Guitar amp

[argonrepublic]

I have a problem with guitar amp. It is a Randall RG200. At idle it has a louder than usual 60hz hum. as I turn it up there is no signal until around "7 or 8". It kind of crackles through loud and powerfully. At full it will almost run. Signal does not decay it just drops out. Could someone please point me in the right direction? I would like to solve the problem myself (with help). I will try to upload jpegs.

[mjf]

hello.
perhaps a (volume-)pot is damaged .have a look for bad soldering points............clean the pot with a contact spray.
greetings.....................

[argonrepublic]

Thanks for input. I checked pot fully and continued to inspect Amp. Found two blown resisitors. R68 and R69. I can't upload jpeg. if Anyone is willing to look at it I could email jpeg.

[spirod]

Did it get worse over time or did it just happen suddenly?
What happend at the time it died?
I've seem guitar amps with T03 ouput devices sometimes the leads get dry joints as they see a lot of thermal stress. this is why i ask the above.

but heres my 2 cents worth:
You could have lost output from one rail by the sound of what you are saying. but the input cct may be fine.
I'd measure for:-
Dc at the output, should be close to 0 volts. ie across the speaker.
Also the VAS transistor voltage across the CE.
if you can measure the bias current would also be good to know if it still exists.
and post you findings.

also if you have an incircuit transistor tester it would come in hand.
but by the sound of it I would guess probably a driver and an ouput device have failed at the least, other than the 2 blown resistors.

[Artie]

Originally posted by argonrepublic
Found two blown resisitors.

Ok. How do you define "blown"? Its like saying . . . My car goes thump, thump, thump. Found one flat tire. What else should I check?

[argonrepublic]

As soon as I power up Amp, resistor begin to smoke until open. To of them are comming off tube heater circuit.