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Old 8th April 2004, 12:28 AM   #11
subwo1 is offline subwo1  United States
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What are those other parts mounted on the heatsink? I suspect those are bad.
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Old 8th April 2004, 01:23 AM   #12
lubbie is offline lubbie  Canada
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how did the old transistors measure??

You may have a shorted transformer or a shorted PSU filter Cap..

Ralph
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Old 8th April 2004, 01:45 AM   #13
K-amps is offline K-amps  United States
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Check the transistors prior to the burnt driver, chances are they are shorted too. A shorted input diff pair will cause boatloads of DC to be amplified at the outputs.
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Old 8th April 2004, 02:04 AM   #14
shajan is offline shajan  United States
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The old transistors WERE shorted.

I don't know what the other components on the heat sink are, may be a negative feed back trans?? I'll see if the replacements are available at Electronic City.

Transformer is fine, I measured +/- 38V.

Which would be the PSU filter cap? and how would one test it with a DMM?

Please help!! If you need more pics, let me know.
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Old 8th April 2004, 02:08 AM   #15
K-amps is offline K-amps  United States
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Don't worry about the caps they rarely fail, first go for the transistors.

The other things that are usually mounted on heatsinks are:

Bias transistor and
Amplified bias Diode.

Both do thermal compensation for bias. Check those as well.

Then checl all resistors on the bases and the emmiters of all burnt transistors.

K-
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Old 8th April 2004, 02:19 AM   #16
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Default What's that IC in the middle?

It looks like an IC Power opAmp?!

If the outputs were shorted, chances are VERY high that whatever drives them is bad. If that multi-legged thing in the middle is an LM3886TF or similar, replace it. I can tell from the PC board there is no discrete driver section, and the only other audio component is a 4558 OpAmp.
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Old 8th April 2004, 02:41 AM   #17
lubbie is offline lubbie  Canada
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You must still have faulty transistors. Also watch the emitter resistors, they have a tendancy to burn out when the transistors go, but may still look good.

The PSU caps and bridge will short, although seldom. But always a consideration when outputs are still good.. But big psu caps are unmeasurable with a DMM.. I use a Cap checker..


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Old 29th May 2004, 03:40 AM   #18
shajan is offline shajan  United States
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(Update)

I was finally able to get it working

The big white resistors were shorted (lubbie's suggestion) which caused the output transistors to burn out, also the multilegged thingy. Replaced them all and now the amp is working well. Thank you all for the help.

Shajan
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Old 29th May 2004, 03:53 AM   #19
lubbie is offline lubbie  Canada
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Glad all is working. May it thump for years to come and drive all your neighbours nuts...
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Old 29th May 2004, 04:36 AM   #20
karma is offline karma  Canada
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nice
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