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Fender Frontman 65-R Power Amplifier Distortion

Started by smilan, October 03, 2024, 12:53:51 PM

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smilan

Hi,
I have a Fender Frontman 65-R with distorted output.
I need some help troubleshooting this amp.
The output from the "PRE OUT|" is clean so the problem is somewhere on the power amplifier.
The signal seems to be OK at pin 3 of U-7A but at the output of this op amp (pin 1) the signal is very distorted.
With Q6 disconnected, the output on pin 1 of U-7A is clean.
I took out Q6, tested it outside the circuit and it tested good.
What should I test next?

Here's the schematic:


g1

The differential pair of Q6 and Q7 are a very common problem in this series of amps.
They need to be well matched and will fail in ways that do not show up in testing on your meter.
I think you have already localized the problem to this area so you should just go ahead and replace them, then continue troubleshooting if the problem is not solved.