Thanks for the reply
dried up capacitor in the power supply
I have already gone over every single cap in the whole unit. I ended up having to replace roughly 3/5th of them, but the rest look fine and tested very good. I suppose there are some freak occurrences of caps the look perfect and test perfect but are bad, but I kind of doubt it; I'm not a fan of shotgun replacing caps.
Q1: What made you suspect it was the motor in the first place? Is there something else, like the motor itself directly emitting a matching noise?
Ok. I think I miscommunicated a little bit here. If you can humor me for a moment I'll try to explain.
There are
two motors; the first motor is the one I'm currently working on. I know the noise is coming from the motor because it sounded good until the original motor went bad. The oil dried up and the brush contacting surfaces got too dirty and it turned into a resistor and started creating a big old humming sound. I was able to confirm it was the motor by whacking it and the noise went away; pretty cut and dry.
So I removed the motor and took it apart and cleaned it out and replenished the oil in the reservoir. I reinstalled the motor and it was more or less okay.
Then I found an NOS motor for sale and bought it and installed it. It sounded fine for awhile and then things started to sound noisy.
At this point, I'm only assuming that it's the motor getting noisy (I think it's a fairly good bet), and that the old motor refurbished (or new one, whatever) would sound better.
So fast-forward to now, and I'm going to put the old motor back together again, but I measured the caps and they seem out of spec based on what I have to measure them.
I can just put it back together, but I'd rather refresh the caps inside the motor, and maybe improve them a little bit. I reckon that some good MLCC caps might be a little lower ESR and more stable at the low voltages I'm dealing with than the original disc caps. Why not, right?
Now, you might be right, there might be a whole nother issue I don't know about yet. It's possible that one of the remaining 2/5th of the caps bit the dust unexpectedly in the past year or so, or one of the transistors went noisy.
Q2: Are you listening directly to the output of the tape player, or do you have an external amplifier plugged in? Have you confirmed that this external amplifier isn't the culprit?
The amp is fine. I rebuilt it completely and it sounds great. I double checked the inputs with other sources and everything is clean and perfect. The noise is specific to the cassette deck, and it's very obvious that it's tape deck noise; for starters, it doesn't happen until I press play on the tape deck; just putting the amp on and turning the volume all the way up there is zero perceivable noise, it's a black hole
Your schematic shows a discrete DC-DC converter, but not all of the device is using that, so I'd measure the voltage on the capacitors both before it (top-right of schem, C801, C802, C803, etc) and after it (top-left C804-C808)
I rebuilt the DC-DC converter because it had a bad transistor. The other ones were perfect on a curve tracer. Again, all the bad caps were completely replaced.
If you don't own an oscilloscope then in your case it's possible to cheat
Sure of course I have a scope, but I also replaced all the bad caps already.