PPI 2050AM doesn’t pass audio

[miniman82]

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!

 

[Perry Babin]

It may have something like a 2N5638 near the input to the amp that clamp the audio.

 

[miniman82]

Sho ‘nuff, J108’s right near the RCA jacks. I read no volts on D or S, around -52mv on the gate. That good or bad?

 

[Perry Babin]

The gate has to go to at least about -8v to switch off. I think it's taken from the negative 15 regulated. Look in the area of the optocoupler. I think that's what's used for switching.

 

[miniman82]

Both JFETS seem shorted, I read 4 ohms D-S on both with the amp off. Is it a common thing for muting FETs to short like that?

 

[Perry Babin]

They aren't shorted. jfets conduct when the gates have no voltage on them. That's how they mute before the amp powers up fully. Find why there is no negative voltage on the gate.

 

[miniman82]

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]

Actually I only have .9v across the LED portion of the opto, but the data sheet says 1.3 minimum forward voltage. Think it’s actually off?

 

[Perry Babin]

Do you have negative 15v on the transistor side of the opto?

Use the secondary CT for ground reference if the amp has a floating secondary.

 

[miniman82]

Transistor side is sitting at -17

 

[Perry Babin]

The amp will operate without the muting transistors in the circuit. FYI.

With the muting transistors in the circuit, if you short the opto collector to emitter, do you get audio?

 

[jkrokenberger]

It's possible there is a shorted output causing the amp to stay muted. Is the yellow LED lit?

 

[miniman82]

Perry: Yes indeed, it passes audio with the opto shorted
Berger: no yellow LED

 

[jkrokenberger]

The transistor side of the opto on that amp should be around 22V or so.

 

[jkrokenberger]

Yellow LED lit and low rail looks like it's triggering the current roll-back from a shorted output. This will keep the mute latch engaged.

 

[miniman82]

NO YELLOW LED, ITS NOT ON

How is the LED side of the opto driven?

 

[Perry Babin]

Did you confirm (without the muting transistors) that the amp was otherwise in good working order?

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.

 

[miniman82]

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.

 

[jkrokenberger]

I posted before you edited that the yellow LED was not lit.

 

[jkrokenberger]

I don't have a 2050AM here to help you with voltages and readings. I do have a 5075DX that has a 2050AM in it that I could get readings and voltages for you if you'd like.