Fender Bassman 250 muting circuit

[Randall]

EDIT: Initially I thought I had an issue with the mute circuit, but have since realized the issue is after that.

Bassman 250 in for no output. It is totally silent. I trace signal from input jack, through FX in/out,thru the Master pot circuit, thru the connector to the PA board and up to the base of Q200. I see a very faint sine on the emitter, 100mV, but that's where I lose it.

So now I am looking at the thermal circuit to the lower left of Q200, and I measure -0.04vdc at TP 11, and -56.5v at TP12. Both diodes test good in circuit. Can I get a hand sorting this out?
fender_bassman_250_fba_schematic_rev-a (1).pdf

[Enzo]

So no output. Even no background sound? And we tested the speaker and wires?

C35? You generally cannot measure signal at the input pins of op amps, so look on pin 7 of U9, if signal is there, then count on it coming through C35.

Do the headphones work.

[Randall]

Speaker and wires are good. It's on my bench with my test speaker rig, same outcome. It sometimes will make noise when I probe the outputs. Headphones do not work.

Barking up the wrong tree here I suppose. Now not focusing on the mute circuit, I see an attenuated signal that follows the Master control appears on the base of Q200 in the PA section, but that's where I lose it. Also notice a faint and fading 1K tone thru speaker when I probe the emitter with my scope probe.

I could use some help with the thermal circuit in section A6.

[dmeek]

Looks like a problem with the thermal circuit around Q224-Q225. Check that you have the -16V , or C214 could be shorted.
Or a bad transistor. TP11 should be around -10V.

This circuit controls current flowing into the base of Q226. No current = no signal through the input stage, disabling the power amp.

Doug

[Randall]

I lifted the 3 diodes and 2 caps, and they tested good. The 3 transistors tested good in circuit, so I left them alone for fear of heat damage getting the little buggers out. I pulled RT1 and it measured 60 ohms, and rose in value when I hit it briefly with the heat gun, so I took it to be working. Then I measured what was left, which was only a few resistors, and voila! R256 18K was open. I replaced it, and now it passes signal. I don't exactly understand how this circuit works, but I found and fixed the problem, so all is good.

Except for those two shafts that pulled out of the pots due to the knobs being frozen to them.