High bias and distortion in one channel of GFA 545 w schematic - diyAudio
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Old 18th January 2012, 01:21 AM   #1
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Default High bias and distortion in one channel of GFA 545 w schematic

Hello I bought a used GFA 545

One channel is totally fine,

I've measured a 150mv bias current across TP1 and TP2 in the faulty channel (should be 6.5) There is audible distortion on that same channel There is no extra noise when no music is playing

Can someone help me troubleshoot this problem? Thank you

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Last edited by spice house; 18th January 2012 at 01:39 PM.
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Old 18th January 2012, 01:40 PM   #2
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I should also add both channels have an offset of about 35 to 40mv
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Old 18th January 2012, 02:06 PM   #3
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driver Q621 leaky
thermistor 601-problem
Q 617 - 619 leaky
C607 leaky
VR 601 open or other

Could also be a combination of the above
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Old 18th January 2012, 02:30 PM   #4
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Quote:
Originally Posted by sakis View Post
driver Q621 leaky
thermistor 601-problem
Q 617 - 619 leaky
C607 leaky
VR 601 open or other

Could also be a combination of the above
Thank you! R645 (Q625 E pin) is reading 150mv, thats where the bias is coming from, no other output emitter resistors have that voltage does that narrow it down? It's receiving proper voltage, so that means its q625 gone bad?

Last edited by spice house; 18th January 2012 at 02:35 PM.
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Old 18th January 2012, 03:10 PM   #5
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I desoldered Q625, reseated, and soldered it back in, cleaned it and it magically works again
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Old 18th January 2012, 04:37 PM   #6
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Well, I was going to add cold solder to sakis list, and apparently that might have the case on Q625.

If possible I'd check and/or re-heat all solders on the amp, or at least on the faulty channel. It might have been an assembly problem.
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