Film-Tech Forum ARCHIVE: QSC Amp red lights on

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Posted by Frank Dubrois (Member # 3042) on 04-21-2007, 12:46 AM:

I have a QSC mx1500 series amp with a SF-1 (I think)sub crossover. We had the Clip and Prot lights come on, and stay on, yet I hear no sound difference. The sub seems to work just fine when movies are playing, however, when I run the CP500 diagnostic pink noise through each channel, the sub cannot be heard. Is it possible that the sub doesn't work but still have fantastic bass? I played a reel of trailers through as well, several with good bass, and all seems well. The lights come on every morning after the amps have been shut down for almost 12 hours.

Posted by Jeremy Weigel (Member # 4208) on 04-21-2007, 12:58 AM:

Sounds like you have a short on the output side of the amp. Disconnect one of the speaker leads on the back of the amp to see if it is short in the speaker wiring or the subs themselves (voice coil could have shorted). Yes you can have "fantastic" bass without the subs if you have good screen speakers, but the bass will usually lack "punch". If you have a spare channel on another amp you could also swap the subwoofer (LFE) input signal leads and the speaker leads to the spare channel and then run the pink noise.

I once had a QSC mx2000a shoot flames out the front followed by smoke when it went bad. Scared the you know what outta me.

Posted by Frank Dubrois (Member # 3042) on 04-21-2007, 02:48 AM:

If I disconnect the speaker wires, should the red lights of off if its a short in the wiring? If I attach the wiring to another amp, don't I need to switch the crossover card too?

The bass definately did not lack any punch. It sounds fantastic. The only way I knew something was wrong, was because the lights came on.

Posted by Mark Gulbrandsen (Member # 72) on 04-21-2007, 08:27 AM:

If the red lights stay on with the speaker and input disconnected then the amp needs repair attention. Yes, I too have heard of local theaters that had flame spitting QSC's but fortunately have never seen one go myself.

Mark

Posted by Gordon McLeod (Member # 33) on 04-21-2007, 11:52 AM:

Disconnect the load and the input and see if it powers up normally
If not then I would suspect it is either the clip circuit or the powersupply
Some QSC use a DC detecting circuit to change the bias of a opamp to shut it down and light the clip light
Often on the 1400 serries it is the filter caps have dried out and failed
If it goes off under no input and load then I would suspect either the load or that there is some DC in the input to the amp