OK, just done working a few hours on the thing....
1)
Checked the +10V rail that feeds all the motors and the laser... read 15V and 700mV pk-pk of ripple ?!
Looked at the PSU schematic... the 10V rail is not regulated at all, actually it taps directly the main filter cap right after the full bridge rectifier ! So voltage level is dependent on the transformer only, and ripple is... well it is what is given the load from all the circuitry. So I guess whatever voltage level I see, is "normal"... so that means Philips calls 10V... and 15V rail. Go figure...
As for ripple, I guess it doesn't matter much for the motors, though I though it would not be wonderful for the stability of the current in the laser LED ?!... but what do I know.
Replaced the cap, 6000uF 16V.... yes, 16V cap for 15V rail... are they kidding ?
In situ, the original cap read... open circuit. So replaced it. Had an almost exact replacement in my collection of 50 year old cap. It read spot on, I charged it, hold a charge very well... so I soldered it on. Absolutely no change, not even in ripple.
Tested the original cap (Nichicon...), and oh... now it does register something, and reads spot on ! Bummer...
So, red herring I guess. Still, Thumbs down to Philips for putting a 16V filter cap on a 15V rail, and labeling in the schematics, the rail as being 10V....
That, or... the secondary winding of the transformer is partially shorted, less effective turns so higher voltage ?! I am not really in my comfort zone here, so might be talking BS sorry...
However I have to say that there is another rail that also reads quite a bit higher than its name suggests : the " -13.. -15V " rail reads more like 18V !
So if there is indeed a defective transformer and the rial is really supposed to be 10V... then I guess a 16V rated filter cap is fine.
Hmmm.... need your view on this ! Faulty transformer or not ? :-/
2)
Tested all the components of the Laser circuitry, all fine, resistors spot on, NPN transistor tests OK (though it might be tired/weak, who knows). The pesky 33uF reads just fine at 37uF. No it's not a blue philips axial, it's a "normal" cap, a Nichicon. On the decoder/servo board there are 20 caps or so. Only 3 blue Philips, but at the other end of the board, near the bit ICs. The 17 remaining caps, in the analog part of the board we are most interested in, are all Nichicon.
3)
Spent a lot of time on the circuitry that drives the turntable motor. Fired up the scope to check waveforms all over the place, while the player was trying to load discs. I think in the last 4 hours I have ejected and loaded the cassette more times than I have in the years I used the player back in the day !
I first tested the motor, feeding it directly with my lab power supply. I of course manually inserted a CD into the drive, from the bottom of the unit, so that the motor is loaded, in case it weak and might spin without a load but not with a load.
At the nominal 10V (assuming it is indeed 10V not 15...), motor/disc starts on the button, and after 2 or 3 seconds it has reached its full speed. at 15V it's... very strong to say the least...
So, I don't think the motor is at fault, nor a stiff spindle or any mechanical stuff.
So I then moved on to the driving circuitry. Started backwards : from the totem pole final driver then upstream.
Right there, it's fishy : the two transistors of the totem pole... both of them... their emitter-collector is shorted !
Also, there collector is not directly connected to the positive and negative rails. They connect via small resistors, 4,7ohms .... and.... trouble here again ! these resistor read almost open circuit, half a meg for one, several Megs for the other !
Given the schematic, it should be possible to measure them in situ reliably, no ? Plus, even if not, any parasitic circuitry would cause the reading to be LOWER then nominal, can't possibly make it read HIGHER ! Never mind Megaohms !
So... doesn't look good ! Will desolder these trannies and resistors to measure them properly anyway, to be 100% sure, but I can't see how it would be any different.
Then I probed the rest of that circuitry, to see what was going on, see if the player was at least trying to spin the motor or not.
Hard to say... because it's not easy to understand how it all works, it turns out !
As you can see (see schematics posted earlier), aside from the final drive / totem pole, there is a dual op-amp, and two control signals, TC and TCMP, and some circuitry between the two op-amps. TC signal is coming from the left, issued from the decoder chip. TCMP comes from a micro. TC, appears to be a "clock" signal... well, I mean it's TTL level, periodic, 50% duty-cycle... looks like a clock. It is present at ALL times, even when the drives is idling, doing nothing at all. Frequency is about 88kHz...hmm... interesting number....could that be related to the 44kHz sampling rate of CDs ?! .....
Then this signal is turned into DC voltage via a low-pass filter, then it feed the first op-amp, set up as comparator with some hysteresis, if I am not mistaken. Then it goes through that block of discrete components, diodes, resistors, caps... whose role escapes me 100%. The TCMP signal is mixed with that blob of component, again in some mysterious way. Then that goes to the second-op amp, set up as... an integrator or something ?!
So I probed all that area at lenght. The output of the comparator does show activity during the 2 seconds where the read head is trying to focus/ jiggles. during that same time, TCMP signal shows two nice solid 1 second long pulses, corresponding to the two movements of the head. So that's promising. At first I though it was strange to drive the motor for such short pulses, since one second, from my experiments above, is not enough to get the motor up to speed. But then I figured that the drive might not have to care : if the disc is not at the correct speed it's not a problem. As long as it's moving, it should be enough for the read head to sense a bit stream. bit stream that would make zero sense but it's not the point : if there is ANY bit stream, then it means a disc is present or else the photo-diodes would just read a constant/flat signal. Then, once the the player sees that there is a disc, it can give the motor more time to stabilize at the correct speed, and actually try to make sense of the data, and try to retrieve the TOC. So, just to say that maybe what I am seeing on the scope actually could make sense and be encouraging....
THE PROBLEM : TCMP signal output of the first op-amp looks promising, but... this results in... no signal on the input of the second op-amp ! Zero volt, constant. So, also nada at the output of the op-amp, so totem pole is not being driven (assuming it were not blown from to bottom that is ! ).
So I think maybe there is a bad component in that bloc of analog goodness between the two op-amps. I don't understand at all what it does exactly, how it works, doesn't help really... so I guess I will have to use brute force and just check every one of these components one by one, hoping to find a faulty one. Also, I guess it's possible that the op-amp itself is faulty ? Maybe it's input is defective, and shorted to ground ? .. though I guess this is unlikely since the op-amp in this case doesn't even know what "ground" is, doesn't mean anything to it, since it's powered by symmetrical supply.
4)
Laser power adjustment : I noticed that the trimmer (wired as an adjustable resistor, not a pot) that adjusts the laser output, is set pretty much all the way to one end : reads 400ohms for a 4,7k track. I don't know which around it works, so either it is set to maximum power, or to minimum... if set to minimum that might explain why the spot appears so weak to my eyes ?!
I will ask my dad if if he messed with the laser and focus trimmers....
5)
I got hold of a "proper" audio CD in place of my home made CDR ... didn't make any change, player still reacts 100% the same.
Anyway, next : will reconstruct the totem pole driver and check the bunch of components sitting between the two-op-amps.