Amplifier problem

[billybob]

I have a qsc gx3 with a blown surface mount fuse reading 10A. It is near one of three inputs from the power transformer. The fuse blew away some of the pad on the pcb and that is going to have to be fixed. It is also by a rectifier. I will send pictures later. The amp turns on and the LEDs light up but there is no signal at the outputs and the input was clipping/peaking. Any suggestions on what my first step would be?
If you need clarification or more information about it please let me know.

 

[KeepItSimpleStupid]

it's a very unusual amplifier. Schematic at Electrotanya.

 

[Nigel Goodwin]

it's a very unusual amplifier. Schematic at Electrotanya.

It's pretty well identical to Behringer amps - who probably went direct to QSC's Chinese suppliers In fact, as Behringer are VERY difficult to find schematics for, it's usually given as a suitable schematic.

As you say, it's an unusual design, with the speaker connected to strange places.

There's two 10A fuses shown on the schematic, one for each channel - presumably one of the channels has blown?.

'Possibly' the bridge rectifier - but much more likely would be output transistors (and all the rest), a typical catastrophic DC coupled transistor amp failure.

 

[billybob]

It's pretty well identical to Behringer amps - who probably went direct to QSC's Chinese suppliers In fact, as Behringer are VERY difficult to find schematics for, it's usually given as a suitable schematic.

As you say, it's an unusual design, with the speaker connected to strange places.

There's two 10A fuses shown on the schematic, one for each channel - presumably one of the channels has blown?.

'Possibly' the bridge rectifier - but much more likely would be output transistors (and all the rest), a typical catastrophic DC coupled transistor amp failure.

So you think it was a chain of the transistors failing? How can I test that? Are they mosfets or bjts?

 

[billybob]

Turns out both fuses are blown

 

[Nigel Goodwin]

BJT's - easily checked if they are short with a multimeter on ohms or diode test.

Did you kill the amp?, or did you obtain it already did? - what heppened to it?, it's a bit suspicious if BOTH channels are blown.

 

[billybob]

Took out one of the C3263 transistor and it appears to be shorted from emitter to collector.
Would it be worth it to replace all the power transistors?

 

[Nigel Goodwin]

Took out one of the C3263 transistor and it appears to be shorted from emitter to collector.
Would it be worth it to replace all the power transistors?

At the absolute very least!.

However, DC coupled amps are like chains of dominoes - a faulty device almost anywhere can blow much more, and it's essential to cure the cause and not just the symptoms. Very often if you just change the outputs, they will blow again - either instantly, or in a short time.

 

[billybob]

BJT's - easily checked if they are short with a multimeter on ohms or diode test.

Did you kill the amp?, or did you obtain it already did? - what heppened to it?, it's a bit suspicious if BOTH channels are blown.

Well my friend and I thought it would be a good idea to to try fixing some broken amps and reselling them. We bought this amp already broken and it's looking like this might be our last amp to fix. Cause business is not good and we have so many unexpected surprises that eat up our money

 

[unclejed613]

Well my friend and I thought it would be a good idea to to try fixing some broken amps and reselling them.

since these are generally used as PA amps for live bands, and there's not a whole lot of live performances going on right now, you might be better off fixing a few and holding on to them until demand begins to get back to normal.

 

[unclejed613]

Would it be worth it to replace all the power transistors?

better to only replace what's actually broken... there are probably some blown parts in earlier stages of the amp, so check everything, especially flameproof resistors, and transistors..... be aware QSC uses an odd design where the power supplies are floating, and the "output" of the amp is grounded... the center tap of the power supply is the actual speaker output terminal.

 

[billybob]

We ended up distributing the components among us since we would only make about 15 dollars profit and we payed 40 dollars for it.

 

[billybob]

Besides pro audio equipment, what electronics would be a good choice for buying broken and reselling?

 

[Nigel Goodwin]

We ended up distributing the components among us since we would only make about 15 dollars profit and we payed 40 dollars for it.

Ouch - you paid a LOT for a completely destroyed unit.

 

[billybob]

Ouch - you paid a LOT for a completely destroyed unit.

yep. live and learn