Sherwood S-7100 Magic Smoke

[atrelstad]

Hi AK’ers, I’ve got a magic smoke story to share, and I’m hoping for some advice on how to fix it.

I’ve been working on a Sherwood S-7110 for a bit now, I got it in pretty rough shape with no right channel output, and after some unhelpful attempts to “fix” it. After a full recap, a replacement volume/power pot, and new output transistors, there was still no right channel to be found.

So today I’ve been signal tracing and testing transistor voltages, which were clearly still off in the right channel amp, and with one slip I shorted an output transistor (TR609b) and saw a lovely poof from somewhere in the power section of the board. I didn’t see exactly where from, but there were two suspicious looking diodes I swapped (D805 and D802), and I replaced said transistor. The main fuse was also blown, so I replaced that.

This is 100% my fault for not having it on the dim bulb, a dumb mistake I definitely won’t make again. But now it either blows the main fuse immediately, or lights up the dim bulb like a Christmas tree. Any advice on where to go from here?

-Smokey the Bear

 

[SkxawngGirl]

Did you check the driver transistors, the other output transistor and their associated resistors? The drivers are in a Darlington configuration with the outputs, so they may well have suffered damage. If any driver has shorted, it's a good idea to check the bias and 'class A' transistors also.

 

[crispycircuit]

I'd disconnect power to the power amp section and try powering it up again.....

 

[atrelstad]

Did you check the driver transistors, the other output transistor and their associated resistors? The drivers are in a Darlington configuration with the outputs, so they may well have suffered damage. If any driver has shorted, it's a good idea to check the bias and 'class A' transistors also.

Thanks, I ordered replacements for the rest of the transistors in the area, along with a few suspect resistors. I figure if I'm pulling them to test I might as well just refresh them all. I still had a channel out too so maybe I'll kill two birds with one stone!

I'd disconnect power to the power amp section and try powering it up again.....

Not sure that's possible on this amp, it has a "tuner board", and an "audio board" and that's it. Otherwise that'd definitely help.

Appreciate the help!