I found the source of the fault in C-2a #1. The symptom was an intermittent "popping" or "thumping" sound, which was difficult to spot because it occurred infrequently.
The faulty parts were R540 and R539 (same part on each channel - pretty interesting), which are base resistors on the tone control board (as suggested by clinic-audio - thanks!). I found them by patiently giving each resistor a gentle tap on the side with a non-conductive tuner alignment tool, which caused the fault to appear.
After pulling the resistors and replacing them, I did a small experiment. I hooked each resistor up to a multimeter set to ohms and used my soldering iron tip, on my Weller's lowest setting, to introduce some heat to the resistor. I didn't have to bring the tip too close to the resistor (this wasn't an excessive amount of heat) to get the thing to go to the 20M+ range, then completely open, going back to 1.2K once the soldering tip was removed from its proximity.
It turns out that a small amount of heat caused these resistors to go intermittently open, which can be assumed that in-circuit, it was the heat of their own dissipation. I'm going to measure the drop across them to figure out the current though them.
Other items of note:
I disassembled the stereo/mono switch and found its contacts were coated in verdigris. I cleaned that up. I'll check out the other switches as well.
The glue that was used to secure the capacitors, and is now dried up is conductive. I figured this out through pure fault finding curiosity. I found lengths of it going from capacitors to neighboring resistors and jumpers that measured <500K ohms. I think that cleaning it all up (gentle, controlled scraping with a plastic spudger) cleaned up some of the light noise, but this may be purely psychological - I have no test to back up that claim.
As it stands, the preamp sounds breathtaking through my Yamaha M-4 amplifier and Paradigm Studio Reference 100 v2 speakers. It is cleaner sounding than my Yamaha C-6 (which is also decent), but is more pleasing to the ear overall. I like it A LOT.
There is slight hum in the phono section that I have to figure out.
I've gone a little bonkers over trying to fix this unit. I'm very glad I've had success, and I appreciate you all for participating in this thread!
Adam
