Question about QSC MX1500A 2-Channel Amplifier

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Peavey CS-800 PA power amp 1 channel is out

Channel ''B'' works fine. Channel ''A'' is out. I have checked virtually every component in channel ''A''. I have test equipment. I have compared resistance and diode meter readings between the good and bad channels I have been an electronics tech for over 30 years. This one has me stumped. The -85V is low at about -55 volts only on the bad channel. I have a schematic, but I can't figure this one out. I have meters, a scope, and also a curve tracer. I have metered all the output transistors - they checked good...someone has replaced 2 of the output transistors previously as they are not the same as the others. Channel ''B'' ' s outputs are all the same like the majority of the parts in channel ''A''. I am hesitant to swap the outputs or the driver boards as I am afraid to damage anything in the good ''B'' channel therefore losing my ability to compare 1 to the other...I have about 10 or 12 hours into this now and I am about to return it to the owner and tell them I can't fix it. any help is greatly appreciated Thanks Bill

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Hi Bill,

It's a log time ago that I repaired those amps., but I give it a shot.
There's a speakerprotection after the finals, what I remember it's a triac (SAC187, or something like that).
You will find it at a small pcb at the back of the amp. Normally it has three legs but the centre is cut away. If it's down you will measure 0 Ohm at the output, also with a switched off amp.

Thats all I can remember, it was about 25 years ago.

Maybe it helps,
Gerard

Posted on Jan 30, 2009

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Hi. i have a peavey cs-800 serial 5A-02115255. when i power on then the fuse blow. anyone here have any ideal what is the problem/?. i do not have a schematic if any one here have could you email it to me at Nam.Nguyen@bnsf.com. thank in advance.

thanks
Nam Nguyen

Posted on Dec 16, 2009

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I work on these too.
I would replace all the high-watt emitter resistors(in the bad channel) and if you have a REAL transistor tester , check the whole "bad string" of transistors for gain match
Peavey also uses dual series diodes that they wire thru the heatsinks(at least on the guitar amps). Thats a usual suspect.
If you ask peavey tech support nicely..they'll get you a schema if you dont have one

Posted on Jan 27, 2009

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