I'm rebuilding a set of Beolab Penta MK2s. The amp keeps burning through the NPN output transistors when powered on (on a dim bulb, nonetheless). I've recapped the board, changed adjustable resistor (R95), and fixed a few broken solder joints. I *think* the broken solder joints on TR28 were killing my output resistors, but I'm not sure of that. Would it be safe to power up the amp without the output transistors installed just to check voltages? I don't want to kill yet another set of transistors. Or perhaps turning R95 to max would effectively "turn off" the outputs?
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Beolab Penta MK2 Amp Rebuild Question
2 replies
It is not necessary to install all 6 output Transistors... 2 are enough for testing the amp. Or remove/disconnect 4 of the 0,47 Ohm resistors. The other 2 resistors can be replaced by cheap 100 Ohm for testing wether the amp is stable and symmetric.
It is also not necessary to power up the amp to +/-68V. +/-15V with a regulated power supply with limited current will do the job
Why is R95 a spindle potentiometer? These mods are the reason for the instable "no load current" adjustment burning the amp. Potis with an oxidated/burned middle contact are the reason for problems.
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