gallien krueger 700rb bias issue

[metalmaniacs]

Hi,I have a problem with g/k 700rb(not Mk2)

Symptom:Adjust bias 5mv at P2, and about 5 minutes later this voltage decrease step by step.
After more 5 minute this voltage 0.5mv.

Voltages at TPs are looks like normal.(Reference to schematic from web.)
I changed almost all transistors in power amp circuit except Q2.

Is there anyone had such a experience like this?

Regards.

[Enzo]

On any amp, set it more or less, and watch it, then recalibrate after maybe 20 minutes. After the amp has fully warmed up. Bias normally adjusts to temperature.

[metalmaniacs]

Thank you for reply.

But 20 minutes later bias adjust pot(R23) does not work at all even if I turn it full clock wise.
I changed bias adjust pot but same as before.

[The Dude]

Try some freeze spray while monitoring bias current. You might be able to narrow down which parts is taking a dump.

[Enzo]

First guess is the bias transistor itself.

[g1]

Is this the correct schematic?
http://music-electronics-forum.com/a...ueger700rb.pdf

[metalmaniacs]

I changed bias transistor(Q23),but same as before.
Q23's vbe is 0.5v〜0.6v.

Yes,it is the schematic of my 700rb's.


Regards.

[Enzo]

See post #4.

Get out the freeze spray and cool areas until you find one that affects this.