Loud hum and blown fuse

[Steve Conner]

OK, so looking at "schem2.jpg"

The ballast resistors are R17, R18. They show as 0.075 ohm, which seems incredibly low, so I'd want to keep this idling cool to reduce the risk of thermal runaway. Also, it's a compound pair output stage, and these don't need much idle current at all to get rid of crossover distortion. As long as it's more than zero it'll do.

So, I'd stick my meter across either R17 or R18, and adjust the bias pot R14 for 1mV (0.001 of a volt) which works out about 13mA of idle current. Assuming your meter can read millivolts accurately... If not, anywhere between 1 and 2mV will probably do.

I should mention this is best done with the speaker disconnected, no input signal, and the master volume turned down to zero.

[ksjazzguitar]

Thank you for everyone's help and patience.

I chose the "voltage drop across the resistor" option since it seemed to be the most simple - I'd have to splice open a power chord to measure the current. I adjusted it to 1.0-1.1mV and now the amp is working great. I'm going to let it stay on for a while now to make sure it won't crap out on a gig. I'll probably do the same with a signal going through it tomorrow (while the wife is out.)

Thanks for the help. The whole repair cost $20, much less than I would have paid if I took it to a shop, and I got to learn some things about amps.

Thanks so much.

Regards,
Kevin

[Enzo]

I am glad it works for you.

Just a note, to measure mains current, you do need to put a meter in series with the mains wiring, but you need not cut the cord. Just remove the mains fuse and connect the ammeter in its place. Or turn the power switch off and connect the ammeter across it

[ksjazzguitar]

Good points.