Peavey PV14AT bad channel

[VintageValves]

Hey all. I'm an out of work live sound engineer and am taking on some electronic repair projects in my down time. I'd say that I'm an advanced novice. Good with a multimeter, scope, smd rework etc.

I've got a Peavey mixer on the bench with one bad channel. The clip light is permanently illuminated, no signal or compression lights function and it does not pass audio. Attached Schematic....

The LED is being illuminated by 14v at the 3PK1 Diode (D101)

Suspecting a bad q46 or q47 tranistor, I removed them, but the clip light is still illuminated.

Voltages on U10 are suspect but I'm not sure if the opamp is at fault or if they are originating else where.
1.) -12.5
2.).33
3.)0
4.) -14
5.) 5
6.) 4.4
7.) 13
8.) 0

Voltages on an adjacent working Channel:
1.)0
2.)0
3.)0
4.) -14
5.)14
6.)7.6
7.)0
8.)14

Also attached datasheets for relevant devices. Any ideas on what to check next would be appreciated!

[The Dude]

Your op amp is missing supply on pin 8. Are you sure your meter probe made good contact? If that is the correct reading, you may have a burnt trace to that pin of the IC. You'll have to trace out why there is no supply on that pin. You also have supply on the output pins 1 and 7 indicating a shorted op amp. Those should be 0VDC like the good channel My guess is that the op amp shorted and burnt a trace or link to the supply pin.

[Enzo]

Whenever I see maxed VU, or permanent clip light, I immediately look at power supply. That almost always means you are missing a supply. Dude said that, you are missing +14v at the one chip.

[VintageValves]

Thanks for the replies. I double checked and I do have that +14V on pin8 upon rechecking. So, probably just a bad opamp as I suspected?

[The Dude]

Yes. If you have both supplies and DC on the op amp's outputs, the op amp is likely shorted. When you pull the chip, it's not a bad idea to see if the DC is removed on the pads that go to pins 1 & 7 before installing the new part. That will tell you if it's coming from the chip or elsewhere.

[VintageValves]

Thanks The Dude. I'll definitely report back. I hate finding threads like this with no conclusion!

[VintageValves]

Ok, new chips came in today.

Voltages as follows with the chip out of circuit:
1.)-0.5
2.)-0.5
3.)0
4.)-14
5.)7.3
6.)7.3
7.)-0.5
8.)14

The Clip/mute light behaves normally with the opamp.out of circuit.

So it seems as though the voltage was going through the dual opamp? Curiously, I'm not finding any continuity through the opamp to pins 1 and 7 with a meter out of circuit.

I bought a pair of the opamps (just in case). I installed one and the clip light problem returned. Maybe I torched it while installing?

[Enzo]

Where is that 7v coming from on pins 5 and 6? You can always compare to one of the other channels.

The op amp is a complex circuit inside, there will never be a measurable path through it.

One bad IC doesn't mean there is not another.

[The Dude]

Yes, more than one part may be at fault. You said you removed Q46 & Q47. Did you check them before reinstalling? The voltage on the output pin 7 of your op amp is different and closer to 0V after replacing, so you can be pretty sure it was bad.

[VintageValves]

I tested q46 and q47 (good) but replaced them for good measure since I was ordering parts.

[olddawg]

Order 2 or 3 opamps. They are cheap. Just in case the fault blows another. If the board has phantom power check the defaults. There seems to always be a way someone can screw up a channel with phantom power settings and the wrong mic... or driving the channel with something humongous like an extension speaker out.

[The Dude]

Did you check D96 & D97? What are the voltages on Q46 & Q47? How do they compare to a working channel?

[VintageValves]

I Soldered in the second new U10 opamp. This time more carefully and let it cool halfway through. Happy to report that the channel is now functional. Thanks for the sounding-board and advice!

[VintageValves]

Well, an interesting development. It's dead again. I noticed just now after putting the whole thing back together (A LOT OF FIDGETY screws and whatnot), that the power button is malfunctioning. When switched to the off position, ALL the clip indicator lights come on across the mixer. The button is not real crisp in it's latching. The power header comes into the board close to channel 3. I think that whatever the power button is glitching is finding it's way through my opamp. Screen grab of the power section. I'm not seeing much that would cause this behavior since the off position goes to NC's. Perhaps finding a ground some other way?

10 more opamps on order.

[The Dude]

It's not uncommon for lights to momentarily come on when powering down, especially if supplies don't decay simultaneously. I don't think that's related to the problem. I'm more apt to believe you might have a connection problem on that channel. I'd try chop sticking the area to see if you might be able to find something intermittent. I'm thinking that flexing the board or connectors to reassemble the mixer might have revealed some connection issues or bad solder joints. Also, re-seat any connections- ribbon cables, etc.