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

George Back said:
Hi Roy,

Here the whole process is shown. As far as I saw, the guy did not mention, that the new caps have to be put in in reverse direction.
Click to expand...

Please see posts #3 and #5 above by Brian Ingwell. I don't think any of us were aware of this issue until he mentioned it. I certainly wasn't. This is one of the problems with making schematics and service bulletins secret, as in the case of QSC and most other companies these days.

However, from an electronics perspective, the polarity situation makes complete sense... These are effectively blocking caps in the circuit design. If you are expecting to block a large positive DC voltage (eg phantom power) with a polarized capacitor of any type (in this case electrolytic), then the PLUS terminal of the capacitor MUST go to the point / connector / pin where you expect to receive this + voltage. Polarized capacitors are easily destroyed by applying reverse voltages. Under certain conditions, the explosion can be quite spectacular. In other cases, the top will bulge, and it merely stops acting like a capacitor.

If this doesn't make too much sense to you / you don't have an electronics background, then at least show this thread to your tech, I'm sure he will agree that this makes sense.

I reviewed the video, it is correct, other than the tech continued with the new caps installed backwards. At this point, I believe installing the caps backwards will only come back to bite you if the mixer gets connected to a FoH board with phantom power enabled. I do not imagine any other situation where a significant positive DC voltage would be applied to the AUX outputs... but there are others on this forum more versed in modern sound reinforcement practices, thus I could be wrong.

The Touchmix +48 only goes the the input jacks on a case by case basis if enabled in the software. It is never routed to the output jacks. Most likely just a coincidence that your auxes failed in a time period when you used +48v. My TM8 was stored (climate controlled office) for the entire pandemic, it went bad just sitting around doing nothing. Electronics can be funny that way.
 
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Hi all,

In the meantime I brought my TM 16 to my technician of trust. I also gave him all information I got here and in the video. So he changed all caps (10). He also put the new ones in reverse direction as recommended here. He could confirm that the caps were mounted in the wrong direction before. The old caps obviously were all ok. Anyway he changed then the caps.

Unfortunately the problem is NOT FIXED! AUX 1, Aux2, and 5 and 6 are still not working. I have low volume on all channels and a musty sound. Aux 3 and 4 still working well and as expected. Also the main Outs.

I am frustrated and disappointed ☹!

Does anybody have any idea what else to do and how to fix?

Now I have to go to an official service center with the hope they can fix it. I so wanted to avoid that, because my local shop will now send this to the service center and I suppose I have to wait for weeks.

Best regards
George

PS. @Roy I don't have any elctronic backround at all! So waht does this mean what you said do the new caps have to be installed reverse or not? I was wondering that the old caps were totally ok. So my problem is obiosly somewhere elso to fix?!
 
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Hi Paul,
thanks for your reply.
I did not a factory reset. Is this required after you changed components?
I have to ask my tech if he did additional checks....
 
Whenever I have odd issues or off sounding audio, the first thing I do is a reset because there may have been a glitch from static shock, or someone messing with the setup, and sometimes it can be a setting that isn't obvious to find. When I know everything is back to factory and it's still doing it, then I can confidently start diagnosing knowing I eliminated that possibility, which has happened on occasion.
*** I always do a copy of the profile/setup first in case someone spent time setting up a show and didn't save it, if resetting didn't work, then I put it back on after fixing if I know it's needed.
 
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100% agree on doing a factory reset & reloading the default profile. It's easy to get settings messed up that might affect output levels. If that's not it though, I do have one other idea for you...

The TM16 I worked on was badly brutalized by a caveman (I can't even call whoever this person is a "tech"), and after I replaced the output capacitors, I was still getting low volume on the right main output. Since I had needed to install a number of jumper wires to bypass ruined plate through holes, I was worried that there was yet another plate through hole that was disconnected, and that I might never get the board running again.

However, under close inspection using a large magnifying loop, I was able to see that the caveman had apparently been using a rock to remove the capacitors, and while doing so had also unintentionally bashed R15_26 right off the board (only half the resistor remained). This is a 1mm x 2mm 10K ohm SMD resistor, and it is somehow part of the output circuit. There is a similar resistor and set of transistors on each Main & Aux. I believe that the resistor biases the transistor, and that these transistors are either an electronic on/off switch for each channel, or the final drive stage for each output. (I could be way off base on that, but they definitely are related to the output signal path). I think it would be worth checking these resistors and the associated SMD transistors closely, since they could conceivably go bad if the backwards mounted caps are not protecting them properly. It's a bit of a long shot, but without schematics, we don't have much to go on.

I apologize if the picture of this brutalized board offends anyone's tech sensibilities, but it is what it is. The mixer is working perfectly & being used in concerts regularly now!!


Touchmix-R15_26.jpg
 
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Thanks Roy,
I am not a caveman :) my TM 16 was always mounted in a rack and used very sensitive, Also my tech is not a caveman :) But of course this can be an issue, He also mentioned that without schematics finding a solution is challenging... Unfortunately the Mixer is put together again. And the question is what is the best solution now.
Obviously to bring it to the official service centre?! The will have the schenatics (hopefully)
 
Yes, I guess you are at the point where going to QSC official service is the only option. I don't know if they do component level repairs, or if they only do board swaps. Since QSC seems to be out of stock of just about everything electronic (definitely the TM replacement main boards, I asked because I nearly gave up on this one, and many of the powered speaker boards), you might want to call & ask them about exactly how they repair Touchmix devices in your country. If they do component level repairs, then you should be fine. If they only do board swaps (like Apple), then you could be in for a long wait (they are suffering from severe supply chain issues like many other companies that build electronic devices).
 
To be honest! I am totally confused!

I just had a call with my tech. Unfortunately, we had a communication problem. He thought the caps are related to the inputs. In this case the reverse installation would make sense.

In case we are talking about output caps, he completely confirms that what Roy said in post #21. In his opinion the original caps were mounted correctly. Beside that all original caps were still ok. So my problem is somewhere else to solve.

He now highly recommends reinstalling the caps again an install them in the original direction which from his opinion was the right direction. Otherwise, the risk is to high that the caps will definitely blow!. Also the guy in the video did not change direction....

Beside that I contacted QSC in Germany, what I did already in the last year. I never got a response. Let’s see now.

Greetings George
 
Roy Andrews said:
Yes, I guess you are at the point where going to QSC official service is the only option. I don't know if they do component level repairs, or if they only do board swaps. Since QSC seems to be out of stock of just about everything electronic (definitely the TM replacement main boards, I asked because I nearly gave up on this one, and many of the powered speaker boards), you might want to call & ask them about exactly how they repair Touchmix devices in your country. If they do component level repairs, then you should be fine. If they only do board swaps (like Apple), then you could be in for a long wait (they are suffering from severe supply chain issues like many other companies that build electronic devices).
Click to expand...
As I understand it from talking to different manufacturers over the last few years, component level repair is a thing of the past. It's possible higher end and specialty units may still do it though.
 
George Back said:
To be honest! I am totally confused!

I just had a call with my tech. Unfortunately, we had a communication problem. He thought the caps are related to the inputs. In this case the reverse installation would make sense.

In case we are talking about output caps, he completely confirms that what Roy said in post #21. In his opinion the original caps were mounted correctly. Beside that all original caps were still ok. So my problem is somewhere else to solve.

He now highly recommends reinstalling the caps again an install them in the original direction which from his opinion was the right direction. Otherwise, the risk is to high that the caps will definitely blow!. Also the guy in the video did not change direction....

Beside that I contacted QSC in Germany, what I did already in the last year. I never got a response. Let’s see now.

Greetings George
Click to expand...
Possibly, but did he confirm the negative wasn't going to ground with continuity measurements before replacing the caps "backwards" before doing it ? Because as Brian mentioned at the beginning of this forum thread, it's just older ones that MIGHT need them "reversed".
I believe he also posted pictures of the ones in question, not all caps are the issue. They show the actual bulging caps, it's a TM8, but still same area.

I compared one of our older ones with one of our newer ones, and he is correct. And an authorized QSC service center told him this.

Did you end up doing that factory reset ?
 
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And the end this device was not worth the money! Never QSC again. Also the service here in Germany is very bad. I already tried to connect the service here in Germany last year. Never get any response! Now again telephone, email nothing. I also spent now 70€ for nothing.

The good thing my tech now found out the real reason ( and I spent the afternoon with my daughter in the mall 😊)

There is no operating voltage anymore on Aux 1&2 and on Aux 5&6! This ends up in a low output level. On 3&4 it is and here is everything fine.

So nothing to do with any capacitors! But now there is really an end. Without any schematics the problem cannot be solved! At the end the change of the board costs me 500 bucks. I will not invest this. Let’s see if there is any other way and maybe there is still an happy end!
 
Lets take a moment to recap (ha, pun intended) the situation:

Where did the polarity reverse info come from: In posts #3 and #5 above by Brian Ingwell, he confirms that there is an official QSC service bulletin regarding the possibility that the output caps may be installed in reverse polarity in some mixers. This info would not be widely available since QSC keeps both their schematics and their service bulletins secret. (side note, this is why I do not believe companies should be allowed to make service bulletins secret)

What do these caps do?: Even without a schematic, it is obvious that these capacitors are DC blocking capacitors. This is an AUDIO circuit, and as such, we do NOT want to have DC voltages flowing in or out of this device, because such a situation could either A) damage connected equipment, or B) damage the Touchmix itself via faulted or incorrectly configured connected equipment.

Where could these DC voltages come from?: They could either come from inside the Touchmix, or outside the Touchmix from connected equipment. I have double checked the Touchmix output circuitry with a precision multimeter. The Touchmix itself does not generate more than 11 MILIVOLTS of DC bias on the Main & Aux outputs. This is 0.011 of a volt. While not completely insignificant, it is negligible, and certainly does not justify the use of an electrolytic cap rated at 63v. Given that the installed capacitors are rated at 63 VOLTS on a circuit that has a mere 0.01 volts of DC on it, the only logical reason to use capacitors rated at such a high voltage would be to protect against +48v phantom power.

How / Why would +48v Phantom power get applied to a Touchmix OUTPUT?: A "small" mixer like the TM8 or TM16 may be used by drummers, keyboardists, or anyone else with a bunch of instruments that they want to do a personal mix on, before sending that personal mix to Front of House (FoH) using the Touchmix's Mains and / or Auxes (which are then connected to a FoH mixer INPUT). When a small mixer is used as a "sub-mixer", there is ALWAYS a danger that the FoH mixer has phantom power turned on. On many large FoH mixers, phantom power is an all or nothing proposition. All channels on or all channels off. Other FoH mixers control phantom power in blocks of 4 channels (or more). It is trivially EASY for +48v to be accidentally applied to the Touchmix in a situation like this. Phantom power is applied to microphones (some direct boxes need it too) via pin 2 of the XLR jack / plug. In order to properly protect a Touchmix (or any other sub-mixer) from an accidental zap of +48v, we would need the POSITIVE side of a capacitor rated at least 10% higher (preferably much more) than 48v connected to pin 2 of the XLR connectors on all the OUTPUTS of the sub-mixer.

Edit: (George - oops, I see you posted again while I was posting this - good luck in your quests)

I hope this makes sense, and also helps any future readers of this thread. I really don't know what else to say regarding this subject. I've done my best to explain in simple terms why I think the QSC engineers built the product this way, and why an assembly error where these caps are installed backwards is a grave fault that should be publicly acknowledged.

My final thought on this subject is: If you (the generic you, to anyone reading this in the future) ONLY use your Touchmix as a MAIN mixer in situations that you control closely, and you have complete confidence in all the equipment that you attach to the OUTPUTS of your Touchmix, then you can install these capacitors in any orientation you want. The DC bias of the Touchmix output circuit is approximately 0.01 volts, and it is generally accepted in the electronics industry that electrolytic capacitors can survive for extended periods of time, possibly their entire lifespan, with up to 0.5 volts of reverse voltage applied. So install them how you want... just be absolutely sure that +48v phantom never makes it to your Touchmix if you decide to go with the capacitor negative to XLR pin 2, which is how some boards have been incorrectly assembled by QSC.
 
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Hi Roy,
thanks for your reply. We have a samll 3 people band. I never used the TM 16 as a submixer in the three years I have the mixer. Always used as main mixer! Phantom power is only used in two of the inputs for drum overheads. The aux outs go to an active monitor box and to two in-ear devices, The main out go to two active PA subwoofesr and from there to to a Head PA box each.
So again in my case the capicitors were fine and not the problem. As my tech explained to me. No more operating voltage anymore on Aux 1&2 and on Aux 5&6! On 3&4 yes, and here is everything fine.
 
Regarding the reset, I pressed the button multiple times, but nothing happened?! Only the message windows closed. I also loaded the default preset. Problem still occurs. All the scenes are stiil available to load. They will not be deleted within a reset?
 
The reset button seems to reset technical settings of the mixer. For example, in my case if I set the mixer to 48KHz sample rate, reset puts in back to the factory default of 44. The manual describes it like this: Reset Mixer – Resets all mixer controls to factory default. This does not erase any user presets on the mixer or on a USB drive.

There is another button called "Erase internal storage" that will get rid of all your scenes stored inside the mixer, but not on any USB drive in the rear slot. Finally there is a "Format USB" button that would get rid of all presets stored on external USB, as well as any recorded sessions.

I also noticed that they talk about an output preset called "Reset", which is exclusively for resetting the output parameters. Found by tapping the "Main" button above the main output slider and going to the presets tab & recalling it.

George, I completely understand your frustration regarding this situation. However, we are not in "normal times" at the moment. Touchmix is easy to use, compared to say, Behringer's Byzantine XAir Edit, and I can say with confidence that the original K series & KW series amplifier modules are some of the best designed & constructed powered speaker amp modules ever made (I live in the 3rd world, have not seen the K.2 series yet & have no opinion on them). I will not fanboy for any product or company under any circumstances, but I will call out superior or inferior electronics engineering when I see it. I do not see any obvious inferior electronics engineering in the Touchmix 8 or 16, other than they made an assembly mistake on some mixers, and they absolutely should have done a recall on this. In your case, it seems to be a genuine, legit electronics failure of some other components.
 
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Hi Roy,

thanks for your reply and your support! At the end Roy too many issues. It’s a combination of the QSC buggy product and my local store where too many demotivated people worked.. It started all with the power supply issue. Within a gig the mixer closed down. God bless it restarted after some time. Too much communication then with my local supplier. I had to bring them the whole mixer. It took years to explain them that the problem is the power supply. They did not really care about my case. So I did the research and I opened a customer case at QSC. Then I got a new power supply, soon. So far so good. Then in the last year, directly after the warranty the problem with the low out put on Aux 1and2. This happened suddenly while starting a rehearsal from one week to another. Same situation with the local vendor. If I had bought the unit at Thomann or a larger store via Internet I had one year more of warranty here in Germany and much better service and real skilled people. And now unfortunately the same with Aux 5&6. I was sure that I find the solution here and in the video. Unfortunately not! So already an new problem!

So what happens next? I have no trust more in this product.

Compared to the price too many issues, bad service (in Germany) no more software updates…

Maybe I try to contact the international service again. If I remember right, the answer in the last year was “contact your local service” ….

Greetings

George
 
Totally understand, George. Once you have a bad experience with a product, not matter what it is or how good it theoretically might be, you never want to go back to it.

Just as a FYI, I use Behringer XAir mixers frequently, specifically the XR16, XR18 & X32 Rack. There is a steep learning curve with the XAir Edit software compared to running Touchmix on a cheap Android tablet, but the mixers are solid. The XR16 can record the stereo mains to a USB stick, while the XR18 can not, but the XR18 can record all channels to a DAW via it's built in USB interface. The X32 can do it either way, and if you want you can add a $250 card to the X32 to allow all channels to be recorded to a SD card. The XR16 is around half the price of a Touchmix16, the XR18 is usually only a little more expensive than the XR16, and the X32 Rack is about the same price as a Touchmix 16. In all cases, the built in WiFi must be augmented by a real domestic WiFi router (even a basic one will do) connected to the Ethernet port, in order not to lose connection to your mixing & management tablet / computer when used in a bar or commercial location. I wish you all the best in finding a solution that works for you for many years to come!!
 
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Roy,
3-4 years I exactly made my choice between the Behringer and the qsc. Now I will come back to the x18 or better the Midas mr18 with slightly better components.

In the meantime a made did some research, QSC seems to be dead in Germany/EMEA. As I said I don’t get any feedback from the office in Germany. Also two shops said that the situation with QSC is tricky. Thomann the biggest store in Europe has no QSC units anymore to sell.
Looking back I made the wrong decision. Regarding the features (included screen, form factor, channel presets, feedback wizard) I still would prefer the TM16. I am a musician not a mixing guy :-). At the end spending a lot more money not always guarantees better quality and services.

Greeting

George
 
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