Noisy Solid State

[mykamatic]

Hey Everyone,
I'm new to the board, so forgive me if i should be in The Newcomer's Forum!?
I have build a few little ruby type amps based off of the LM386 chip & built/
moded a few pedals as well so I have some knowledge I have a question about an old
1980-81 (I think) Randall rb-120 bass head. It started sounding distorted
and there isnt an overdrive. I'm not talking scratchy pots or high gain, it just sounds like
a fuzzy "queens of the stone age" type distortion? Someone told me it could be the capacitors?
I just don't want to replace everything on the circuit board for no reason trying to make the
problem go away....So I just wanted to know if anyone had any ideas why a SS amp might
start sounding distorted?

Thanks!!

[Enzo]

A million reasons.

First, connect the amp to some other speaker.  Does it sound the same through that?  That tells us if it is the speaker or not.

If the amp is truly at fault, then check the powr supplies.  I don't know what voltages it has inside, but there will be both positive and negative.  The powr amp will work on something like 40-50vDC.  Are both of them about the same voltage?  Flip your meter from DC volts over to AC volts and measure the same two supplies.  You are now measuring the AC ripple on these DC voltages.  it should be very low, like a volt or less.

The preamp will probably run on +15 and -15 volts.  Find those, are they both about the same voltage and close to 15?  Also measure them for AC and we should see zero AC volts.

SInce I don't know the amp, old as it is it could be transistor instead of ICs, so in that case there may be only one preamp supply.  Still should be free of ripple.

And go through the stages l;ooking for unwanted DC offsets at each stage.

[mykamatic]

Thanks Enzo!

I'll try those tips out!