x

[fenderguy81]

x

[shaun]

I doubt I can be of much help, but the problem with many modern hybrid amps and pcbs is that they are a pita to work on. You kind of have to know the specific amp pretty well before pulling out the boards, for example. At least, that's been my experience, and I won't even attempt repair on them, especially because even if you can get a schematic, they're extremely detailed compared to vintage gear, and are often full of micro-components. I like the analogy of vintage cars - you can look under the hood and find the carb, the points etc. Diagnosis is easy compared to a modern vehicle that has to be plugged in to the right computer for diagnostics. Which means taking it to the dealer or manufacturer. It's the same with many modern amps, so repair becomes very expensive unless you live next door to the manufacturer.

But my wild guess is that it could be a cap, maybe in the preamp section. Or a switch going bad. Or a solder joint on a tube socket. Spray all connections with electronic cleaner/lubricant. Maybe search other forums to see who is replacing entire pcbs in your model of amp. Finally, throw it off a bridge in frustration.

[PRR]

User manual:
https://www.manualslib.com/download/597468/Markbass-Tte-500.html

[fenderguy81]

x

[thetragichero]

is v4 a cathode follower, possibly one for each half of a push pull output stage after a v3 long tail pair and before the solid state output stage, similar to an svt?

[fenderguy81]

x

[Latole]

This will be a very difficult problem to fix in my opinion.

If you don't have the schematic, you absolutely need an oscilloscope.
Reading the voltage is unlikely to help in this case.

Could you show us some good pictures of the circuit?

[fenderguy81]

x

[Latole]

Thank's for the pictures
No schematic , no scope. lots of micro-electronics !!!

 I can't see how, you or somebody, can fix it

[sluckey]

Run away quickly!

[Latole]

Run away quickly!