GK 800RB no 300 watt no sound

[pontiacpete]

I have 2 of these amps here that do not passing signal thru the 300 watt side of amp. Is anyone familiar with these amps?
This is what is happening, when plugged into the 100 watt speaker jack I'm getting sound.
When plugging into the 300 watt jacks there IS the pop sound at turn on out of the speaker as if the power amp is working but no signal passes from input.
Since there is a pop sound when power on am I safe to assume the power amp section is working and the fault is somewhere in the pre amp?
When plugging into the return there is no sound, nor is there sound when jumping the pre amp out of the send to another amp.
The effects send and return work normally when using the 100 watt output.

Does anyone have an idea what is going on here?

[Jazz P Bass]

Have you opened it up?

I would check for Volts dc on the 300 watt output jack.

[52 Bill]

Are you in full or biamp mode?

Have you tested the power supplies?

[pontiacpete]

There is no DC on the output.

[pontiacpete]

I have been trying both modes

There is +/- 76 vdc on the main filters

[52 Bill]

Bad master volume pot for the low amp? Try injecting a signal at cap C85.

[pontiacpete]

I injected a signal at the top of C85 and there's huge signal coming out.
Does this mean the MV pot is bad?

[pontiacpete]

Or a solder
All 3 solder joints on this pot were broken.
Thanks 52