11-24-2014, 06:55 PM
Well, looks like your output transistors are gone.
I would check also at least the previous stage, two complementary one (PNP and NPN).
It is strange the diodes are gone, the fuses should've protected them, diodes do not blow that easy, but then I cannot find datasheet on SRIDM-8.
I also wonder why they used Hi-voltage transistors considering the B voltage is 25V (or so I see in the sch). One could use much better lower voltage higher current transistors.
When adjusting the amp, first turn VR3 and VR4 to short, then when done with making sure the amp works, use these to set the minimum quiescent current (to remove the crossover step distortion).
Best done with a scope.
Of course if those were not touched you could try to leave them in place.
Good news is, more often than not the damage of the output amp is limited to the output transistor pair, but you should check all you can to be sure.
I would check also at least the previous stage, two complementary one (PNP and NPN).
It is strange the diodes are gone, the fuses should've protected them, diodes do not blow that easy, but then I cannot find datasheet on SRIDM-8.
I also wonder why they used Hi-voltage transistors considering the B voltage is 25V (or so I see in the sch). One could use much better lower voltage higher current transistors.
When adjusting the amp, first turn VR3 and VR4 to short, then when done with making sure the amp works, use these to set the minimum quiescent current (to remove the crossover step distortion).
Best done with a scope.
Of course if those were not touched you could try to leave them in place.
Good news is, more often than not the damage of the output amp is limited to the output transistor pair, but you should check all you can to be sure.
![[-]](https://philcoradio.com/phorum/images/bootbb/collapse.png "[-]")
