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Old 27th October 2010, 02:32 AM   #1
barnes is offline barnes  United States
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Join Date: Oct 2010
Default Beginners Issue: Failing PA Amp

First post here!

I have a failing PA amplifier I've started to attempt to fix. I'm studying electrical engineering, but never really worked with audio amplifying circuits so I'm in need of some help.

The issue is such: After about 30 minutes or so of amplifying, the output degrades to clicks. I can tap the cable to get some noise but no amplifying of an actual signal.

My completely ignorant view of the circuit makes me suspect it is a shot FET, as I see about 7 FETs lining rather large heat sinks. As soon as the FET takes on to much heat (after about 30 minutes) it fails.

I'm curious if the problem is common, or if there is an obvious solution. Any help would be great.

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Old 27th October 2010, 02:45 AM   #2
llwhtt is offline llwhtt  United States
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Location: SoCal
Knowing what brand and model sure would help.

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Old 27th October 2010, 03:01 AM   #3
barnes is offline barnes  United States
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Join Date: Oct 2010
It is a Phonic 410 100W.
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Old 27th October 2010, 05:44 AM   #4
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Join Date: Mar 2010
There are no FETs in this. It uses ICs and bipolar transistors.

This is a job for an oscilloscope, voltmeter, audio generator, and of course schematic diagram.

Don't try to fix it by asking questions here. You can eventually get enough information that way but it will take a long time. No substitute for understanding how the circuit works and probing around to see what isn't doing what it should.
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