Acoustic 150 help needed

[Bishop]

I have an Acoustic 150 that has a major hum, unaffected by the controls..

If I remove the "signal in/out" wire from between the first channel (17-10) and the power amp (17-12) the hum goes away and the second channel sounds great (they basically enter the power amp at the same point).

Simple enough, but what I can't get my head around is that if I ground that same signal wire while it's connected, I get no change in the hum whatsoever. It kills the signal fine, but not the hum.

If it were something being introduced through the preamp signal wire, how does grounding it not kill the hum? If it's something from the power amp, how is it only effecting one channel? Someone please tell me I'm not crazy.

The pertinent pages on the schematic are pages 16 and 20. The signal out from the preamp (C) goes directly to the signal in on the power amp (C)

Already replaced the two e-caps and Q103 (MPSA09) on the preamp board.

Thanks!

[Enzo]

Perhaps a ground loop. Is this 60Hz hum or 120Hz hum?

Is there DC on the output of the 17-10?

The two little polarized caps outputting from Q103, are they tantalums? Always suspects on my bench.

When th orange wire is connected, you get hum. But while it is humming, does the 17-11 channel still work? And does the 17-10 channel work, though humming too?

[The Dude]

Also, check ground connections throughout. If disconnecting input signal to the amp gets rid of hum, but grounding it doesn't, I suspect a missing or bad ground connection.

[Bishop]

Thanks guys, that makes sense on the grounding. I was telling myself that even a grounding issue that manifests itself on the signal output would still be affected if I grounded that output. I did check the grounds on the preamp, but haven't gone over everything.

No DC, oddly enough one of the tantalums was bad in a different issue I already solved. Both channels work over the hum, which sounds like 120 to me.

[Enzo]

You have a scope? 60/120 is easily determined visually. Hard to tell 120 from 60 with overtones by ear.

[Bishop]

Spot on guys. A major grounding point is the second channel's input jack, it wasn't making good enough connection despite measuring 0.0 to the chassis and being tight enough for use.

Much appreciated as always!