Philips CDM-0 laser

[mole42]

Hi,
I have a Revox B225 with a Philips CDM-0 swinging arm drive. It works for three or four minutes then stops playing with symptoms similar to a 'no CD' .

I checked the voltage to the laser today and found it was set way too high, adjusting it down to the correct 500mV got the machine to play for about ten minutes before it stopped. I was monitoring the HF output on the 'scope and the laser voltage on a DVM at the time it stopped playing the CD, the HF stopped and the laser voltage went up to about 1.5V.

Does this sound like a faulty laser, or something in the control circuit?

Thanks for any input,
Richard

[amc184]

I wouldn't immediately blame the laser mechanism, the CDM-0 is a very robust unit. Lasers also tend to work better with increasing temperature (so that the player will only work after some warm up time), rather than what you are experiencing.

The first step I would take on a player like this is replacing all of the electrolytic capacitors. They will be faulty by now, as these have a service life of about ten years, and could well be causing the fault with your player.

[mole42]

Yes, I already replaced all of the electrolytics. The ones in the PSU were particularly bad - I tested a couple for fun and they were very leaky! There's a crude voltage doubler in there too which wasn't working until I replaced the caps. There's a scattering of nice Philips tants which I haven't changed, they seem to be more robust!

I've replaced all the 33µF caps on the CD control board this morning with no improvement.

I'm wondering if I can put the CDM-1 from my Philips CD104 into the Revox as a test. Would it be a direct swap?

Richard

[amc184]

Okay, just to confirm, you replaced every last electrolytic capacitor, including the three or four on the PCB attached to the laser mechanism itself?

The next step I would suggest is probing various important points on the PCB with an oscilloscope (such as power supply rails, HF signals, etc), and note any changes between when it will and won't read a CD. My guess is that your fault will be due to a faulty passive or IC that is failing when it heats up. Unfortunately, I can't think of much more than this, as these first generation Philips players are so complex, there are so many things it could be.

You can try swapping in the CDM-1 from your CD104 if you like. The two are compatible, the B225 actually could have come with either. The only problem is that some of the newer CDM-1s can with different connectors to a CDM-0, which will complicate installing it.

[erin]

Did you resolder the ground "eyelets" that join ground from the top and bottom of the PCB?

[amc184]

Originally Posted by erin

Did you resolder the ground "eyelets" that join ground from the top and bottom of the PCB?

No such thing in this CD player.

Any progress Richard?

[mole42]

>erin: This is not a Philips player and doesn't have the eyelet problem. It does, however, suffer quite badly from expired electrolytics!

>amc184: No real news yet. The good weather has precluded any playing with important things (we install photovoltaic systems at work!) but I have discovered:

1. Swapping the CDM-1 from my Philips didn't work - I haven't had time to investigate why. The connectors are the same on both, the only real difference seems to be the turntable motor drive circuit. Might just be a different type of motor.

2. I have bought a Thurlby logic analyser which I'll be using to make sure both the microprocessors are doing what I expect them to do. Might be a bit easier than probing all the data and address lines with the 'scope! It's a long time since I did any troubleshooting with microprocessors though.

3. I did try to buy another, working, B225 on ebay so I could do board swaps but the price went too high :-(

I'll post again when I have more news!

Richard

[amc184]

I have bought a Thurlby logic analyser which I'll be using to make sure both the microprocessors are doing what I expect them to do. Might be a bit easier than probing all the data and address lines with the 'scope! It's a long time since I did any troubleshooting with microprocessors though.

Sounds good. I don't think you'll need the logic analyzer though, I'd consider that superfluous when repairing CD players. Sometimes you can have problems with a logic bus (like I did a week ago with a Philips CD150), but nothing you can't spot with a scope. The first thing I'd do is 'scope the power supply rails on each daughter board.

[Mooly]

The laser power control circuit is usually pretty much self contained... is it discrete on this, a couple of transistors with just a Laser on/off line from the servo IC.

If the laser power is going haywire that suggests the "servo loop" from photo diode to power control may be being broken... something as simple as the flexiprint or connector with a break etc, or failure of one of the transistors... assuming it's discrete.

Have you a circuit showing that part of the player ?

[mole42]

I do have the full Studer / Revox service manual. Unfortunately, the servo control has a microprocessor in the loop so although 'self-contained' there are other functions associated with that processor which do work. One symptom is that the user display shows a 'no cd' indication when the turntable motor stops and the laser voltage rises.

I suspect there may be a break in the servo loop. I thought it was a thermal fault, but I haven't been able to find one in spite of using most of a can of freezer spray!
As you say, it could be something simple, but so far I'm unable to find it!!

Oh well, it keeps me out of trouble!