What caused the problem?
Hopefully, you haven't:
.... because the negative binding post on that amplifier must stay floating at all times... and there's no short circuit protection (nor any type of current limiting) to save the transistors... you know.... the ones that you coincidentally reported as blown.
Hopefully, you haven't:
- Connected the negative binding post to ground
- Used a type of measurement equipment that would make ground connection via mains cable earth pin, if an alligator clip or similar was connected to the grounded measurement equipment case... a non-differential (bog standard) oscilloscope probe comes to mind.
.... because the negative binding post on that amplifier must stay floating at all times... and there's no short circuit protection (nor any type of current limiting) to save the transistors... you know.... the ones that you coincidentally reported as blown.
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Sadly I have no clue of what caused to problem. I got the amp already broke. The last owner told me that it just stopped.
I've not been involved in powered on live measurements so no risk of accidental earth connection while measuring.
Inside, S6 was blown : it is the fuse of the +20VL.
I've traced that the +20VL_AMP1 section got a short : I've unsoldered the resistors of calibration (R119, R120) to +20VL_AMP1 and +20VL_AMP2, removed two jumpers (P7) and then no problem. Then, I've reconnected one at a time. When +20VL_AMP1 was connected, the fuse was blowing.
I've found that Q78 was E-C shorted : the 2SB1184 I am trying to replace becoming short, I guess it just pumps all the voltage of the positive half of the push pull, and so draw a lot of current.