Please help me repair a stereo receiver

[68rustang]

Hello, I was hoping somebody could help me figure out how to fix a Sony surround sound AV receiver my little brother blew up while at college. This is more of a learning excercise than anything else. What it is is a Sony STR-D790 Pro-Logic AV receiver. The story I got from him was he had a party and they needed more speakers. Well they hooked up god knows how many to the stereo outputs and it worked for he says 10 minutes then nothing. As it sits now when you push the power button a small LED indicator lights up for surround sound but nothing else. I opened up the case and found a blown fuse. This fuse is located on a board where the main power cable comes into the unit. It is one of two. The blown fuse is a 6.3A 125V fuse, the other non blown fuse is a 3.51A 250V. I tried replacing the blown fuse but it blows right away as soon as the power is turned on. Where do I begin troubleshooting to even see if this is fixable. Like I said it is more of a learning excercise than anything.
Thanks in advance for any help.

Regards,
Eric

 

[jbeng]

Sounds like an amplifier output transistor (more likely an amp output module) has a short in it. You'll likely need a service manual and some test equipment ...
JB

 

[68rustang]

by test equipment I ASSume you mean more than a digital multimeter? What do I need? Does anyone know of a source to download a service manual?

 

[zachtheterrible]

google.com

type in your stereo name and search.

EDIT: funny thing, now i'm searching for a certain VCR schematic and my "go to google and type it in" didn't yield any results. you might want to try schematic websites. go to google again and type in electronics service manuals and you'll find a bunch of websites that specialize in service manuals

 

[panic mode]

It should be easy to identify output circuit. you can trace connections
from speakers back to PCB etc till you find large IC or transistors,
probably mounted on a heatsink. google is your best friend.
it is very common that one or more of output transistors is shorted
due thermal overload (probably both - or all four, six... depending on
design they might also be part of the IC). Because they are shorted,
fuse pops right away, before you even hear a note from the speakers...
if you are feeling lucky keep changing semiconductors (starting with
largest) until fuse stop blowing (good luck...) :?

 

[Nigel Goodwin]

First off - he did a really STUPID thing connecting loads of speakers, he was almost guaranteed to destroy the amplifier!.

If the fuse that's failed is F902, it's in the primary of the mains transformer for the amplifiers - so there's a possibility it's killed the transformer?. There are two fuses in the secondary, F971 and F972, I would expect those to blow with a dead amplifier.

The main output transistors (fronts) are 2SC4387 and 2SA1672, you 'may' be lucky and only require those replacing, if not there are drivers in front of them, plus a large IC which feeds the drivers.

The centre channel uses 2SC4431 and 2SA1684 for outputs, and the surround is an IC, SI18752N.

Your first problem is to find which amplifier channels have been killed!. Simple ohmmeter tests on the transistors should do that!.

 

[68rustang]

First off - he did a really STUPID thing connecting loads of speakers, he was almost guaranteed to destroy the amplifier!.

You knew this, I knew this, and now he knows this. Experience is a great teacher

Thank you for the input I will look at those when I get home this evening.