Amp heats up

Michael F

Michael F

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Jul 3, 2013
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Montreal Canada
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Hi All,
I came a across a QSC MX1500 that seems to be working ok but the left channel heats up. Would this be a biasing issue?
Where would the testing points be? I could use the good channel as a reference.
thx
 

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Measure across one of the .15 ohm 3 watt emitter resistors. Set bias for 15mV. Measure it before you adjust. If the bias seems normal, the heating can be a dangerous high frequency oscillation. Check R49 and R50 for burns.
 
That did it:)
It was way off at 80mv. Channel 2, while running relatively cool was at 25mv so I brought it down too.
How did you arrive at 15mv?
Thank you kindly for your help.
 
Made the number up assuming it was for PA use. If 25 mV is running cool then for home use I would use that for both channels. Your amp isn't old enough, but for stuff made in the '60s, we are spraying the bias pots with Deoxit. You will find random changes in bias are common when they oxidize.
 
I have a Kenwood direct drive table (KD-5070) and it wouldn’t hold speed...

Turned out to be dirty trim pots.

I think, while you’re in there, cleaning them is a good idea.
 
Turntables are the worst! On analog tuners, if you get a bunch of static when you tune, spray one of the bearings towards the dial string with deoxit. Keep it away from the last two bearings, or you will launch the dial out into space.
 
Michael F said:
Hi All,
I came a across a QSC MX1500 that seems to be working ok but the left channel heats up. Would this be a biasing issue?
Where would the testing points be? I could use the good channel as a reference.
thx
Click to expand...
Those Mx series amps are TANKS! once you get it fixed, I doubt you can break it again for another 20 odd years. I’ve pushed my mx3000’s bridged at 4 ohm to near clipping for hours and the only thing that melts or even get remotely warm is voice coils!

I do have a USA900 that’s heating under light load though... that needs attention.
 
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