Soundcraft 600 output issues diagnosis help

[The Scum]

Well, the usual advice is to start upstream, and work your way downstream. What's the last point in the circuit where everything is fully functional? That's why I asked about how the master insert points were behaving. Are the inserts configured pre or post fader?

Then. what's the next place you can inspect the signal, and what goes wrong in between? Identify and address the issues.

Then repeat. Work methodically, find the next broken stage, and address it. Until you've solved the problems with the mix out, don't worry about the later stages.

Doublecheck all of the physical stuff is correctly assembled. This won't be the last desk that's not working right because a jumper fell off somewhere, a normalled jack is too dirty to make connection, or a multipin connector has been inserted incorrectly.

Being that it's a stereo device, and one side is more functional than the other, you can compare between them.

Again, I'll strongly advise that you get an oscilloscope...preferably with 2 channels - it makes those channel-vs-channel comparisons very easy, and gives you the means to identify the stage where things go wrong. You'll need to make up some externder cables, too, so the module can be connected to the rest of the desk, but you can get to the PCB to probe signals.

You say that the mix out is out of phase, but the control room outs are functional? I'm not sure how that's possible - the CR outs are fed by the mix outs - if one was wrong, they both should be.

I will warn you about that 2 opamp output stage. It uses crossed feedback...and double-inverted feedback becomes positive feedback. They're prone to misbehave, in sometimes puzzling ways. Religilously ground the - output if you're running them unbalanced. The test I'd present would be to ground the - output, observe that the + output doubles in amplitude. Then ground the +, observe doubling at - output, 180 deg off from the input signal.

Doug Self has written about that stage before...see figure 6 here:
http://www.douglas-self.com/ampins/bala ... nced.htm#4

[sam humans]

Problem solved!! That said, the only explanation is a rather odd one.

After taking great pains to return the Master Module to stock, I began working backwards from the Mix Out, tracing the signal path from component to component with nothing but the continuity tester in my multi-meter. Far as I could tell everything was fine, so I looked at every connection a little closer... and blamo!

Turns out the Mix Out Left signal had it's polarity reversed. The positive and negative outputs for the Left Mix OUt connect to the negative sides of C21 and C22 respectively. At first, seeing continuity between the points I thought nothing of it, until I realized they were the wrong points. C21 was connected to the negative side, C22 to the positive side. Reversed from what the schematic tells me. What's so damn strange about this is that it appears to be wired that way on the PCB. As in etched that way by Soundcraft. Could this be the case? Did they make a mistake with my particular Master Module? It's an earlier one, Issue 2, if that makes any difference.

Regardless, I ended-up cutting 2 traces and wiring them correctly myself, and viola, I am now in-phase. The CR outs sound great, and the Studio Outs are working 100% as well. Having rebuilt the AFL/PFL circuit, it now also works as it should, which will be handy for sure.

So that's the story. Don't quite get it, but I did fix it, and am happy to know my mixer all the better for it.

[The Scum]

Any chance it was a pin 3 hot issue?

Can you remind me how it's wired to the output connectors? There's a ribbon cable that goes to a PCB behind the connector panel, right? Is that PCB intact?

Tell ya what, if you help me clean my garage, so we can get into the crate, we'll be able to check it against my master module...

[Jim Williams]

I have an issue 3 and an issue 5 on the bench now. I checked the outputs and wiring and both are correct. IC1B inverts phase and then the output 5532 opamp driver 1C5A inverts it back into phase. There is no phase inversion on either module here nor any of the dozens I've reworked here in the past.

Maybe you need to trace that ribbon to the rear connector pcb?

On another note, the positive feedback caps C17 and C19 should be removed. That creates an instability in the design.

[sam humans]

Ugg. I'm about ready to shoot this thing! Seems I jumped the gun a bit, because while the Mix Outs are now in phase, the CR and Studio outs are not. I know this doesn't make much sense.

Today I traced the connections from the Master PCB through the ribbon cable to the various I/O on the rear connector. Everything matches up. Pin 3 is negative, as is the ring on both the CR and Studio outs. Pin 2 is hot, as is the tip in both the CR and Studio outs.

Not sure where to go from here... probably time to call in some more experienced brains than mine.

Jim: expect an email.
Scum: I'll happily help you clean your garage, hell, I'll even clean your bathroom, that's how frustrated I am!

[The Scum]

the CR and Studio outs are not. I know this doesn't make much sense.

Actually, makes perfect sense. The CR & studio outs are fed from the mix outs. You traded the signals such that the mix outs seem right, but now the other two are flopped.

I'd double/triplecheck the path from C21/C22 to the mix output connectors.

And maybe compare the mix output versus the CR output. They should have the same polarity, as the CR is fed from the Mix. If the CR is correct, then at least internally, the mix is too.

[Drone]

Hi Sam, not to hijack your thread, but how much was the recap, I have a 500 in need of a good servicing?

[sam humans]

Hi Sam, not to hijack your thread, but how much was the recap, I have a 500 in need of a good servicing?

Short answer: don't know.

Long answer: All I had done by the local shop was the Master section and the PSU. In retrospect, now that I've done a bunch myself, I would still pay someone to do the PSU as ordering the correct size caps (height in particular) can be tricky, but should have sent the master to Jim Williams from the get go. Considering what he charges and the upgrades that come with it, it's really a no-brainer.

I did all the channel's, groups, and aux master myself, with a normal Weller soldering iron and a bulb-style solder-sucker. All the caps, high quality Panasonics, only ran me in the $60 range. Other than that it took a few hours a day for a week or so, and done. No issues, just repetitive labor. HUGE difference. HUGE. The group modules, however, were a total pain in the ass at times. They, like both masters, have double-sided PCBs which make them harder to work on with the tools at my disposal. These might warrant the added expense of a Hakko desoldering tool and/or paying someone else to do them.

Hope that helps!

[Drone]

Ta, I don't really use the busses on mine much, I only have 8 ins and outs, so I though bussing down to the groups would be the way to go, but in the end it was easier to just take direct outs from 8 channels, and feed 8 lines back in, and take from the master buss during mixing.

Did you have any problems with the ribbon cables running between the modules? Just removing and reseating those conectors made a lot of my noise issues go away.