fender frontman 212 distorted output

[g1]

What JPB said above, reduce your signal at the PA in jack.

[pontiacpete]

I put 5mV into the PA in jack I get 50mv at pin 7 of U6.
Pin 1 of U6 has 1.9mV

[g1]

Does U6 have proper +/-16V at pins 8 and 4 ? Do D16 & D17 check ok?

[pontiacpete]

There's the +/-16 v on pin 4 and 8
The diode are not shorted with the diode test on the meter
They both read ~260k both ways with the meter leads, in circuit. I wonder if they are bad, maybe I need to lift one leg out and measure?

[g1]

You shouldn't have to lift an end to get the lower forward biased reading. If anything parallel circuits would make it read lower, not higher. On diode test function you don't get a lower reading one way? Sounds like they are both open, although that would be a bit of an odd fault.

[pontiacpete]

They read the same both ways red and black probes, On the diode function i get the churp and .605vdc.
Open?
Trying to decide if I should flip the board...

[Jazz P Bass]

Well sure the diodes will read the same both ways .
Look at how they are connected. (in parallel/ reverse polarity)

Is there any dc voltage on U6/ pin1?

You may have a duff opamp.

Or the problem is further down the line. (like Q10)

You really should diode check all of the transistors.

Some voltage readings would help.

[pontiacpete]

This is what I have so far
U6 pin 1 has .2vdc
Q10
c = 40vdc
b = 0 vdc
e = -.55vdc

[Jazz P Bass]

Well, seeing that U6 pin1 is not putting out 33 mv's, as is pin7, maybe it's bad.

[pontiacpete]

That did it. U6 changed and it's working.
Thank you Jazz so much for helping me with the repair.

[Jazz P Bass]

Hey, you did all of the work.

It sure is helpfull when they list test point voltages, isn't it.

[pontiacpete]

I'm back to working on this amp. Fortunately it does not belong to a customer. After changing out U6 and the amp worked I put it under my bench with plan to post it for sale and finally get rid of it. Then one of my customers comes by I take it out to demonstrate it's wonderful sound and it's back to sounding like it hell. So I put it back under the bench with the hope of returning to it.
The weird thing now is that I'm finding that if I touch my probe on one of the leads of the diodes in the string D21-23 the amp works correctly, distortion gone.
If i randomly probe components around the area the distortion comes back and the bottom half of the wave goes askew. Can't seem to be able to do this on command but I can correct the problem by touching one of the diodes.
Any one want to take a stab at this.

[Jazz P Bass]

My guess is a bad solder joint.

You may not be able to 'measure' anything that may be off if touching the diodes makes it work.
(Heisenberg's Uncertainty Principle)

Do you have an analog meter? That may help.

At any rate, try to measure the voltage drop across each diode.

[pontiacpete]

I tried re-soldering the area, it doesn't seem to be responding to my hitting the board or tapping the components, only the touch of a probe.

I do have a VTVM and an analog VOM, will give them a try measuring the voltage drop.

[Jazz P Bass]

Was Q9 ever replaced?

If you do decide to change it, change it's brother (Q10) also.