QSC amp problems

rkc7 · 15th November 2005, 05:37 AM

I bought a broken amp (QSC USA400) and am trying to fix it. Here is the problem:

R21 of the posted schematic gets VERY hot when the amp is turned on. I haven't tested for sound or functionality, but I figure this isn't a good thing as it starts to smoke after about 30 seconds.

When I got it, this resistor was missing and the lead holes were scorched, I replaced it with the same rating (2W). It had 3 blown filter caps (out of 8). A MJ21193 transistor of Ch 1 (Q3A, not the one with the missing R21) was bad. I noticed there were MJ15023 transistors in place of the MJ21193s for the second channel (Q4B, Q6B). Could the bias setting be inappropriate for these transistors and be causing the overheating resistor?

Also, the service manual says for bias to adjust TR3 while measuring DCV across the emitter resistors for Q1 and Q2, would those be R15 and R16? It's supposed to be 100mv, but I don't get any voltage across those resistors when I turn it on (only for a few seconds, so I don't burn out the resistor).

All other components seem to be OK.

http://i13.photobucket.com/albums/a2...aniac3/sch.jpg

amplifierguru · 15th November 2005, 06:26 AM

Hi rkc7,

Yes that's R15/16 to measure.

Check that D5 is in the right way round and not shorted.

Cheers,
greg

rkc7 · 15th November 2005, 04:11 PM

Well, D5 wasn't bad, but D6 was! I replaced it and now it seems to work fine. Speaker output sounds good, I was able to properly adjust the bias, and the resistor still gets warm, but doesn't get really hot like before.

BTW, R21 was measuring 100vdc across before, now it's 50vdc.

Thanks for the inspiration!

15th November 2005, 05:37 AM	#1
rkc7 diyAudio Member Join Date: May 2004 Location: Toledo, OH	QSC amp problems I bought a broken amp (QSC USA400) and am trying to fix it. Here is the problem: R21 of the posted schematic gets VERY hot when the amp is turned on. I haven't tested for sound or functionality, but I figure this isn't a good thing as it starts to smoke after about 30 seconds. When I got it, this resistor was missing and the lead holes were scorched, I replaced it with the same rating (2W). It had 3 blown filter caps (out of 8). A MJ21193 transistor of Ch 1 (Q3A, not the one with the missing R21) was bad. I noticed there were MJ15023 transistors in place of the MJ21193s for the second channel (Q4B, Q6B). Could the bias setting be inappropriate for these transistors and be causing the overheating resistor? Also, the service manual says for bias to adjust TR3 while measuring DCV across the emitter resistors for Q1 and Q2, would those be R15 and R16? It's supposed to be 100mv, but I don't get any voltage across those resistors when I turn it on (only for a few seconds, so I don't burn out the resistor). All other components seem to be OK. http://i13.photobucket.com/albums/a2...aniac3/sch.jpg __________________ Disclaimer: I may be wrong, I may be right. I'd rather believe that I do know what I'm talking about, but am always willing to be told I don't.

15th November 2005, 04:11 PM	#3
rkc7 diyAudio Member Join Date: May 2004 Location: Toledo, OH	Well, D5 wasn't bad, but D6 was! I replaced it and now it seems to work fine. Speaker output sounds good, I was able to properly adjust the bias, and the resistor still gets warm, but doesn't get really hot like before. BTW, R21 was measuring 100vdc across before, now it's 50vdc. Thanks for the inspiration! __________________ Disclaimer: I may be wrong, I may be right. I'd rather believe that I do know what I'm talking about, but am always willing to be told I don't.

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15th November 2005, 06:26 AM	#2
amplifierguru Account Disabled Join Date: Aug 2004 Location: Earth	Hi rkc7, Yes that's R15/16 to measure. Check that D5 is in the right way round and not shorted. Cheers, greg

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