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I'm trying to adjust the internal trimmers for the gain stages. I looked up the service manual it says adjust trimmers for 1.4 v on pins 1 and 7 on the opamps. But my voltmeter says 15v it will not adjust to 1.4v any body have any knowledge with these. I was trying to adjust the trimmer on the compression opamp it says 45mv but im getting like 15v on it to. I just bought this used with the foot switch since this is 34 years old i dont know if anybody has messed with the trimmers inside. Just wanted to try to set them to the factory specs.

Measure if the -15V is present at pin 4 and if ground is connected to pin 3 of the OP-Amps.

Ok thanks I’ll check

The trimmer is for setting the *AC* signal gain. On the schematic you put in an AC signal then you adjust the gain so the AC signal level matches the adjustment spec.

george giblet wrote: ↑18 Apr 2021, 04:40 The trimmer is for setting the *AC* signal gain. On the schematic you put in an AC signal then you adjust the gain so the AC signal level matches the adjustment spec.

You are right I overlooked thatJmpj45 tried a DC setting with the potentiometers.
The outputs of the mentioned OP_Amps should be ~0 VDC.

So I’m going to need a Oscilloscope to get this done? I don’t have one what about one them cheapy Oscilloscope on Amazon would that work.

jmpj45 wrote: ↑18 Apr 2021, 17:40 So I’m going to need a Oscilloscope to get this done? I don’t have one what about one them cheapy Oscilloscope on Amazon would that work.

For the measurements I meant, a multimeter is enough.
Measure the voltage between +15VDC and PIN3 of the op-amps which are connected to ground according to the schematic,
repeat this with -15VDC. If the rail voltages can be measured, the ground connection is OK.
Then measure the voltages on PIN4 and PIN8 to ground.
This applies to the op-amps U5,U6, U14,U15 and U16.
It is enough to measure on one op-amp, because the voltage rails and the ground are connected on the PCB.
For audio repairs you can use a cheap oscilloscope.

So do I set my multimeter to ac or dc. And what pin is -15vdc. And what voltages am I’m looking for pin4 and pin8 to ground. And what pins do I use to set the trimmers. Thanks just trying to wrap my head around this lol

jmpj45 wrote: ↑18 Apr 2021, 21:47 So do I set my multimeter to ac or dc. And what pin is -15vdc. And what voltages am I’m looking for pin4 and pin8 to ground. And what pins do I use to set the trimmers. Thanks just trying to wrap my head around this lol

Set the multimeter to DC.
Pin4: -15V to ground
Pin8: +15V to ground

Ok ill check it out thanks.

I think we are missing something here?

If you are trying to set the voltages noted on the Schematic posted above, then the instructions are at the top left corner.
Ideally you need a signal generator set at 400hZ with .5 volt output to take the measurements (take note off where to set controls first)
You can likely get close just by using your ear. If it distorts too quick just back it off until it cleans up,,
Not sure how well that will work but I'd try.

You need to set your meter to AC to make any useful readings.
The test points and pin numbers are shown on schematic.
Hope it helps, Phil.

I have tried everything with no luck. I’ll try what you said I will have to get a signal generator thanks for the information

jmpj45 wrote: ↑21 Apr 2021, 17:10 I have tried everything with no luck. I’ll try what you said I will have to get a signal generator thanks for the information

There should be no +15VDC at the output pins (1 & 7) of the op-amps in the signal path, what values did you measure the measuring points I suggested.
A signal tracking by means of signal generator and oscilloscope will not help if the OP-amp biasing or the power supply is not correct.

Let me check the voltage again on pin 7 of u10 and the voltage on pin 1 and 7 on u14. I’ll get back with you.