1980's Marshall Bass 12 - Hum Problem

[Barnsleyboy]

Do you have a scope?

I'm intrigued about using a scope to identify the problem. I'd presume that you feed a signal into the amp with a sig gen, then examine the waveform at various points along the signal path until you find where the hum is introduced into the circuit? Not sure if I have oversimplified this! If you could talk me through the basics of how to connect it up, suggested locations along the path to check etc. I would be very grateful.

Usual caveat applies regarding high voltages, extreme caution, not ending up being a charred and smoking guitarist on the end of a probe connected to the scope etc.

[Helmholtz]

No sig gen required here as the hum is your signal.

Start with scoping the speaker output.

Also make sure there's no DCV across the output.

[Barnsleyboy]

I've just dug out the wife's scope - haven't plugged it in yet, so don't even know if it works. So when you say make sure no DCV across the output, I have connected my multimeter across the speaker wires and I'm getting the reading on the photo. Doesn't vary when I twiddle volume or gain knobs, or when I plug a lead in. Forgive my absolute newbie questions, but what is this telling me?

[Dave H]

It's saying there's a -100mV offset on the output which is OK.

[Barnsleyboy]

ok - great! I've connected the scope to the output, but I'm not really sure what I'm supposed to be looking at!