QSC K12 Powered Speaker

[DGstrat]

Hi,

I am looking for the schematic to a QSC K12 powered speaker. I'm chasing an intermittent with pops/static. Doesn't seem to be heat related, nor cracked solder joints.

thanks for any pointers,
Dave

[Enzo]

Contact QSC and see if they will send it to you.

Meanwhile...

Isolate the problem.

This is biamped, so is the noise in both woofer and horn?

You have two input channel, each with a control. Does turning either or both controls to zero stop the noise or does it remain?

Does the MIC/LINE switch alter the level or tone of the noise?

You have a post gain line out. Run it to some amp or mixer so you can listen to it. Is the noise on the line out?


Ball up your fist and whack the top of the speaker hard. Does thatr make the noise either stop or start? Does it make any noise of its own when whacked?


When you say doesn;t seem to be heat related, what does that mean? That it does it when cold or when warmed up? That is probably a valid conclusion if so, but that doesn't mean thermal measures are useless. This is a perfect place for freeze spray. Freezing parts sends a thermal shock through them, and it might affect the noise temporarily. If we have a noisy op amp and freeze it, and the noise peaks or stops, we have found a part with some issues.


I assume this is all surface mount inside. Use a wooden chopstick or something insulated, and while running, push down on various components to see if nudging them affects the noise.

I don't know the innards on this, so I cannot tell you if you can disconnect the preamp from the power amps for example.

Once you know where the problem is in the system, it is easier to cure it.

[DGstrat]

Thanks for the reply and the suggestions. Here's where I'm at:

>>Contact QSC and see if they will send it to you.

They replied that I could have it if I was an authorized partner and logged into the partner website... No luck there.


>>Meanwhile...

>>Isolate the problem.

Agreed.


>>This is biamped, so is the noise in both woofer and horn?

Still trying to determine this. I may have to physically separate the speakers to be able to tell.

>>You have two input channel, each with a control. Does turning either or both controls to zero stop the noise or does it remain?

The noise remains with both at zero.

>>Does the MIC/LINE switch alter the level or tone of the noise?

No effect.

>>You have a post gain line out. Run it to some amp or mixer so you can listen to it. Is the noise on the line out?

Good point. I need to wire up an XLR to test this.


>>Ball up your fist and whack the top of the speaker hard. Does thatr make the noise either stop or start? Does it make any noise of its own when whacked?

One of the first things I tried :-) Has no effect.


>>When you say doesn;t seem to be heat related, what does that mean? That it does it when cold or when warmed up? That is probably a valid conclusion if so, but that doesn't mean thermal measures are useless. This is a perfect place for freeze spray. Freezing parts sends a thermal shock through them, and it might affect the noise temporarily. If we have a noisy op amp and freeze it, and the noise peaks or stops, we have found a part with some issues.

The noise happens whether its just been turned on or if its run for a while (and the fan has come on). I agree on the freeze spray idea. I have some on order.

>>I assume this is all surface mount inside. Use a wooden chopstick or something insulated, and while running, push down on various components to see if nudging them affects the noise.

Yes, that is how I started my debug. I've poked and prodded the boards and ribbon connectors, etc. with no effect. Hopefully the freeze will help me identify the culprit.

>> I don't know the innards on this, so I cannot tell you if you can disconnect the preamp from the power amps for example.

Right... That's why I was hoping for the schematic. Given that this is a 1000W amplified speaker, my thoughts are that a problem in the preamp would result in very loud static/noise, but what I'm hearing is not deafening. I keep thinking bad capacitors, because when it happens it seems to trail off slowly, like a cap charging or discharging. You get a crackle or crash of noise and then it trails off to silence.

>>Once you know where the problem is in the system, it is easier to cure it.

Absolutely.

thanks again,

Dave

[Enzo]

I was hoping there might be more than one board in there. In similar mackie units, for example, there is one main board that is the power amp, but there is also a smaller board with the level controls and input jacks. It connects to the main via a ribbon. A ribbon I could unplug.

[DGstrat]

You are correct. There are 2 boards. THe power amp board with the output transistors bolted to the aluminum box as its heatsink and the input board on the back panel with the controls and inputs. I can try running with that ribbon disconnected. Just not sure if the main board has to unmute the power amp or not.

[blindboybenton]

Hi Dave Did you ever get the schematic for the K12. i got one in today and the psu aint starting up

BBB

[audiopower1000]

Look this :

[audiopower1000]

Update for service manual of QSC K series :

[blindboybenton]

Thanks. I will keep it on file. Customer paid for a whole new pcb.... Very expensive

thanks

BBB

[DGstrat]

Thanks so much!

I still have this unit sitting off to the side. At least now I can try scoping my way through it once it actually starts acting up... It's very random. The last thing I tried was blasting it with freeze spray and that had no effect on creating the problem.

Dave