OB-8 noise issue / low end (last post about this I promise!)

[WhinyLittleRunt]

I need to revisit an issue I have been having with my OB-8 for several years now - there is a weird, 'digital' sounding hiss that comes through when any key is pressed, regardless whether or not the oscillators or on or off. In fact, when they're off you can just hear it... the more keys you press, the louder it gets...

I had brought it over to an acquaintance of mine (I'm thinking now more than a year ago), who owns an OB-8 as well and thought I'd let him have a shot at seeing what is causing the noise problem. Well, we basically swapped the entire innards of his OB-8 with mine one at a time and never could seem to find the root cause. His OB-8 does not have the noise issue. I found that very strange. I'm not positive we swapped over the main board with power supply; it's been some time now, but I do know he was unable to find the cause. He did fix a tuning issue I was having on one of the voices by replacing a chip (forgot exactly what it was; not a CEM).

So now I'm left with some questions. 1- if we're going to be looking at possible dried-up or malfunctioning caps, what could I use to measure them in circuit? If I'm pulling them out to measure, I'm just replacing them. Physically everything I've seen by eye looks fine. I know what a leaking cap looks like,

2 - what exactly does the 5V bridge rectifier do and does it have any effect on audio?

3 - Could there be an op-amp that is starting to malfunction?

The noise has gotten significantly worse since I've owned it, but only in small increments. I can tell it's gotten worse just by knowing the sound.

I'm perfectly fine with swapping parts and I'm more than capable / knowledgable to do so, but I think it's all guess work at this point. We did have a scope on the output and various parts of the audio path originally and still could not track it down.