QSC K12 repairs help

[Rajan Paliwale]

QSC K12 is cheap but good monitor. Generally out mosfets, driver ics few smd resistances in amp section replacements do the job done. In power supply section mosfets & driver ic replacements replacing helps you.A new case came to me with the complaint no power complaint,found the fuse blown, offcource smps mosfet short circuited (STW26NM60) .Replacing both fuse & smps mosfet machine started working good! Put on trial with 20% volume on side .After 35 minutes it stopped. Meter checks revealed the same components failed again.A quick inspection to out put section for leaky components ,Q16 2SC1655 found leaky, replaced,smps STW26NM60 & the fuse replaced. Furthermore trial showed same behaviour works for around 30 minutes. I dont know what I realyam missing.

[vintagekiki]

Please upload schematics for QSC K12

thanks

[nickb]

My first suspect would be excessive voltage on the main transformer primary.

I think these use a UCC2893 controller. If that is correct it is an active clamp meaning that Q13 gets turned on a just the right time to prevent excessive voltage on the main switching MOSFET. A bit tricky to check for pulses as the components keep blowing so you'll have to work blind.

Check UCC2893, D23, R57, C128, T3, C137, D35, R64, D35, D40, R165, C116.

[Jazz P Bass]

It is recommended that both mosfets be changed if one goes bad.
qsc_k-series_k8_k10_k12_Power supply.pdf

Full schematic set: qsc_k-series_k8_k10_k12_sm_full.pdf

[Jazz P Bass]

Here is the PS startup sequence: K Series startup sequence.pdf

[Rajan Paliwale]

Thanks for startup supply sequence off.It has become easier to understand.

[Vicus]

I experienced that if Q14 not work (always off) the output voltage is very low and for example PSU-BIAS never came up. In my case the D23 diode was short.

I have another problem, after repair the module now all work fine but the psu produce way too heat. Also at idle the fan start to spin and after some minute it reach the max. If you apply a signal, at low power the output stage produce some bump signal. I don't know what check. Some ideas?

[chickenwatt]

Hi, I am repairing a K12, is turning on but only sounds the tweeter, I measure the output mosfets and only the tweeter amp is switching voltage, then Low amp mosfets are not switching, I replace IRS20957S gate driver thinking is was bad but the same thing, do somebody has had this problem ?

[Vicus]

Maybe the integrator in the feedback look. Or the woofer itself.

[rasmus.lepik]

Hi!
I decided not to make a new topic, I will write about my problem here. My K12 speaker also has a burned out mofet Q14. The problem, however, is finding a spare part. The STW26NM60 MOSFET appears to be discontinued. It is not available in major European electronics online stores. Can anyone tell me where to get this mosfet at a reasonable price, or what it can be replaced with?
Thx!