Noisy preamps on Touchmix

[Kelstertx]

Update to my previous post. The same circuitry must be used for both TRS and XLR inputs on the combo jacks, because plugging line devices into the TRS jack didn't bypass the problem. I still have hiss on those channels even at lower settings (around 10:00 instead of 2:00). It's quieter hiss, but still loud enough that it spoils the sound too much to use those channels. With the motherboards costing $295 and labor to put them in (locally) is $100 per hour, I believe it's almost "totaled", to use car-speak.

Recommendation to others who might buy these Touchmix units used: buy from someone that offers returns, and check all your channels within the warranty period. Don't plug or unplug things without turning off the phantom first!

 

[s0c9]

Update to my previous post. The same circuitry must be used for both TRS and XLR inputs on the combo jacks, because plugging line devices into the TRS jack didn't bypass the problem. I still have hiss on those channels even at lower settings (around 10:00 instead of 2:00). It's quieter hiss, but still loud enough that it spoils the sound too much to use those channels. With the motherboards costing $295 and labor to put them in (locally) is $100 per hour, I believe it's almost "totaled", to use car-speak.

Recommendation to others who might buy these Touchmix units used: buy from someone that offers returns, and check all your channels within the warranty period. Don't plug or unplug things without turning off the phantom first!

This is probably the same on most consoles that support XLR/TRS inputs.
But.. going back to your OP on the topic (excerpts I quoted below) and thinking about the root cause..

The first happened when I plugged in a Rode VXLR adapter for a Wireless Go mic while power was on and phantom was on. I saw the meter peg and immediately noticed it was noisy after that.
The other two went south sometime after I plugged in VXLR adapters on those channels.
.
.
but no preamps got noisy until I started using Wireless Go mics.

You may have overloaded the inputs and caused some damage. It's ALWAYS a good idea to mute a channel before doing anything.. especially with phantom power on!
Noisy preamps with wireless mics may NOT be the preamps, but the output gain settings on your wireless mic receivers creating a noise floor.

 

[Kelstertx]

This is probably the same on most consoles that support XLR/TRS inputs.
But.. going back to your OP on the topic (excerpts I quoted below) and thinking about the root cause..
You may have overloaded the inputs and caused some damage. It's ALWAYS a good idea to mute a channel before doing anything.. especially with phantom power on!
Noisy preamps with wireless mics may NOT be the preamps, but the output gain settings on your wireless mic receivers creating a noise floor.

No, it's definitely damage. I tried the inputs with line inputs and regular mics and there's noticeable hiss. I need to go back and try a line input without phantom, in case it's the phantom circuit and not the preamp, but those channels are essentially off limits for me now.

 

[4Mal]

I never, ever hot plug inputs with phantom power on... bad stuff can happen. so end of the night, phantom power is turned off before I out the mixer to sleep. In my band we use 5 channels with phantom. 3 Audix VX-5 vocal mics 2 active DI's ...

 

[CBBass123]

Protect your equipment. Always mute channels and turn phanton on channels last. Been a happy tm16 user for a while. Excellent mixer.

 

[Proformance]

I also have channels on the QSC TM that have gone bad. It's the poor pre-amp quality, and the noise renders the channel unusable. Phantom power amplifies the problem. While plugging things in hot is a risk - it's a known risk that all quality manufacturers account for. Good consoles aren't typically murdered simply by careless techs.

I don't buy the "corroded motherboard" story. The last stage of circuit board manufacturing is the "rinse" which is dipping it in water. I've personally recovered gear that was dropped in the ocean and a fresh water rinse is the first step in getting it back on line. That being said - a spill while the mixer is powered on is an electrical problem not a corrosion issue. The mixer fails within minutes if not seconds.

Ultimately the Touchmix is more computer than audio device. Most of the mixer's cost owes to the on board touch screen and control. Given it's price point I suspect the pre-amps or other associated components are of a poor quality and not that robust. The TouchMix 8 and 16 are probably an all-in-one construction with few (if any) serviceable parts. Replacing the mother board is probably the QSC solution to everything because I doubt there's much else they can do.