High bias and distortion in one channel of GFA 545 w schematic

spice house · 01:39 PM

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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spice house · 18th January 2012, 01:40 PM

I should also add both channels have an offset of about 35 to 40mv

sakis · 18th January 2012, 02:06 PM

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

spice house · 02:35 PM

Quote:

Originally Posted by sakis

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?

spice house · 18th January 2012, 03:10 PM

I desoldered Q625, reseated, and soldered it back in, cleaned it and it magically works again

carlmart · 18th January 2012, 04:37 PM

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.

18th January 2012, 01:21 AM	#1
spice house diyAudio Member Join Date: Nov 2010	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 ImageShack® - Online Photo and Video Hosting Last edited by spice house; 18th January 2012 at 01:39 PM.

18th January 2012, 02:06 PM	#3
sakis diyAudio Member Join Date: Jan 2006 Location: nea makri athens greece	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 __________________ SERVICE ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr

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18th January 2012, 01:40 PM	#2
spice house diyAudio Member Join Date: Nov 2010	I should also add both channels have an offset of about 35 to 40mv

18th January 2012, 03:10 PM	#5
spice house diyAudio Member Join Date: Nov 2010	I desoldered Q625, reseated, and soldered it back in, cleaned it and it magically works again

18th January 2012, 04:37 PM	#6
carlmart diyAudio Member Join Date: Mar 2001 Location: Brazil	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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