Presonus StudioLive AR12&AR16

[shortcircuit]

Hello friends at MEF it's been awhile
I have a friend with these two mixers, both have the same issue, they lost channel 2, you can crank it up and hear a faint distorted signal. No such thing as a schematic that I've found. I haven't accepted either mixer yet so I don't have pics either. I'm just wondering if anybody has any suggestions from past experience with these, or should I pass on them all together. Thanks all!!!

[The Dude]

I haven't been inside one of those that I recall. There are usual suspects- burnt resistors off the input jack, shorted op amps, shorted transistors, etc. It probably wouldn't be too difficult to figure out without a manual. Mixer inputs are generally similar. Plus, you have working channels to work from as far as measuring voltages and identifying any burnt parts. I imagine it's like most common build mixers these days in that there is probably one main board for the whole thing, which means lots of disassembly and reassembly. It will likely be time consuming and expensive whether there is one channel blown or 5.

[shortcircuit]

Thanks Dude, I'll get at least one of them in and post more info from there.

[shortcircuit]

I'm finally back with this, what I've got is a Presonus AR16 USB , the XLR jacks 1,2,3 no signal, 4 works fine. The 1/4 inch jacks all function as normal. Presonus is no help, no schematics, no parts , no tech advice, for a 2016 !! mixer. Here are some pics of the module I find no popped caps, nothing burnt, no bad connections. I suspect the caps, any suggestions before I start pulling caps to test?? Always appreciate the knowledge base here!!!!!

[The Dude]

I realize you don't have a schematic. But, if you have a scope, I'd insert a test tone and follow circuit board traces. See where you lose signal.

[xtian]

And you can measure for +ve and -ve voltages at each of the op amps.

[g1]

I'd start with the ch 3 and 4 part of the board, it's nicely sectioned with both channels having same layout. It doesn't have the extra DI stuff for the instrument jacks.
Looks like there are a bunch of transistors (likely mic pre for xlr's) and what is probably a dual op amp, one half for ch.3 and one half for ch.4.
You should be able to follow the signal through the working ch.4, and see where it is getting lost for ch.3.
Once you get ch.3 solved, I'd wager the same issue will be present for ch.1 & 2, as someone was probably connecting something bad, and quit after they blew 3 channels.

[shortcircuit]

Thanks for the input, dealing with just ch3 & 4 , diode test mode, IC's seem fine match every other. Signal trace (no scope) , using a mic and a diy tracer , red dots on pic show signal , cap test set at 200u , ch4 cap 60u(circled in red) , ch3 0u (circled in yellow) same with ch1&2(are these the culprits?). I'm to old for this micro b.s. does this make any sense to you? schematic would be golden

[The Dude]

I can't tell for sure from your pic, but those look like bypass caps and one side goes to ground or pin 1 of the XLR (check with meter to see). I doubt those would cause your problem. I'd be looking for open resistors, or shorted preamp transistors.

[g1]

I'd be looking for open resistors, or shorted preamp transistors.

It looks like those caps maybe go from pin1 to pin3? Then a shorted transistor from pin3 to ground might make the cap test read 0u there (depending on cap tester)?

[The Dude]

Yes. A resistance measurement from pins to pins on the XLR might be more informative. My general experience with microphone input circuits is that there are often low value series resistors that burn up particularly with hot DI inputs. It's common in bass guitar channels where signal is taken off of a DI on a bass amp. If not that then shorted preamp transistors or sometimes op amps. This is partial speculation without a schematic, but in the pictures in post #4, I see what looks to be preamp transistors ahead of the op amp. My guess is that those are for the mic input and the line input goes directly to the op amp. Since the line input works, the op amp is likely good and we are looking for a problem with a gain stage prior to the op amp. There really isn't much there in front of the op amp, Probably just a simple differential transistor amp circuit, so it shouldn't be too difficult to trace even without a schematic. A scope would make this much easier, but just checking transistors for shorts and looking for open resistors would probably get you the answer.

[shortcircuit]

Thanks for the direction, I think The Dude has the cause, getting more info from the owner, they were running a mic into #1, a DI from a guitar amp Marshall silver jube to #2, and kick drum mic on #16 using phantom power, phantom power is global on this unit I believe.
I'll keep at it, and let you know, thanks again

[shortcircuit]

res across xlr pins 1-2, 2.2M and climbing, 1-3, 2.2M and climbing, 2-3, 13.5k. Going through resistors, found R23C, R23D open, both 10ohm

[shortcircuit]

Follow up/ final thoughts
replacing resistors solved the problem, g1 right on the money ch.1 & 2 same issue all resolved. what I've learned with this one

1. The talent/experience and knowledge of MEF is phenomenal !!!! We knew that
2. I'm horrible with smd, strip of 20 resistors, I replaced 6, I have 6 left , ruined 2 installing, the others are in the twilight zone somewhere.
3. you wish they were 1206's , but they're 603's,
Once again thanks to all!!!! Have a great weekend, especially for you fathers out there.