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djfoundation 8th December 2009 07:02 PM

shortcutting protocol
Just a quick check for some guidance, before I spend hours swapping parts from side to side.

I have a QSC amplifier that I'm almost positive is oscillating at the output.

Long story short, I found it with multiple speakers plugged into both channels and the bridged terminals all at the same time! They said it "just stopped working" and after pulling clumps of dust and debris to clear the heatsink/get the fan spinning again it's up and running. Props to QSC, all output drivers read ok.

Problem is (and what makes me suspect oscillation) is when I turn it on, channel one clip light goes on. All I have to do is unplug the speaker for a split-second and it's normal.

I remember back in school that sometimes even the slight change from my probe lead was enough to knock an errant oscillation out, but it was already bedtime so I didn't get a chance to put the 0scope on it, just some basic DMM checks were run.

Should I just start replacing all the small (coupling?) caps near the output or is there an easier way to track this down?

Thanks in advance

llwhtt 8th December 2009 07:06 PM

Which QSC?

Netlist 8th December 2009 07:16 PM

I'm not underestimating our members but great help can be found at QSC Audio Forum • Index page
They have first hand tips and tricks most here can't offer.
Thanh Nguyen is your man. :)


djfoundation 8th December 2009 07:22 PM

hmm, ISA series -I'm guessing the 450? (I'm at work, waiting to get back to fun land.)

Thanks for the lead, Net!

wg_ski 8th December 2009 07:38 PM

ISA series? Multiple speakers plugged in? Perhaps they were driving a 70V line in addition to a pair of low-Z speakers. They are supposed to be able to do that - as long as the effective load doesn't go below 2 ohms/side.

Too much load and/or a clogged fan, OTOH, and it will shut down.

djfoundation 8th December 2009 07:48 PM

ha, no logic. He's a buddy of mine, and this is him just plugging in whatever speakers he can find. Frankly I'm surprised the poor thing didn't blow a channel - I replaced the drivers on one side already a year or two ago when he blew it that time..

just grabbed a fresh copy of schematics from the QSC forum, I'll have to browse and simmer for a bit. my knowledge of electronics after years of blind experience is almost purely intuition.

wg_ski 8th December 2009 09:36 PM


Originally Posted by djfoundation (
ha, no logic. He's a buddy of mine, and this is him just plugging in whatever speakers he can find.

If he's going to do that he needs a Crown DC300(A) or 1977-era CS800, not a modern "pro" amp.

djfoundation 11th December 2009 07:46 PM

1 Attachment(s)
checked it with the scope, and there's no DC on the outputs of the bad channel.

While probing around, I noticed that R133, 135 were really cooking, and dropping a lot of voltage across them. Is there a reason they would be conducting as much as they are? It just looks like a feedback line to me..

I also discovered that the resistance across the speaker outs was about twice what the good channel was reading.

Any help/pointers appreciated.

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