Electronics > Repair

Stereo Repair

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staze:
Okay, so a friend purchased a stereo receiver, largely untested, from a Goodwill here in town, and came to find out when he got it home that the left channel is largely gone (distortion, etc). Knowing I've taken up electronics, he handed it to me and asked me to fix it.

So, brought it home, hooked it up to the FG and Scope, and attached are the results. Bet you can guess which channel is which. My thinking is most stereos are push pull, so one set of transistors handle the positive side of the wave, and another set handle the negative. Obviously, the positive side is gone, and it appears to be oscillating. My thought would be the rail to those transistors, or maybe just a blown one, and the "oscillation" is just noise. I also note that the left channel has a negative DC offset relative to the right channel.

Any thoughts? I did manage to find the service manual online, so that helps. But holy cow, it's complex. For info's sake, it's a Pioneer VSX-452. Obviously it's not WORTH a whole lot of effort money wise, but experience wise...

Thanks!

David_AVD:
Always check for the obvious causes first; blown rail fuse, cracked joints, etc.   :)

Psi:
Does it have a amp chip or is it built from discrete components?

staze:

--- Quote from: Psi on July 29, 2013, 11:22:28 am ---Does it have a amp chip or is it built from discrete components?

--- End quote ---

Most of the circuitry is discrete. There are a few big ICs that mainly handle things like Dolby, etc. But the power amps, etc, are transistors.

dr_p:
apply the 1st rule off ee troubleshooting: Thou shalt check voltages!  :-DMM


I mostly mean the power supply, maybe load it up a bit, see how it copes with that.
I'm by no means an expert at this, but IMHO you can probe for that sine wave from the input towards the output stage and see where the shit hits the fan. That should narrow it down.


le: also, shorted semiconducors are a dead giveaway :D

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