QSC TouchMix 16 sound problem fixed (other touchmix versions may also get this)

Rick Smith

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May 28, 2022
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Is there absolute certainty that the capacitors are reversed? Is it possible that the negative side of the caps connect to pin 2 of the XLR because the 48v originates from an external source and not the TM16?

Has anyone obtained the official Service Bulletin from QSC on the capacitor issue? It would be nice to see it. Also has anyone found a service manual for the TM16 or TM30?
 
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Paul Lewendon

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Oct 8, 2019
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Is there absolute certainty that the capacitors are reversed? Is it possible that the negative side of the caps connect to pin 2 of the XLR because the 48v originates from an external source and not the TM16?

Has anyone obtained the official Service Bulletin from QSC on the capacitor issue? It would be nice to see it. Also has anyone found a service manual for the TM16 or TM30?
3rd post down from the beginning explains it. And it's not every one of them, just certain versions before they caught it, so make sure before just changing.
 

Rick Smith

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Thanks for your reply. I did read and understand the entire post. But my concern is that this post seems to be the only source of information on this issue that I can find. And to be clear, my intent isn't to challenge anyone's character, but rather to authenticate the information before I modify the several mixers that I own. Two of them had swelled caps. So as prevalent as the issue seems to be, you'd think there'd be more out there besides this post and videos instructing how to replace the caps (in the same orientation). I just put a call in to QSC in hopes to get confirmation directly from them. The rep I spoke to was unaware of the cap issue on any boards manufactured, but assured me that he'd research it and get back to me with an answer.
 

Paul Lewendon

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Oct 8, 2019
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Yes I know what you mean, it's understandable, it seems like a big error, and head scratcher how it left the design, factory, etc. without being caught.
Any repairs they'd just swap the board, and problem solved.
The thing is that with most things these days, just like automakers with recalls, etc.. Admitting a problem opens you up to liability.
As long as there's no phantom power hooked up to the outputs instead of the input, the problem won't show up.
After he mentioned that, after I had already fixed ours, I took that one and another couple apart, I compared them and they look almost identical, except the markings, when I measure the continuity, it confirms what was said.
Putting reverse voltage to a capacitor will make a cap go bad.
If you have full control over your equipment and prevent power to be hooked up there, you'll be ok.
 

Paul Lewendon

Member
Oct 8, 2019
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British Columbia Canada
To add a bit more to this story - we have a TM8 that was exhibiting the same problem. Low and uneven frequency response on Aux 3. I found this thread and took ours apart to find bulging blocking caps. Here's where things get interesting: I spoke to a friend of mine who does authorized service for QSC. Apparently in early production units of the TM's, the circuit boards were labelled backwards for the blocking caps, so although they appear to be installed correctly, they are backwards, meaning that if you send any of those outputs into a console with phantom power on, there's no blocking going on and the caps will fry.
So if you go to the effort of replacing the caps, check to make sure they are in correctly (positive side of the cap should make continuity with pin 2 of the output XLR - ours are backwards currently).

Hope that helps anyone else with this problem.

Cheers,
Brian
New problem caused by these bad caps, it sounds like a train going down the tracks just in hearing distance, fine on Aux out.
This is another one of our touchmix-16 units with bad caps and I did a continuity check, and sure enough the negative was at pin 2, but I was expecting the + of the cap to be at pin 1(ground), so for blocking it's a in series cap the signal just passes through, not attached to ground(parallel). It fixed the train noise problem.
 
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