OK, let's be quick... 21H00 here and must go to work tomorro morning at 5AM, so alarm clock set at 3H30 AM ! I should already be in bed
So quick...
Worked a little bit on the player this evening, on the CDM this time.
1) First tried to oil the top bearing of the pickup... to no available : somehow I just cannot find my bottle of machine oil ! Now I understand that my bench right now is a complete cluster fuck, I get that but still, it's big enough that I should be able to see it eh ? I swear I saw it yesterday just standing right in front of me by the scope.... but spent 15 minutes searching for it, no luck. It just has vanished in thin air, I am baffled... I am sure it's right there looking at me, screaming " Hey I am sure, can't you see or what ?! ". I am sure.
2) So next item on the list, was to check the blue Philips cap on the laser drive. Cut on leg of it so I can measure it reliably. Was nto disappointed. As you suggested these caps like to lose capacitance, indeed ! Supposed to be a 47uF one, measured at 20uF !
So happy to have found the problem, I replaced it with an equally old one from my stock, but not regular radial one not a Philips of course... and it measured at 51uF so I called it good enough, at least for a quick test. It's only rated at 16V when the Philips one was rated at 25V but since it is fed by the TTL output of the decoder chip, I very much doubted that it would see anywhere near 16V anyway. Not ideal OK, but still, for a quick 2 minute test, good enough. Was very disappointed when I tried to play a CD and it made no difference whatsoever !
3) So next item in the list was laser drive. I moved the laser adjustment pot just a hair CCW, and it made things worse. So turned it CW a hair... now works just fine, I have audio coming out !!!
Maybe during shipment the trimmer got disturbed a tiny bit, enough to upset the player.
However I am not happy about the fact that it turns out that the trimmer is turned ALL THE WAY CW ?!
I only barely moved it to get the drive to work, I swear, so that means it was already 98% turned CW.. strange ! I guess it's designed so that at the factory the player works with the trimmer set about half way, would make sense no ? Maybe some douche bag "serviced" this player in the past, " screwdrivered " it, worn the laser out doing so, so now you have to turn up the power to the max just to get enough laser output for playback to work ? Hmmm... not looking good. Poor CDM is on its last leg then, I guess. .. could die any day, makes me sad.
Anyway, there is still one BIG problem with the CDM :it works.... but only for 2 minutes at a time ! Ever encountered something like this guys
I mean, it does a weird thing, here goes :
1) Press PLAY, starts on the button, plays music just fine.
2) After a couple minutes or so, sound cuts out abruptly. No skipping whatsoever, it just cuts out all of a sudden
3) Then the turntable motor loses its mind and the disc starts to accelerate, accelerate, like a jet engine wannabe
4) After a couple seconds, the disc stops, fast (brakes are applied).
At this point, I can press PLAY again, immediately, and the player will play the music just fine, as if nothing weird at all had just happened !
... but after a couple minutes again it will speed up the disc, then stop it... then I can press PLAY again and the cycle repeats !
It is very repeatable. I didn't time it, but it feels like it always takes the same amount of time for it to cut out and accelerate the disc. At a rough guess I would say 2 minutes or so. Could time it if need of course...
Also, I swapped transport, and the black one for the Toshiba, does not misbehave. I let it play for like 10 minutes, changing tracks from first to last, then back to middle, to try to upset it... but no, it just works.
So, the white CDM is most likely at fault, not the main board. Making progress none the less !
At least now I know the pickup is probably just fine (other than the laser being close to dead), since it plays fine for 2 minutes, then plays fine again as soon as I press PLAY again just after the mishap.
Also, even if the pickup was somehow showing this weird behaviour, it still would not explain why the disc speeds up, would it ? I mean if I were the decoder chip, and the pick-up was sending me garbage... I would just stop playback and stop the disc, that's all... how would accelerating the disc make things any better !
So, my first guess is that the problem is not the pickup, but simply the turntable driving chain that's bad. OF course it's all a closed loop, everything is interdependent, I get it but... since we know the problem lies in the transport, not the main board, and since the pickup works fine for the first 2 minutes... my bet would rather be on the PCB that's at the back of the CDM, since it controls the turntable motor, and nothing else. Service manual gives the schematic for this board. There is plenty of stuff on there, so one of the components must be bad.. would you agree with me, thus far ?!
I could swap the control board with the black / Working CDM to confirm this. Had a look, looks quick and easy and does not disturb the alignment of the pickup. It's well separated, great.
If it is confirmed that the board is bad, then I can start to trouble shoot this board. There is a lot of stuff on it so could be anything.. 4 op-amps, 4 bipolar transistors,, a dozen resistors, 6 caps but all sub uF / low value ones, so probably ceramic or film, not troublesome electrolytics. Still, have seen film caps go bad, an ceramic caps as well, given the right circumstances..
...but there is also I see, two HALL EFFECT sensors ?! Hmmmm.... these things do go bad, in general... don't know about CD transports, but they sure go bad in laptop computers where they detect the closure of the lid, and they sure go wrong in the ignition system of all my old cars !
Now, the fact the the problem has a definite "timing" aspect to it, fails after 2 minutes, every time, then "reset" itself once the motor is stopped and I press PLAY again....I guess excludes any thermal effect ? I mean the timing would be more random I guess. Plus, it probably could not reset itself so fast, pretty much instantly. So it looks to me as if this timing aspect would more due to a capacitive thing going one somewhere, that would be reset once the main board cuts power to the control board. So one of those small caps might be very leaky or something.
I really don't know, trying to make sense of this weird behaviour is not easy with my very recent and limited experience.... so please speak up my dear friends, tell me your thoughts !
Hmmm. ... just thinking of the laser drive circuitry. Guess I was wrong, laser output pin of thje decoder is probably not logic level, has to be analog and varying since it's in a closed loop using the "monitor" diode. So yeah maybe the trimmer being all the way CW to get normal operation is not necessarily a worn out laser, maybe it's just some problem in the driving circuitry, something is off, causing the trimmer to have to be fully CW even with a healthy (enough) laser. Worth investigating... though it won't cause my main problem discussed above, so I won't spend on it for now. Fix the control board first.. laser driver second.
Anyway, please tell me your thoughts on it as I could really use your help !
Now rushing to bed, almost 10PM already, not much sleep left for me, oh boy, gonna be tough getting up at 3H30, ohhhhh dear ....