Adcom GFA-1A - How to lower DC offset?

absolve2525

Super Member
I don't see a trim pot anywhere, so I was wondering which transistors need to swapped out for better matched pairs? Some of them aren't labeled, so I'm not sure what I need to replace. Thanks!
 

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Also looking for one of these switches to control the fan.
 

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There are four dual transistors per channel, two NPN and two PNP, both of which are obsolete. So you will have to come with eight matched pairs to do both channels, 2SC2240BL and 2SA970BL would be good choices if you can find them. But there are several others from which to choose.

Craig
 
I don't see a trim pot anywhere, so I was wondering which transistors need to swapped out for better matched pairs? Some of them aren't labeled, so I'm not sure what I need to replace. Thanks!
How much DC offset are you talking about ?
 
I think one was around 60mv and the other 30mv. It comes out of protection, just figured I could lower it some. Thanks!
 
2SC2240BL and 2SA970BL would be good choices if you can find them
Oh you can get them. Kinda pricey though. Especially to get matched pairs. You would need maybe 20 or more of each to obtain Hfe matched pairs.
 
Stay away from chinese vendors and 99% of Ebay vendors or you will get exactly what you pay for.;)
 
Thanks guys. It sounds really good as-is. I may just leave it alone. If I had the transistors already, I'd give it a shot.
 
Good news! After opening it up (which involves removing around 50 screws) and more thoroughly cleaning off all the dust build-up inside on the heat sinks, I'm getting very respectable DC offset measuments of around 20mv on each channel! Woo hoo. Guess I didn't need to replace the transistors afterall. I snapped a few pics of this crazy piece while I had it opened up. Big potted toroidal transformer is neat. Fortunately the fan is pretty quiet.
 

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What do the clear PTFE covered wires do to? Are those the STV-3H diodes? My GFA (not the -1) does not seem to have those parts.
How about another pic of the circuit board if you don't have those 50 screws put back in...
I know all about those.
 
Hi there, sorry I didn't see your message earlier. I'm not sure which wires you mean exactly. The wires in the one photo appear to go into the two bridge rectifiers. I can take a few more pictures, if that helps. I can't show the full circuit board, since the heat sinks are attached in the center, but the sides are pretty visable.
 
I’m asking about the clear wires in pairs that head to the back side of the heat sink near the holes in it.
 
Oooo now I see. Yeah, they look like diodes. They are probably listed in the service manual? I tried to get a better photo. There are four total.
 

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