Peavey Pacer 100 (45w solid state) amp repair second time

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Wasoofa82

Joined Jan 24, 2021
44
Hello everyone I have an old vintage 1975 peavey pacer 100 (45w solid state amp). I am fixing for a friend of mine. Back in summer 2020 it went dead. I replaced all the electrolytic caps and two of output power transistors, used deoxit on all the pots. It worked great after the 1st repair. Now it won’t go into overdrive crunch with the master volume gain knobs. My friend said he can still get distorted sounds with his pedal
board connected thru the input jack
I am thinking maybe one of the output power transistors are fried. The reason for this maybe is because I recall not getting a matched pair perhaps. I am also wondering if I did not apply the heat sink grease correctly. It uses mica insulators that are sandwiched between the chassis. Here is a link to a schematic and some pictures https://www.audioservicemanuals.com/pdf-download.php.
 

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Thread Starter

Wasoofa82

Joined Jan 24, 2021
44
I was thinking in order to check the supply voltages. Should I input a 1khz sine wave at 300mv peak to peak to simulate an audio signal. I’m guessing I could not check these voltages with the unit just powered on and at idle. Maybe I could probe just the positive terminal of the speaker with a scope probe and the earth lead connected to nothing, being that the amp and the scope is already earthed to thier respective chassis’s. By doing so I could crank up the distortion and see it’s going into clipping on the output. I also already tested the large can transistors with the diode feature out of the circuit with a multimeter and they seem good.
By looking good at the schematic these seems to be a lot of discrepancies, I found the following
(6): Motorola sps953 npn audio transistors
(1): Motorola spf558 n-channel FET
(1): Texas Instruments EP430 NPN transistor
(1) 2n4248 pnp low noise transistor

I’m guessing it has to be a cap that is faulty and not is coupling between the stages in the amplifier . How would I go about safely checking the stages with a meter or a scope. I’m really lost now!!
 

MrChips

Joined Oct 2, 2009
26,478
Take one step at a time.
First thing we do is check the supply voltages with a DMM with no input source.
Now that we know that you have an oscilloscope that opens up a whole new game of diagnostic testing.
Let's do the voltage checks first.
 
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