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Old 19th November 2015, 01:10 AM   #1
diyAudio Member
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Thumbs down qsc mx1500 1 channel runs hot

im working on a qsc mx1500, repaired some issues the amp was having.
just noticed the channel 2 run hots. At idle with no signal going into it the channel get hot.
probably 40-50 degrees, while the other channel is stone cold,
ive ran it amp sound fine, ran it over an hour, did not have any audible issue.
gotta put the scope on the channel see if i can see anything, i check the speaker terminals for dc voltage, both channels test the same.
does it sound like a bias issue? or what else could cause this?

thanks in advanced
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Old 19th November 2015, 05:05 AM   #2
Enzo is offline Enzo  United States
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Location: Lansing, Michigan
Yes, it sounds exactly like a bias issue. At idle, meaning no signal, it ought not to warm up much more than room temperature.

I am not at my shop, so I can't look it up, but is there not a bias adjust for each channel?

DC on output would warm it up if there was a load. Try it without a load. If it still heats up, then I'd bet my lunch money on bias adjustment.
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Old 19th November 2015, 10:01 AM   #3
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VR601 is the bias adjustment for Ch B, 501 is for Ch A. DO NOT adjust 602 or 603, (502 or 503 for Ch A)!
Support for Flying Mole Class D, Allen & Heath, Valve amps, older designs & 2CVs. www.jonsnell.co.uk
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Old 19th November 2015, 05:43 PM   #4
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awesome thank you guys very much, i will check that tonight when i get home
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Old 20th November 2015, 02:43 AM   #5
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well it was defiantly the bias, all fixed now, but the pot's were not mark as mentioned, possibly that was the newer mx2 1500, this is the OLD mx 1500, anyways. its all fixed up now thank you guy for your help
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Old 21st November 2015, 02:56 PM   #6
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Those are great sounding amps BTW. Enjoy!
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