If I were in your shoes, I'd get a high-impedance (crystal) piezielectric ear plug, like we usta use on crystal radios, tie one wire to
DC minus through a .47 mfd 100 volt capacitor, then go fish with the other wire. It is almost the reverse of signal injection, and
has led me more than once to the defective board, stage or component.
Filter caps in power supplies (electrolytic) do a better job on 60 hz than higher frequencies.
When you say there's no ripple on the DC, is your meter true RMS, and does it really count "audio" frequencies in its RMS value?
If there's any high-frequency AC in the DC, you'll hear it with the probe I describe. You can hook it right to the DC rail and
listen to the power. To hear that the noise gets in as early as the first card points to the power supply as well.
Next question would be is the pwr supply linear or switching mode?
Switching mode has lots of reasons to interject whines into the power when they begin to fail.
I'd rather hear hum myself. I don't really like switching mode power supplies much. Too many things to fail, too many parts
undersized, poorly heatsinked, hot leads causing solder to fall out of wave soldered circuit boards causing opens, etc.
Other possiblities are a poor DC continuity form any/all boards back TO the power supply.
This would impress other noises (from control circuits) into the audio.
Does anything change the noise? Operator interaction? Screens/displays on/off dim, etc?
Have you reseated all connector/cards and maybe even sprayed Deoxit in the sockets?
This seldom hurts.
My audio processor was dead one week not long ago, maybe three weeks.
I came home and only thing on-air was the reverb, still being fed audio, and into a second channel on the modulator.
I saw that of two passes through the ART Pro VLA ( ahem "affordable" ) compressor, the first was passing audio, but not pass 2.
This box is 10 months old, but I tore the cover off since I fix electronics for a living, and within 5 minutes found
a multi-wire conector, with crimped end meeting the circuit board w/signal from input/output. A wiggle put back the signal.
After re-crunching the cable end gently and reseating the connector, the problem was (has been so far ) fixed.
I would hope Orban would be engineered beyond such simple failures, but I fear the cost of truly hardwired gear would be
astronomical, so we have plugs and connectors and sockets. These points of contact often develop high resistance
and if these particular (DC) connections are comon to audio and control circuits, audio from the control current changes
appear in the audio currents.
