Peavey PV14AT bad channel

[Enzo]

OK, I have a problem, and it could be me, hey I'm old...

Both rails work, but you have 0.2v on the "gnd" pin of the IC and 0 on the bottom of C131. And that is directly wired to the -15 output. So we either have -15 or we don't. The -15 output and those two tested points ought to read the same.

Since we are working on the power supply, I urge you to ground your meter to the supply. All those zero ohm guys for example. If that is not convenient, sometimes I tack solder a small piece fo wire to the point I want and clip my meter to that. And from there, measure resistance with power off to the chassis connection you used before. Something is wrong here or I am missing something.

The IC wants to make 15v. But we want -15v from it. So they connect what would have been its output pin 8 to ground, and so what would have been its ground becomes -15.

I just want to make sure the mixer has proper power to the circuits before we move on to the mixer.

[VintageValves]

You're not old, you're experienced! I truely appreciate you sticking with me.

At your request I've switched to referencing off the ground terminal at the power inlet. For what it's worth, that point meters 0.002ohm to the point where I took the readings originally.

I am comparing the voltages of the mixers in the OFF state.

On the working mixer there is 0.17v on pin 6. (When turned on there is -14)

The malfunctioning 'stuck on' mixer is showing -14 on pin 6 (gnd) of U67 with the power switch completely removed. If I simulate a switch closure with a piece of wire the -14 supply disappears on pin 6 and all the clip lights come on.

[Enzo]

And in the working mixer, the On state is with the open switch?

[The Dude]

^^^^ Yes, that. When the switch is closed, what are you reading on the +15 supply? When the -15 supply goes to 0V, so should the +15 supply.

[VintageValves]

Oh my! Upon inspecting the working mixer, the contacts are OPEN when it is in operation. Contacts closed=off. So I've been chasing my tail a bit here, my apologies.

The problem with the mixer is that when switched 'off' (contacts closed) only the -15 rail turns off, while +15 remains on. Any thoughts on that? I've suggested to my friend to not worry about it and leave it with no power switch, but I've not heard back yet.

[VintageValves]

I can take measurements with it in the 'off' position tomorrow if that would be helpful.

[The Dude]

I'd first check for a shorted Q58. If that's not shorted, does the base of Q58 voltage change when the power switch is closed?

[VintageValves]

Thanks Dude - I will revisit this in the morning and report back. This is making a lot more sense now.

[Enzo]

Q4 turns on and off Q58, so check it too.

[VintageValves]

Thank you Dude and Enzo. Q58 is shorted all directions out of circuit. In circuit Q4 appears to be good. I'll order up a new Q58 (or 3).

Do you guys think that the -14 supply turning off while, leaving the +14 on could have anything to due with the dying opamp in that one mic pre? I would think that it would burn out other opamps as well since there doesn't seem to be much unique about channel #3? The new opamps for that arrived today.

[Enzo]

I think channel three is just a failure, not part of some larger systematic thing. Channel inputs are by their nature exposed to the real world.

[The Dude]

I agree. I don't think the power supply issue would take out an op amp on 1 particular channel. If anything, it would be the other way around- a shorted op amp might draw excessive current and take out the power supply. I'm not saying for certain that's what happened. In more expensive mixers, there is a low value resistor from the supplies to each op amp supply pin so that if the op amp shorts, the resistor burns saving the PS. In this mixer, the supply pins of the op amps tie directly to the supply.

[VintageValves]

Final update!!! New Opamp in channel 3, New transistor in the power supply, new power switch and it's back together and working 100%. Thank you dude and Enzo so much for the guidance! I'll buy you guys a beer and invite you to a show next time I'm in Sioux Falls and Lancing. Not even kidding!

[Enzo]

Great, I happen to live right next door to a brewery.

https://www.badbrewing.com/