Peavey bass amp hum.

[Boo Levard]

I have a Peavey series 400 bass amp (UK issue), which started farting last night. The noise was intermittent but built up to a loud & distorted crackle & that's when I switched off. When I switched back on this morning I'm just getting hum & nothing else. The amp was unused for about 18 months prior to last week. What could the problem be? I'm not an electrician but it would be nice to know some possibilities - I can solder joints, components & other stuff but the only time I use a multimeter is for setting the ignition timing on my motorbike. It would be useful to know how much it would cost to repair from the worst case scenario of having it repaired in a workshop to replacing the transistors myself. The light comes on & there has been no burning smell coming from the chassis but a sharp rap with the knuckles on the metal backplate sometimes causes the light to go out & the hum to stop like there is a loose connection but a visual inspection revealed nothing untoward aside from a lot of dust which I carefully hoovered out. Any ideas fellas?

[Enzo]

OK, if hitting it makes it work for a moment or at least stops the loud hum, then I suspect it is not blown up.

The rear panel is the power amp. Does your amp have the row of 8 black plastic covers hiding power transistors across the rear panel? or does your amp have heat sink fins on the rear?

Either on the circuit board on the rear or possibly on the floor of the head cab, you should see two identical filter caps side by side. They are probably either blue or silver. These are cylinders very roughly 30-40mm diameter and 80mm long maybe. If on the circuit board, look at the solder under them. There should be like maybe 6 tabs sticking through the board. Any of them cracked free? In particular the two in the center of each.

If yours are mounted on the cab floor, ther will be wires from them back to that circuit board. A loose wire or connector could cause the same thing with those.

Failing that, plug a speaker cord into the rear of the amp, but don;t plug the other end of the cord into a speaker. Set it in front of you. Take your meter, set it for DC volts, and a scale that will read up to 50v or more. Now turn on the amp and measure for any DC voltage between the tip of the plug and the shaft of it.

You could take these readings inside the amp, but using the speaker cord as a test point, you don;t even have to open the amp head.

If you get 40-50v DC, your amp has a blown output.

[Boo Levard]

Thanks mate, I'll do the checks & mine is the model with 'the row of 8 black plastic covers hiding power transistors across the rear panel.' I did notice those cylinders - the wiring seems to be intact. I am not getting any voltage at all across the speaker cord jack tho...

[Enzo]

If ther is no DC voltage across the speaker terminals, that is a good thing. It means the amp is not blown, and you likely have a power supply filter issue involving the large caps.