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Old 10th August 2007, 06:12 AM   #1
Ampdoc is offline Ampdoc  United States
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Default PC 2350 power supply - a little help

Hello.

I have a PC 2350 I'm working on. Normally these amps are no problem but this one's being a pain, and I THINK I know why but I'd like to make sure I'm in the right area. This amp came in with all the switching fets smoked, several burned resistors, and the trace from the supply gnd to the case open. I replaced the RFP70N06, resistors, U6, U3, and U9, just to make sure all would be well. I also found a rectifier, BYW29-200 shorted, and replaced it. Now here's the problem, I do some work for a small electronics shop, and instead of sending me the original diode, they sent an NTE597. Put all parts in, powered the amp up, and got high idle current (10ish A) and a bad odor.

After pulling the bottom back off I found the trace from T1 pin 5 to C169 and R78 had burned. All other parts in the amp check OK.

Is it at all possible that since the original diode has a Trr of 35ns and the NTE part's Trr is 60ns, that the slow cutoff could be inducing noise? The RC circuit appears to be a filter to remove switching noise from the secondary side ground. The remaining diodes are all original, only one was replaced.

Also, R172, which supplies b+ to U3 and U9, was burned beyond recognition. I believe it was used as a fuse for those IC's, so I put a 5 ohm 1/4w flameproof in that location, but would appreciate the proper value just in case that has something to do with the problem.

Thanks,
Jammy
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Old 10th August 2007, 10:10 AM   #2
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Unless the burned trace was tiny, there's no way the RC network could have burned it.

The most common problem with slow diodes is heat. They will run hotter than the faster diodes.

R172 is a 10 ohm resistor. It's the same as R253 next to it.
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Old 10th August 2007, 12:57 PM   #3
Ampdoc is offline Ampdoc  United States
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Aye, the trace is tiny. Thanks for the info :-)

Jammy
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Old 2nd September 2007, 03:50 AM   #4
Ampdoc is offline Ampdoc  United States
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Join Date: Aug 2007
Almost forgot to report back on the success Turned out the transformer T1 was shorted primary to secondary. That's what ate the filter / trace. Rewound the transformer and got it going :-)

Thanks,
Jammy
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