Let's say I have an electronic device, inside which is a power transformer that outputs a known DC voltage. Can I bypass the transformer and wire in a jack to accept the correct DC voltage from an external source instead?
Here's the context: I have a nearly 50 year old hifi spring reverb unit (Sansui RA-500) that has a 60 Hz hum problem even after I replaced its capacitors. I believe the problem may be that the power transformer is inside the unit right next to the spring reverb tank, which picks up its magnetic fluctuations. In guitar amps, one solution to this problem is to move the tank outside the amp to get it away from the transformer. While I don't want to move the tank in this case, it would be great if I could "move" the power transformer outside the unit to achieve the same thing. If I could use a wall wart or laptop power supply, that would be ideal. I assume there would be additional safety benefits to this approach as well.
In case it's relevant, here's the schematic. Page 10 of the PDF, top left, shows the power transformer outputting 21V, so if I can do this, I would try applying 21V to the circuit at that point. But a more general answer would of course be useful as well.