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On Sun, May 15, 2022 at 05:07 PM, unclebanjoman wrote:

I don't really know what else to try

Hi Max,
Currently, I'm not where I have my equipment, doc and parts so I'll do without:
I'm pretty sure that Q150 is an often-used FET (151-1032-00?), so it would be easy to replace. I'm almost certain I have a few. You're in Italy, right? I'm in NL, so replacing it wouldn't be a real problem, even if you don't have a replacement. I can check if I have one this Tuesday.
Since these FETs are basically symmetrical, you could swap both by rotating the case 180 degrees, depending on pinout, just to try, also depending on how the case is connected.
R133 isn't in my schematics, so I didn't know where it was until now. It's not surprising that it doesn't make any difference re. your problem.
The DC voltages in the circuit seem fine.
I agree with you that the red diodes and - CR130 - are for protection only.
Are you sure that continuity from the BNC input to R132 is OK?
You can't really trust your 500 Ohm 'scope probe input at the gate of Q150A. Still, I don't quite understand your different observations at the gate of Q150A with Q150 in vs out, although of course, there's Q150A's gate capacitance.

I have a feeling the problem isn't in the area around Q150.
What do you see at U350 pin 14 if you first pull the IC? Again, don't worry, easily replaceable IC, not expensive. Carefully lift each pin (spider leg) a little in sequence and go around until the IC comes free. Do you have another 155-0078-xx available for testing?

Sorry, can't be of much help ATM.

Raymond