Preamp Hum: Plugged into power but powered off

[earth tones]

Hey all,

I've got an old ~50s era Presto Recording (so I believe) tape machine amplifier that somebody wired up as a preamp. If I have the output sent directly to a record-enabled track in Pro Tools and I then power the preamp off, the unit begin to generate a very loud hum on the output signal. If I pull AC power the sound is gone, only to return when I plug it back in. When I shut the preamp off, it can take a few second for the noise to generate, as if there's a feedback loop of some sort growing after power off.

Is this a sign of anything in particular?

The unit has been in storage for a few years. It passes a microphone signal but that is riding on a lot of noise now.

On an aside, I just moved to the Portland, OR area. Do you all know any locals that specialize in maintaining or rebuilding vintage equipment?

Thanks!

[kslight]

You could try a different outlet?

As a quick fix, maybe plug it into a power strip and turn off the power strip when not in use?

[earth tones]

The unit is on a surge protector and none of the other rackmount units on the same surge protector exhibit the issue. I'm certain that there is a problem with the preamp and was just wondering if that specific symptom would mean something in particular to more experienced tinkerers.

[emrr]

Many systems do this, it's simply that the one is powered off.