New through hole Rectifier installed, unable to Bias amp channel now

[batteryman]

Hello everyone

I am wondering if anyone would happen to be able to provide advice on this Adcom GFA-7500 amp channel I am working on?

I replaced 2 bridge rectifiers on one of my amp channels in an attempt to increase reliability as I have had others fail and now I am unable to properly bias it. I powered up using a dim bulb tester and the bias is now .34v verses what it was before .01 which is spec. The bias pot is a single turn unit turned all the way down. I just recapped this unit so I know its not due to stale caps. on the plus side the DC offset is great.

The original rectifiers were 8 amp rated units
http://html.alldatasheet.com/html-pdf/208646/PANJIT/SBU8G/214/1/SBU8G.html

The replacements are 10 amp rated
http://www.mouser.com/ds/2/80/KBU10005-G_KBU1010-G-3822.pdf

upgrading to a higher rated bridge should not have any affect on the amp output should it? After the bridge and filter caps are 8 amp fuses. I just was hoping to get something with a bit of overhead. I should note that while replacing the bridges I replaced the sand cast emitter resistors with Dale RS 1% units of the same value but I dont think that would have any thing to do with it.

Any help would be appreciated. Thanks in advance

here are some links to photos
https://photos.app.goo.gl/m6epxixXOKQ3gALy1
https://photos.app.goo.gl/yOoRBbwP2CI4rqrQ2

 

[awillia6]

Gonna be tough backing out all those changes to pinpoint the cause. Could also be a kicked sleeping dog (broken trace/wire/solder joint). Good luck.

 

[batteryman]

Hello Awillia6!

The only thing I changed before the bias went all crazy was the 6 emitter resistors and 2 rectifiers.

 

[whoaru99]

Of those two I think I'd focus on the emitter resistors for a bias problem, before the bridge rectifiers.

 

[awillia6]

Then that makes things easier. Don't forget to check for the usual gotchas attending any work on old electronics.

 

[llwhtt]

Are you sure the emitter resistors are the same value, they look to be .33 Ohms in the picture.

Craig

 

[petehall347]

i agree .emitter resistors may have been replaced with higher value ones . thats if you are checking bias across them .
p.s 2 outputs need some solder on the pins

 

[Powertech]

The soldering looks very poor on Q10. Check the circuit, but by the look of it this is probably the transistor that set the bias balance for the output stage.

 

[batteryman]

Thanks for the help everyone! I just wasn't sure after having this problem if upping the bridges forward current capabilities would cause any problems (still quite new at this) Whoaru99 you were right, it was the emitter resistors. I thought I confirmed with my fluke they were all in properly but I went out to the garage and touched the solder joints up on all 6 and its back to normal. They were all replaced with the same values (.33 ohms).


Powertech and Petehall347 you are right those are some not so great solder joints done by Adcom. It could be partially be because bottom of the board says "Solder side" on it. I was going to but do you always just apply solder to both sides? All of those transistors are blown on that bare board I showed and am going to have to replace them, not sure if anything else got fried yet or not..

 

[Powertech]

Glad you managed to find the problem.