Continuous low hum (all checks passed) [resolved]

I may have found the issue. Drain wire was disconnected on the C side when I took the board out to get a better look. I am rechecking resistance and voltage now. Is it likely that I damaged anything but missing this during troubleshooting? Just wondering if I'll need to budget for a replacement regulator or anything.
 
Yep - that loose wire was the issue. I feel silly for not noticing it before (my right ear is a little deaf so I didn't realize the buzz was mostly in the left channel or that might have clued me in). Thank you for all the help. Gonna start listening now!
 
Re your question about cutting off one end of the drain wire. Yes, that is proper procedure. The idea is to extend the ground plane of the chassis around the cables by connecting the drain wire at one end of the cable (and thus the foil shield is also connected) to the chassis. If you connect both ends of the shield and drain wire you don't just extend the chassis ground potential, you also create a parallel path for current flow thru the shield and the chassis between those two connected ends. This creates a ground current loop that could increase the noise floor instead of helping to minimize it.

The overly high regulator voltage is a separate issue and should be checked even if the noise is gone. Double check that OB reading and if it is still high something should be done to correct it.
 
The OB reading looked good after I got the drain wire reconnected. It worked great for a matter of minutes and then the right channel lost all sound. I'm tracking that problem down next. The joys of DIY! (Spoken without a trace of sarcasm. I find the problem solving to be rewarding).
 
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