Amp Repair - AVS Forum
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post #1 of 5 Old 06-26-2012, 09:33 AM - Thread Starter
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Disclaimer: This information is intended for entertainment purposes only. I do not want to be responsible if anyone electrocutes themselves or experiences any other inconvenience while attempting their own amp repairs. If you don't know what you're doing, don't try to save a few bucks at the risk of serious injury or worse.

I got this Sunn amp (made by Fender) a while back for really cheap from Craig's List.
The owner was not honest. The ad said it worked.
When I picked it up, the guy's wife said they were not aware of any problems.
I assumed there was a problem anyway, since the price was so low.
So it didn't bother me much when I found a dead channel.
Oh, and the fan was very loud and clunky, sounding like it might start slinging bearings at any moment. I replace the fan.

402


With it unplugged, I used an ohmmeter and found a shorted transistor and replaced it. I know that's not very technical or very interesting, but I had to start somewhere and that seemed a logical place to begin. You'll get a better look at that transistor in a moment.

402


With the transistor replaced, I got sound from the previously dead channel, but it was very distorted.
Unplugged, I began ohming again and found an open resistor (infinite resistance).
The lighting makes it hard to tell the color bands exactly in the photo, but this should be a 22 ohm resistor.
R110 is the culprit, below R109.

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By happy coincidence, I happen to have some 22 ohm resistors.

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I traced the resistor to a transistor and found it was shorted.
I don't know how I managed to overlook the second shorted transistor, but I missed it the first time through.
The transistors are in pairs and I already replaced its mate.
One transistor amplifies the positive half of the signal and the other transistor amplifies the negative half.
Here, you can see the transistor I replaced. It looks shiny and new.
I used a silver marker to mark the one that needs to be replaced next.
It's up two and over one to the left from its newly replaced mate.

402

Now I need to order the replacement part.
Had it not gone so well, I would have had to inject a signal and trace it through the signal path, but that wasn't necessary this time.

Not long after I got that amp, I got this other one for really cheap, also.
At least this guy was honest and said it had a dead channel.

402

Fortunately, I quickly found a bad internal connection in the dead channel.
Now, I can't get it to fail.
Well, that wasn't very interesting.
I hope it was at least entertaining.

402

I've got one ready for Craig's List already.
Well, two actually. Soon to be three.
Around the same time that I got the others, I picked up this amp for pretty cheap, but it has nothing wrong with it and I don't need it after all.
Pink Floyd sang about The Wall through this amp, while I looked at the other two.

402
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post #2 of 5 Old 06-26-2012, 10:03 AM
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Nice going PA! Wow, is there anything that you don't excel at?

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post #3 of 5 Old 06-26-2012, 11:34 AM
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I love learning! That is why I never miss an opportunity to follow a PI thread.

Thanks again for another informative thread PI.

"Half the world is looking for Jesus, and the other half is looking for more bass..."
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post #4 of 5 Old 06-26-2012, 12:32 PM - Thread Starter
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Thanks guys. But, please don't be impressed by this. It really wasn't a big deal.
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post #5 of 5 Old 06-26-2012, 06:01 PM
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Quote:
Originally Posted by PassingInterest View Post

Thanks guys. But, please don't be impressed by this. It really wasn't a big deal.

not a big deal! tell that to the guy who spent x amount of dollars for someone else to fix it wink.gif love to see diy in action!biggrin.gif

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