PPI 2050AM doesn’t pass audio

miniman82

Member
2016-12-30 11:53 pm
USA
Amp has good rails and +/-15v for the opamps is also good. Output stage has a few mv of offset, not objectionable. No output though. Probed around on the TL074 and LM833 hoping they were part of the preamp, but found no signal anywhere when the amp is fed a sine wave. Do these have mute circuits? If not, I’ll need a schematic to chase down where I’m losing the signal. Thanks in advance!
 

miniman82

Member
2016-12-30 11:53 pm
USA
Dammit that’s right, they work opposite normal silicon. lol

Side note: I had been grounding to PS secondary, but just for kicks I tried main ground. From there to gate I got -.6v, but prob still not enough to pinch them off. I’m leaning toward the opto being bad, since that’s what killed my Big Red Sony. Let me replace that thing, see if that don’t clear it up.
 

miniman82

Member
2016-12-30 11:53 pm
USA
Did you confirm (without the muting transistors) that the amp was otherwise in good working order?

I didn’t remove them since they seem to be working as advertised in the results of #13, otherwise it seems fine.

I don't remember but many of the early amps used a simple high-value resistor charging a capacitor for the mute delay. Either an out of tolerance resistor or a leaky capacitor could prevent the charging of the capacitor.

Web shows several possibilities for how it could be wired but most don’t involve an opto, just a 1-meg resistive divider with a cap leading to the gates. I’ll poke around some more, see if I can figure out where the LED side of the opto goes. Remote lead, I assume.