I remembered something about the clean channel being solid state and the overdrive channels being tube driven, so I started with a set of preamp tubes which changed nothing.
Getting back to basics I started with measuring plate voltages and wouldn't you know, there was very little voltage on pin 1 of V2 which receives its plate voltage through R41. The resistor tested OK with my extra high quality Harbor Fright multimeter although not so well with my ten buck analog multimeter.
But the voltage was going in but not coming out.
It was a simple enough matter to snip the resistor and sweat out the cut off stubs without removing the circuit board, check underneath with a mirror to see that all was well on the underside, and solder in a new resistor which I could do because the holes are plated through. Cleaning out the lead free solder from the holes and using some good Kester 44 did the job. To be on the safe side I resoldered pin 1 where it merges to the circuit board. It looked like it could use it.
The overdrive channels sprang to life with all the bowel loosening feces tone Blackstar is famous for, and after checking and adjusting the power tube bias, away it went. I kind of like the idea of a bias balance and level set which these amps have. I mean, I had to say something good about them, didn't I?
The point is, start with the basics-swap out for a set of known good tubes and then some checking to see that voltage appears in all the right places. As it happened no fancy schmancy tooling or scopage was required to set this amp right, and it was not the first time I've seen such things happen.
