Audio centron mixer delemma

[Bigdrums]

Hello all: I bought a AC-416 mixer back in 86 brand new. It has served me very well but now AUX 1 (EFX 1SEND) has no output. I have tried everything I can think of to repair this problem. AUX 2 works fine and is routed through the GROUP 2 board with the same circuit as in GROUP 1 board. I've changed most of the op-amps in that part of the circuit and socketed them. Replaced key capacitors in the circuit. I thought I had it fixed at one point, but alas, no go. So I'm hoping someone might take a look at the schematics and make some suggestions.
Thanks


AC416,424 Main Board Schematics (16-219-01).PDF

I have the rest of the schematics but don't believe they matter right now

[Enzo]

FX1 send comes through a ribbon from the jack board. The signal comes from IC4. So is that working? Touch pin 6 and then pin 2 with a small screwdriver or something to inject hum. Does that come out? Pins 1 and 7 should have no DC, is that the case?

[Bigdrums]

Injecting a signal on pins 6 & 2 does pass to the output (the jack board) but is attenuated considerably compared to FX2.
There is only trace amounts of DC on pins 1 & 7, .039 v and is the same on the FX2 board.

[Enzo]

NAh, I was worried about 10 or 15v. A few millivolts won't matter.

So if you can inject noise into the other AUX drive and hear it, but not here, then teh IC is suspect. First though, monitor the output pin of the IC and see if it is getting signal when you do this. We need to determine if the IC itself is not passing signal, or if the IC is working, but the signal is not making it to the jack board.

[Bigdrums]

The output of the IC does have signal, but like I said is so far down gain wise that it is unusable. I can see it on the scope and I can hear it on the output jack.
I wasn't sure if it was coming into IC4 attenuated, but I recall it being the same as FX2.
So I suppose then it should be IC4 that's bad. I'll see if I have another 5532 to swap.
It may be that I changed it, saw the gain return only to put it all back together and have nothing again.
I wonder if there is something that is taking out the IC? That would explain the problem(s)
+/- voltages are all good so what might destroy the IC?

[Enzo]

You have a good circuit nearby to compare. SO all I wanted to know was if the IC output was there or not. It seemed like before you were looking for output at the jacks. 5532 is specified, but if you lack one, most anay dual op amp will function there. 4558, TL072. They may not technically be "low noise", but at this point it would tell you right away if the IC was the signal level problem.

I wouldn't think anything was taking out the ICs.

[Bigdrums]

I replaced the 5532 and it seemed to be working abet at a slightly lower output than the FX2.
Slowly the output of FX1 declined to nothing over the course of a few minutes
Very careful during reassembly to not pinch wires etc.
This is similar to my last attempt to repair it a few weeks ago.
For the moment I'll just use it as is because I have a session this evening
To be continued
Thank Enzo, maybe we can get it next time.
Perhaps tomorrow

[g1]

I'd suggest swapping that now bad chip around with FX2 chip just to verify 100% it's the IC that's the problem, and not the associated circuit. If the 'bad' IC works in FX2 you can move on from the IC4 itself.
There doesn't seem to be anything there that could make it go bad, aside from a short on it's output.

[Bigdrums]

I socketed the op-amp for FX2 and put the FX1 5532 in it. FX2 worked so it's not the op=amp that's the problem
Put the 5532 from FX2 into socket of FX1 and it doesn't work, so it's something else

[g1]

So it's not killing the IC's, the circuit just fizzles out right?
And a couple times you replaced it and it worked for a bit. So I would be pretty suspicious of a bad connection somewhere in that circuit or the feed in or out of it. Cold solder, oxidized connector etc. Other than that, if there are any series electrolytic caps that the signal passes through, they can also be problematic.

[Bigdrums]

This is very frustrating: When I hook the bus cable and P/S and everything else up properly, everything seems to be working properly except AUX 1 output. I need to have the SUB and Main outs board turned upside down to take any measurements, so I have a ground jumper hooked up as well. I inject a signal into a channel and have MON 1 and 2, AUX 1 and 2 all at 50%. Measuring at the connecter to the AUX 1 board, the only real AC I see is on pin 9. Nothing measureable on pin 10, 7 or 8, or any other pin for that matter in AC. Yet pin 9 is the feed for AUX 1, the only function on the board that's not working. Ac signal can be seen on pin 9 at about .3 volts AC with the channel send at 100%. I'm stumped!

BTW I checked the bus cables and punched down all the wires on all the connectors

And previously I checked all the series caps finding 1 that was suspicious and replaced it already

[g1]

You're not seeing signal where it should be and seeing it where you shouldn't? That implies to me there may be some kind of ground issue with your test set-up (upside down with ground jumper).

[Bigdrums]

The problem is now fixed and board back on the mix table. It was indeed a bad connection (solder joint) Found it by comparing FX1 and FX2 boards with a meter.
Measuring each circuit till finding the high resistance section. removed the solder and re-soldered. Thanks!
Still don't understand why there was no signal on FX2 pin 10, MON1 pin 7 or MON2 pin 8 prior to doing the repair.