Polytone Mini-Brute I hums

[Giaime]

Hello folks,

I have this Mini-Brute on the bench. Syntoms: even without anything plugged in, it hums very loudly both on the "clean" channel and the "overdrive" channel. But when plugging in an instrument (or a signal gen) in the "fx input" jack, using the "fx gain" knob and the master volume at maximum, I have zero hum (I'm referring to the chrome jack under the "FX LOOP OUTBOARD" lettering).

So I think the problem lies in the preamp section before the master volume stage U3A. Visually checked for broken wires, solder connections, leaking e-caps, etc etc but found nothing. Any thoughts?

[pontiacpete]

I don't know if this particular model has it, but I have seen the connector RCA cable that goes from the power amp to the pre cause hum problems with the ground connection lost. But heck, I've only worked on a hand full of these.
pete

[Giaime]

Hi Pete,

yes it has the RCA cable between preamp and poweramp, but the ground continuity is ok. And it should have made hum even with the FX loop input in that case.

The hum is also intermittent. Now I turned it on and can't reproduce the hum, even with all knobs at maximum. Ideas?

[52 Bill]

I'd start by checking the preamp ICs for shorts. Take voltage readings on the input and output pins to see if there are any voltages there.