular types like VAM1201, VAM1202, VAM1204 VAM1252, CD-Pro, etc.).
The first trouble I discovered is sag in the lens suspension which can be corrected by removing the lens assembly from the block, swapping the suspension arms side-side so they sit upside down, and reinstalling.
The second trouble I discovered (and this may actually help correct the first one) is the LDGU (Laser Diode Grating Unit) gets debris on its glass block where the hologram diffraction grating surface is etched. This is just normal dust buildup and smoke precipitation you get on every surface, no matter how remote, as long as it's exposed to the air. I found that on all the units I've tried so far, removing the LDGU from the optical block, cleaning the glass surface, and reinstalling, resulted in immediate restoration of perfect operation even when the unit was prior totally unable to focus (wouldn't spin the disc).
There are obvious caveats to what I mention; this is very delicate work. Thankfully these units do not require very much in the way of alignment; the lens assembly merely must be parallel with the optical block (so that its axis is perpendicular to the light path via the mirror to the LDGU) and the LDGU must be rotated to the correct orientation so that the satellite diodes receive a properly balanced tracking error signal. So far, setting the mirrors parallel and the LDGU by eye has resulted in perfect operation and it was found that manually "misadjusting" them slightly still resulted in good operation, so this design is relatively insensitive to such alignment.
I will continue to develop some procedure and take measurements of the signal outputs of a number of optical blocks in custom built test jigs so that I can show the correlation between the adjustments and the resulting output. What were primarily looking for is optimum focus and tracking error signals. Once those signals are in order, the rest will fall into place as long as there are no glaring mechanical problems, failed diodes, dirt in the optical path, weak laser, etc...
Regarding laser weakness, so far all of the units I've tested except one showed a measured laser current between 35mA and 37mA after warmup of a minute or so. One unit measured 55mA, which suggests that its laser is quite weak and the feedback diode is commanding extra current in order to maintain the desired light output level. Even this 55mA unit still reads discs perfectly, but I pulled it from use as a preventive measure and will keep it for test purposes and to help in the development of test equipment and jigs for further CDM-12 optical block investigations.
I'll share more info and pictures as this goes along. So far, I'm just happy I've brought a pile of completely dead CD players back to rock-solid operation without needing to source new pickups of questionable quality.
The first trouble I discovered is sag in the lens suspension which can be corrected by removing the lens assembly from the block, swapping the suspension arms side-side so they sit upside down, and reinstalling.
The second trouble I discovered (and this may actually help correct the first one) is the LDGU (Laser Diode Grating Unit) gets debris on its glass block where the hologram diffraction grating surface is etched. This is just normal dust buildup and smoke precipitation you get on every surface, no matter how remote, as long as it's exposed to the air. I found that on all the units I've tried so far, removing the LDGU from the optical block, cleaning the glass surface, and reinstalling, resulted in immediate restoration of perfect operation even when the unit was prior totally unable to focus (wouldn't spin the disc).
There are obvious caveats to what I mention; this is very delicate work. Thankfully these units do not require very much in the way of alignment; the lens assembly merely must be parallel with the optical block (so that its axis is perpendicular to the light path via the mirror to the LDGU) and the LDGU must be rotated to the correct orientation so that the satellite diodes receive a properly balanced tracking error signal. So far, setting the mirrors parallel and the LDGU by eye has resulted in perfect operation and it was found that manually "misadjusting" them slightly still resulted in good operation, so this design is relatively insensitive to such alignment.
I will continue to develop some procedure and take measurements of the signal outputs of a number of optical blocks in custom built test jigs so that I can show the correlation between the adjustments and the resulting output. What were primarily looking for is optimum focus and tracking error signals. Once those signals are in order, the rest will fall into place as long as there are no glaring mechanical problems, failed diodes, dirt in the optical path, weak laser, etc...
Regarding laser weakness, so far all of the units I've tested except one showed a measured laser current between 35mA and 37mA after warmup of a minute or so. One unit measured 55mA, which suggests that its laser is quite weak and the feedback diode is commanding extra current in order to maintain the desired light output level. Even this 55mA unit still reads discs perfectly, but I pulled it from use as a preventive measure and will keep it for test purposes and to help in the development of test equipment and jigs for further CDM-12 optical block investigations.
I'll share more info and pictures as this goes along. So far, I'm just happy I've brought a pile of completely dead CD players back to rock-solid operation without needing to source new pickups of questionable quality.
A quick teaser of RFE from pin-9 of the TDA1300 photodiode amplifier chip. This was after peaking the output by rotation of the LDGU. I have seen better RF waveforms, but certainly also far worse. There was noticeable low-frequency amplitude dropout which is often due to issues with the motor, disc clamp, or suspension; in the case of this unit, it was wear/dryness of the upper spindle motor bearing. A drop of oil on the motor bearing entirely eliminated the pattern error; at low sweep rate there is almost no low-frequency amplitude error, just how it should be.
Hi,
I am really interested in your findings on the CDM12 laser. I would like to have a test rig so I can test the diode current or the method I can use to test the current while it is fitted to the board which the motor is also connected to. I have never dismantled a laser like you have and would be interested to see how you do it.
Thank you
I am really interested in your findings on the CDM12 laser. I would like to have a test rig so I can test the diode current or the method I can use to test the current while it is fitted to the board which the motor is also connected to. I have never dismantled a laser like you have and would be interested to see how you do it.
Thank you
It would be nice to post up a photo of the arms you propose switching from side to side if possible, and a photo showing where to clean for those with less experience. This is wonderful that you share what you've learned.
You can easily check the laser current by measuring the voltage across the 6R8 resistor on the flex PCB. It is the emitter resistor for the transistor whose collector drives the laser diode. So the current through this resistor will be very close to the diode current. The 6R8 resistor is the large one next to the laser power adjustment potentiometer as you can see in the photo below. Setting the laser power correctly probably requires a laser power meter (Leader LPM-8000 or equivalent) and knowledge of the correct power setting. Unfortunately, I have neither. Anyone know the correct laser power for a CDM12?
I was able to do the suspension arm swap and LDGU cleaning per Duo's suggestion on my Philips CD920's CDM 12. It went from no-focus, no spin to spinning up, but now it shows a Radial Error 10 in Test Mode 1. Since it uses the optics to tell if a CD is loaded and enable spin-up, this tells me that the laser is not totally dead. Anyone have ideas for the cause of a radial error?
Dear Duo - I hope you see this message; it's been a while since you posted your findings on CDM12 laser units. I've got one in my Wurlitzer OMT CD jukebox. The problem: CDs spin and stop 3 times and then are rejected. I am interested in cleaning the LDGU but have no idea how to get to it without making matters worse. Can you provide me with a simple instruction on accessing the grating unit so that I can clean it? Thanks so much!!