CD473 Elusive Tracking Glitch

I recently aquired a CD473 player which has a niggling problem I am struggling to fix. After a though clean up and replacement of the axial electrolytics and the 6800uF radial I have found that there is a range of focus offset adjustment where the TOC is easily read and usually the first track will play. If the first track does play the CD will play to the end if left alone. The problem comes when pressing 'next track' or 'previous'. Over a narrower range of focus adjustment some track changing works but there is always a number where the tracking is lost and ERR is displayed and the CD stops.

The other puzzling thing is tthat the voltage across the laser power sense resistor is 105 mV when playing and 84mV when not playing. The manual says it should be 50mV +-5mV.

Any suggestions/ideas welcome... . . . . .
 
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Your head may be at end of life. Normally you are allowed a 10% increase in laser current before declaring the head is at end of life. I don't think Philips ever put that in print, but it is a common spec. in CD player world. Original Philips machines had you set the laser power using a Laser Power meter (ex Leader LPM8001), then you would measure the laser current.

Normal 3 beam machines stipulate that you absolutely do not adjust the laser current. Generally speaking, this advice is correct. You could do this to troubleshoot, but it must be returned to exactly the same current. I never increase the laser current. There is no need.

One thing to note is that while increasing laser current, at some point the laser diode will cease to emit a beam. Once you pass this point even briefly, it's all over. You destroyed the head. New head time, period. The same holds true for the Philips heads.
 
Thank you for your speedy reply. The machine was knee deep in dust when I got it, with a visible amount on the lens. Now it is clean I wondered if turning the laser down might be a good move. It is a possibility that someone previously turned it up to compensate for the dust on the lens. So far I have not touched the setting for the reasons you warn against it. Any thoughts?
 
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That is entirely possible. I have also seen typo's in every service manual going.

What I would do is look at another service manual for a model from the same series (it will have the same mechanism) and see what value it gives. They also had manuals on the mechanism itself. Try and find it. I did get trained on these decades ago by Philips.

In general I always suggest that you don't adjust anything. After all, it worked with those adjustments before. Try to set the trimmers to the positions they were in before. So the rule is: Never adjust anything unless you are absolutely positive that thing needs adjustment. The other truth is that corrections to adjustments are typically very small ones. All bets are off if an idiot gets inside (and that includes a lot of "technicians"). The problem is, making an adjustment is quick and easy. The problem is, something that stops working suffered some failure and adjustments do not fail. They drift slowly out (assuming they were properly adjusted from the factory).

We look at things like the eye pattern to determine what needs to be done. I trained techs for years and years, step one is always looking for evidence and gathering information. Then you see where it leads you.

One thing that happens often is the disc table slides down the shaft. That throws out the focus and takes the lens out of it's normal operating range. The table height is critical on all CD players. Use calipers and find the distance somewhere in a manual or the 'net. Check that first.

-Chris
 
Chris,

Thank you very much for that detailed explanation. The Philips manual I have is far from detailed or unambiguous, and to make matters even more baffling is in German (Aaaagh!). I need to spend a bit more time on this and see what else I can find. I'll post any outcome I manage to acheive. Cheers, Dick
 
Hi Chris,

I believe the mechanism is a cdm4/11. The problem now seems to be down to harder to read CD's. If I use a Naxos or DG classical CD in pristine condition it performs as I would expect, but with slightly marked pop CD's the problem remains. Even though my Philips CD604 copes without a hitch.

Thanks for the advice on the platter. I might try some tape to shim the platter up a few thou at a time to see if it helps.

Cheers,

Dick
 
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Hi Dick,
That was a very popular mechanism and I believe the CDM 2 is very close to it.

If you get a chance, measure another machine. This is going way back for me, if I find any notes I made I'll forward you the information. The disc table height is critical. I don't have any in at the moment.

Basically, CD players have servos to compensate for disc impairments. If things are off with the mechanism or electrical adjustment it will impact how well it can deal with defects. Eye pattern quality is the most important test you can perform. That determines the digital error rate as well as tracking issues. What I am saying is that your CD player will sound better when it tracks properly.

So grab your oscilloscope and monitor the RF test point. Any adjustments you make that makes the RF pattern (eye pattern) more stable horizontally and vertically is going to improve performance in all aspects. The pattern should be clear (low noise) on a good disc with a well defined open "eye". Adjustments will also improve this along with the things I already mentioned.
 
Quick update on investgations so far:

I have found adustments to the platter height (using three small pads of tape to shim the CD up), plus appropriate focus offset adjstment, made either no difference or, with enough height, made things worse. I concluded the platter has probably not slipped down.

With a CD playing the eye pattern is fairly stable. However it has a concerning amount of high freqency noise on it. Roughly 15% of the waveform height. So far I have been unable to track down the source of the noise. I am checking the surface mount de-coupling capacitors but this is very tedious and I am slow.

Also, I have reached the conclusion that the Philips design engineers paid no regard to service/repair work as there are numerous poorly thought out mechanical details. The most glaring example is the main PCB which has many inaccesible components under the mechanism (such as the test point for looking at the eye pattern), yet there is a lot of the case area to the right remaining empty with no pcb in it at all.
 
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Okay, sounds like table height is fine - good!

Noisy RF patterns are often caused by bad decoupling capacitors. There are one or two on the CDM, plus a bunch on the main PCB. Change-'em.

Philips design engineers made things cheap. Period. Philips has a habit of not supporting their products, an age old thing with them.
 
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The only other thing I can think of is the flex cable to the laser assembly.

If you have a proper amplitude eye pattern, the laser head should be fine. Noise in the eye pattern is caused by noise in the laser supply, or in the I-V converter (to convert the signal from the pick-up diodes in the head into a voltage that can be processed). The RF amp would also be susceptible.
Have another look around and make certain you got all those small electrolytic capacitors. That includes the axial ones.

Also check all the top to bottom PCB connections on component leads and the "gribblets". Those are small rivets that connect the top to bottom. Remove the old solder using a solder sucker (the big ones, not the small, useless types). Resolder using fresh solder (don't ask what I have seen people do) after the joint gets up to heat.