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Old 24th April 2009, 05:49 PM   #1
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Join Date: Apr 2005
Location: Simcoe Ont
Default Guitar amp

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.
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Old 24th April 2009, 07:55 PM   #2
mjf is online now mjf  Austria
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Join Date: Jan 2009
Location: Austria
hello.
perhaps a (volume-)pot is damaged .have a look for bad soldering points............clean the pot with a contact spray.
greetings.....................
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Old 28th April 2009, 06:59 PM   #3
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Location: Simcoe Ont
Default Resistors Blown

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.
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Old 2nd May 2009, 06:06 PM   #4
spirod is offline spirod  Australia
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Join Date: Sep 2006
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.
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Old 3rd May 2009, 12:48 AM   #5
Artie is offline Artie  United States
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Join Date: Jul 2007
Default Re: Resistors Blown

Quote:
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?
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Old 4th May 2009, 01:16 PM   #6
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Join Date: Apr 2005
Location: Simcoe Ont
Default Blown

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