Blackstar HT60 Bias circuit repair

[Smokebreak3]

I need some help with this one. Amp sounds great, all channels, but super low output. After checking the EL34 grids, I've got -92VDC, so they're barely on. Bias pot does nothing. 500+VDC rail

I read in another thread that Jon Snell says the FETs are known issue. https://www.tdpri.com/threads/blackstar ... s.1124429/

That part of the schematic is on pg 4. But I'm having a hard time understanding how this bias circuit works, and what role the FETs play, and honestly, which ones I should be looking at, at least as far as which ones, that when failed , would cause massive negative voltage at the grids.

I'll attach the schematic and also pic Jon Snell attached and circled, but again, I'm not understanding this bias circuit so not sure if I should be looking at the MOSFETs or JFETs

pi.png

[Helmholtz]

What is the voltage at TP13 (V BIAS)?
What is the voltage "V LTP"?

[R.G.]

Near as I can tell from the schemos and J.S.'s comments, the FETs have nothing at all to do with the output tube biasing. They are DC isolated from the output tube DC bias by C122 and C123. Even if one of these caps were bad, the tube bias wouldn't go to -92V or whatever.

Looks at first glance that the Vbias is always whatever its most negative value is, except that TR4 leaks in some positive current to pull the actual voltage to the tubes up somewhat. The amount of pullup seems to be set by the bias pot PR2. I didn't go ferreting through the whole schematic to find out what ''BIAS_ON" is, but it has the smell of a digital signal to turn on TR4 to allow the outputs to be biased more on. It's not clear why someone would leave the output tubes biased way, way off until a signal says "OK, let 'em run now", but that's what it looks like.

As to how to debug, I would look at the BIAS_ON signal to see if it's actually turning TR4 on for normal operation, and if so, whether TR4 collector is being pulled up to something higher than V BIAS.

[Stevem]

I repaired this issue on one of these like 4 years ago.

I did not have a schematic then for it, but I do recall that as you look into the chassis with the input jack facing you that the bad transistor was close to being inline with the input jack, but more towards the rear of the amp .

[Smokebreak3]

Thanks a bunch for the responses guys. Hemholtz and RG I pushed the amp off my bench in the effort of progress on other things, but will revisit soon.
I totally misread that other thread , in the sense that the OP has the same symptoms as me, then another poster stated he had the same technical problem as me (big neg V), but now I see J.S. was talking about another issue with these amps, that wouldn't produce the neg V problem

Steve, this sounds hopeful, and impressive that you fixed this nightmare without a schematic. The transistor inline with the input is TR7, and controls the footswitch circuit, middle of pg3.
The only other one in the area is TR5, and appears to be connected to the speaker jack. I've attached a pic.
Any of that ring a bell?

IMG_1779.jpg

[Smokebreak3]

What is the voltage at TP13 (V BIAS)?
What is the voltage "V LTP"?

Helmholtz, both points are -97VDC, which is also the same as I have at the grids of the EL34s

[Helmholtz]

What is the voltage at TP13 (V BIAS)?
What is the voltage "V LTP"?

Helmholtz, both points are -97VDC, which is also the same as I have at the grids of the EL34s

Then either R212 is shorted or TR4 doesn't conduct.