Old ss guitar amp repair / volume loss

[Mathias]

Hi all,

I'm looking on some advice on how to proceed with a ss guitar amp repair. There is barely any volume at all maxed out. It's 100 watts, but putting out close to nothing. It's a 70's-80's amp made in the Netherlands. The company still exists but didn't get a direct reply to get a schematic... so that's the first issue to begin with. I've added some pictures in an attempt to show the traces and layout, hope this helps.

- PSU is good, rails are +/-38V DC
- Signal is coming in, gain 1 and 2 give distortion and clipping
- EQ section seems working correct as well
- Volume seems to be working
- 100 (large beige) ohms power resistor is good
- Changed the large IC with a known working one, same result
- Checked for bad connections... nothing to be found
- Speakers measure out 4 ohms (2x8 in parallel)

I'd like to hear your suggestion on what could be the possible cause of low output. The drivers are near impossible to measure from the top.
The outside drivers run quite hot compared to the inside that run cold. Could be something in the pre amp section as well...

 

[pucho812]

shotgunning parts does not always mean your issue is solved.

What you need is a tone generator and o-scope so you can trace the signal; path and see where you are dropping your signal.
This is assuming the PSU is correct.

 

[Mathias]

shotgunning parts does not always mean your issue is solved.

What you need is a tone generator and o-scope so you can trace the signal; path and see where you are dropping your signal.
This is assuming the PSU is correct.

I’ve started drawing the schematic (or at least an attempt) and trace signal with the scope. The input stage goes like this I believe and continues to the tone stack. I see the signal dropping significantly after C3. Could that cause the signal to drop in both channels?
Those 2 output caps measure correct capacitance which makes the whole thing scratch my head.

 

[CJ]

First thing to do is pull the output transistors and check them.

You can use a simple EB and BC junction test with a DMM diode checker,

Or if you are able, do a HFE check with a dual power supply,

 

[Mathias]

First thing to do is pull the output transistors and check them.

You can use a simple EB and BC junction test with a DMM diode checker,

Or if you are able, do a HFE check with a dual power supply,

Hi CJ, thx for your input. These pass the DMM test.

No dual PSU at the moment but reads 15 and 45 Hfe on a component tester.