03-12-2016, 07:30 AM
Took voltage readings of both channels and found a disparity on Q3 (BD235). On the good channel we get E 1.7v C 0v B 1.1v so all looks kushti there however I would have expected some voltage on the collector. I'll take the reading again today to make that's right. Any the faulty channel reads E 1.7v C 1.7v B 1.1v. The faulty channel also has slightly lower DC EG good channel = Q1 (MJE340) E 85.4v C 1.7v B84.1v compared to faulty Q1 etc E81v C1.7v B 80.4v which I think is down to the faulty channel pulling more current.
Anyway, I could be chasing a red herring but after checking all components around Q1A - Q5 the only thing I found was that the resistance on the good channel from R25's non wiper "terminal" to Q4 and 5's connected emmiter is 66k ohms but on the dud it is a few meg. this doesn't make sense, I would have thought a lower resistance would have caused more current to flow. The situation isn't helped by the schematic, I can't find any R24, it isn't listed in the parts list, it's not clear whether R24 ( which doesn't exist) is connected to C22. There is no direct track on the PCB from R25 to the emitters of Q4/5. I need to investigate more.
Andy.
Anyway, I could be chasing a red herring but after checking all components around Q1A - Q5 the only thing I found was that the resistance on the good channel from R25's non wiper "terminal" to Q4 and 5's connected emmiter is 66k ohms but on the dud it is a few meg. this doesn't make sense, I would have thought a lower resistance would have caused more current to flow. The situation isn't helped by the schematic, I can't find any R24, it isn't listed in the parts list, it's not clear whether R24 ( which doesn't exist) is connected to C22. There is no direct track on the PCB from R25 to the emitters of Q4/5. I need to investigate more.
Andy.
