That would be great, what I really need to know is how the opto is triggered. I’m guessing voltage is fed to it by the remote wire, but it could also be from PS secondary.
It's all in the local area. They use the opto so that all can be done from the primary and it will have no influence on the secondary. All components are likely within 1-2" of the opto.
There should be a 2N7000 that is triggered by a 470K resistor, 1N4003 diode and a 10uf capacitor. I can't check the 5075DX because the crossover card is over the area of the opto. I am comparing my ProMOS 25 and it uses the diode/resistor/cap from +rail to control the gate of the 2N7000 to switch the opto.
Did you check for negative voltage on the input of the opto or on the gate of the jfet?
Did it produce audio when you pulled the jfets?
Did it produce audio when you pulled the jfets?
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It passed audio already, when I shorted the opto. The input side JFETs were thus proven good, I just need to track down why they aren't being told to allow audio through to the front end. I suspect the 2N7000 that switches the opto is bad, I'll look there next.
So I traced the LED side of the opto back to the transistor that's controlling it, which turned out to be Q4 (a 2N7000 MOSFET). If I short the drain and source of Q4 I get clear audio on both channels, proving that the opto and JFETs in the muting circuit are good. The drain lead of Q4 goes to the opto LED which is fed voltage through a dropping resistor, source is grounded. Gate has around +.6vdc on it, which comes from the positive rail of the power supply via a pair of diodes (CR1 and CR3). So I guess the question is: is .6v enough to switch Q4 on, or is Q4 bad?
The datasheet says 0.8 to 3v.
If you pull the 2N7000, does the gate drive voltage go higher?
Some of the early amps had a Zener in series with the gate. If this one does, is it within tolerance?
If you pull the 2N7000, does the gate drive voltage go higher?
Some of the early amps had a Zener in series with the gate. If this one does, is it within tolerance?
I'm thinking CR1 is a zener, it's a tan colored body unlike the black more common style that CR3 has. Won't know till later though, I have to pull one lead on both of them in order to read the part numbers.
CR1 was indeed the zener, a 4.7v unit (1N4732). CR3 is a standard 200v/1a silicon diode (1N4003). I'll test the zener for breakdown voltage tomorrow, if that's not the problem I'll replace the 2N7000 MOSFET. I really doubt the standard diode is at fault.
Pulled the 2N7000 and it's shorted from gate to source (13 ohms), meaning all the voltage coming through the diodes was being shunted to ground. I'll replace it and retest.
Replaced the 2N7000 and audio was restored, but I felt the gate voltage was a bit high at 3.8v. The data sheet says 3 max, so I changed the value of the bleeder resistor from gate to ground from 220k down to 150k. The gate voltage then stabilized to 2.6, and everything appears to be happy.