The million-pound question is why? I guess that's the cause of the watery audio
Perhaps - does the frequency of that pulsing match the frequency of variation of the audio? I'd be interested to know the result of feeding the capstan motor with a steady DC from a bench PSU is. As John said, putting your scope on one of the motor drive pins should show something similar.
BTW, is that 1M resistor in the servo loop filter still 1M?
I've just returned to the video with the audio fault: I'm not so sure that it's a servo fault now... I wonder if it, as has been mentioned already, that it's an audio processing fault. The frequency of variation of the audio seems much faster than the pulsing of the servo. However, let's not leap about but be methodical.
BTW, is that 1M resistor in the servo loop filter still 1M?
Yes.
I thought I'd found something that might be of significance, it looks like some sort of leakage, possibly C75, that may have spread to the adjacent link?
However, C75 is not fitted.
As John said, putting your scope on one of the motor drive pins should show something similar.
I tried scoping directly on one of the pins, exact same pulsing DC level.
I'd be interested to know the result of feeding the capstan motor with a steady DC from a bench PSU
Just to be certain here, 12V?
Have you tried doing a recording on this machine and playing it back on a good machine? Might eliminate some of the circuit if it's on playback only. Just a thought.
Ignore my previous stoopid query, I fed via my bench supply, 5.1V as shown on the circuit to the capstan, and we have correct clear audio.
If I tune to get a picture, there is a whomp, whomp, whomp on audio, I assume this is the sync Mike mentioned.
So, I have to figure out why I have 7.9V on pin 7 of IC13 instead of 5.1V
If I tune to get a picture, there is a whomp, whomp, whomp on audio, I assume this is the sync Mike mentioned.
It's because there is now no servo action to keep the timing right.
Going back to the drawings: first of all, I'm reminded of an old Sony Betamax fault where if the control head became magnetised (causing a loss of level) the picture would be muted, unless you put it into picture search whereupon it'd be fine. So my next thoughts are to demagnetise the control head, then look at pin 5 of IC3 and pin 8 of IC17. Both these signals are necessary for the servo to work but a 50% over-voltage to the motor implies the servo is trying to make it catch up with something.
I have no way of demagnetising, no tool to do so.
Out of interest, I looked at the voltages in IC3 and IC17, they are on spec.
IC3-5 reads 11.7V
IC17-8 reads 6.4V
I'm guessing IC13 pin 7 is so much higher than the expected 6.5V as the cct is unloaded due to external capstan supply?
IC13-7 reads 24V
Have you tried doing a recording on this machine and playing it back on a good machine? Might eliminate some of the circuit if it's on playback only. Just a thought.
Good advice. Can't really add much but as the flutter is not there when the motor is powered from DC it does seem to be a servo issue. Where I can help is readings from a good working machine. Pin 7 of IC13 reads 7V DC on mine and attached is the AC waveform which is 0.18V in amplitude sat on the 7V DC component. The ramp portion is about 1.2mS wide and the flat top 2.2mS. Putting slight pressure on the pinch roller to try and stall it to put extra load on the motor causes pin 7 to rise to 7.2V.
Mine is an utter mess in IC13/7
What are the waveforms on those pins I mentioned?
I'll take a look and post back
I see the same but 14V pk:pk and 22V respectively.
Mine is an utter mess in IC13/7
Mmm. If you slow the trace to 20ms/division (I'm assuming you've got it as 2ms/div), what does it look like then?
Is it possible that C75 should be there and that sine wave is the servo hunting due to loop instability?
I think we're now seeing the problem. @irob2345 has a point, unless C75 was deleted in production?
Personally I think it's a red herring, I don't think there ever was a C75, I can't even find it on the circuit. I suspect John @jayceebee will confirm it's not on his working machine either.
C75 is not fitted to my machine either. I wonder if that sine wave ripple is the same frequency as the capstan FG? I’ll measure it as soon as I can.
Right, so it was deleted in production. I would try and clean the PCB with some IPA in the area you photographed as there are some high-value components there and leakage might be an issue.
Going off-line now - I'm going to collect a radiogram from the coast. I understand those. Happy hunting!
Done all that, no difference, worth a try though.
