Progress report since last post. I see a light (?)
Good Day🙂,
Thanks for that reply Craig. I removed and tested the outputs on the 'original' side of the amp that was indicating a shorted output on one rail with my HAMEG and found one to have a slightly leaky junction. New ones are in my future. I believe there was an alignment issue on one of the legs of an output that was causing the short because when I reinstalled them the short disappeared! (2 piece heatsink alignment issue..) While I would normally be concerned about this "magical" fix, I did not change anything else so I am going with that for now with fingers crossed. I brought up the amp on variac and it is idling at about 48 mA both channels connected. I can adjust the DC offset and emitter voltages to the recommended settings on one channel (untouched side of amp). Now to the other channel.. I did not really do anything to the 'modified' side of the amp after I could originally set offset and emitter voltages, but now the offset seems to wander quite a bit more than it did before connecting the other channel to the supplies. I am also having difficulty adjusting the output emitter resistor voltages to 30mV. Before, changing the (new) trimmer's position would easily change the voltage and it would remain steady at 30mV. Now the trimmer makes little difference but I can get it to 30. And before, changing the DC offset would affect the Emitter voltage and require re-adjustment. I had all that working ok and with little else changing, it is a little squirrely. The 2 large 10 Ohm resistors are visibly darkened on that board, but do not get hot since I have started working on the amp. They do measure 10 Ohms.
The kicker - I connected everything up to loads and signal source, brought up the amp and my scope signals look good on both channels!😕 It is not heating up very much under load and I can drive to a decent voltage but I haven't stressed it yet. I connected speakers and it plays music nicely but I haven't driven too loud yet. There is some audible hiss noise on both channels but I don't really hear hum.
So the sum up is; 1. any thoughts on the emitter resistor adjustments 2. the significant wandering of the DC offset on one channel, and 3. the (slight) hissing in the speakers.
Cris
Good Day🙂,
Thanks for that reply Craig. I removed and tested the outputs on the 'original' side of the amp that was indicating a shorted output on one rail with my HAMEG and found one to have a slightly leaky junction. New ones are in my future. I believe there was an alignment issue on one of the legs of an output that was causing the short because when I reinstalled them the short disappeared! (2 piece heatsink alignment issue..) While I would normally be concerned about this "magical" fix, I did not change anything else so I am going with that for now with fingers crossed. I brought up the amp on variac and it is idling at about 48 mA both channels connected. I can adjust the DC offset and emitter voltages to the recommended settings on one channel (untouched side of amp). Now to the other channel.. I did not really do anything to the 'modified' side of the amp after I could originally set offset and emitter voltages, but now the offset seems to wander quite a bit more than it did before connecting the other channel to the supplies. I am also having difficulty adjusting the output emitter resistor voltages to 30mV. Before, changing the (new) trimmer's position would easily change the voltage and it would remain steady at 30mV. Now the trimmer makes little difference but I can get it to 30. And before, changing the DC offset would affect the Emitter voltage and require re-adjustment. I had all that working ok and with little else changing, it is a little squirrely. The 2 large 10 Ohm resistors are visibly darkened on that board, but do not get hot since I have started working on the amp. They do measure 10 Ohms.
The kicker - I connected everything up to loads and signal source, brought up the amp and my scope signals look good on both channels!😕 It is not heating up very much under load and I can drive to a decent voltage but I haven't stressed it yet. I connected speakers and it plays music nicely but I haven't driven too loud yet. There is some audible hiss noise on both channels but I don't really hear hum.
So the sum up is; 1. any thoughts on the emitter resistor adjustments 2. the significant wandering of the DC offset on one channel, and 3. the (slight) hissing in the speakers.
Cris