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Old 5th November 2012, 04:25 AM   #21
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Absolutely, and I did check everything very carefully after the first short occurred. Each device was isolated beyond my meter's high resistance range and the amp powered up OK. But - I suppose something settled under pressure overnight and.. Well... Kaboom!

Fixable, if frustrating.
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Old 5th November 2012, 06:38 AM   #22
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All it takes is a speck of grit to short through most insulators. The outputs use hard anodized aluminum, don't scratch them.
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Old 5th November 2012, 08:42 PM   #23
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Roger that djk, and thanks for the advice. I'm familiar with anodising, and was very careful to check isolation after mounting the outputs. It was the TO220 drivers that undid me Hopefully someone else can learn from my mistakes.

Cheers to everyone for their comments & advice
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Old 8th November 2012, 09:29 PM   #24
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Replacement parts arrived yesterday. I hope I can fit them with a replacement set of care and attention . I'll update once there has been a chance to tinker.

Cheers,

Ed
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Old 10th November 2012, 12:26 AM   #25
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Get a car battery charger. Old fashioned 12 vdc e frame transformer with 2 rectifiers, nothing with electronic sales gimmick devices. Put a 25 v electrolytic cap across it. That is an 18v peak power supply. Solder a 47k resistor to one lead. Put the amps scale of your DVM in series with it. Hook up a negative probe, get some clip leads. That is a leakage tester that finds more problems than the 2v compliance 2megohm scale of your DVM. I found some things with it, including toasted film caps and output transistors that were "okay" by the double diode test but had the habit of exploding when I put 75 VDC on them. When testing naked transistors, short b to e before putting that much voltage on them.
I held the rail voltage down on my amp from +-85 with a room heater in series with the power lead, but if I held it down too much with a 100 W light bulb, the op amp supply wouldn't reach +-15v. the room heater did keep the silicon from blowing absolutely across the room through the steel TO3 cases.
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Last edited by indianajo; 10th November 2012 at 12:29 AM.
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Old 11th November 2012, 01:35 AM   #26
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OK, at last, music without Kaboom! All drivers mounted very carefully with new mica washers & new shoulder washers under the screws, onto heatsinks that had been carefully rubbed down with fine emery. Tested, tested and tested again for isolation. The same with new power devices in the damaged channel.

And now, for some long promised pictures:

Click the image to open in full size.
[url=http://www.flickr.com/photos/16059325@N03/8173815187/]
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Old 11th November 2012, 01:36 AM   #27
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And another:

Click the image to open in full size.
IMGP0778 (640x424)
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Old 11th November 2012, 01:36 AM   #28
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One more:

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IMGP0779 (640x424)
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Old 11th November 2012, 01:41 AM   #29
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Just one thing left, to let it soak in and set the biasing and offset as per the manual.

My listening test was rudimentary, plugged into my cellphone playing from Pandora, and driving a couple of bookshelf speakers in the garage. That was enough to tell me it was operating cleanly, and I could hear the quality limitations of the Pandora feed. Tomorrow I'll finish reassembling the case, check those transistors one last time for mounting shorts, and give it a proper test.

Cheers,

Ed
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Old 11th November 2012, 02:43 AM   #30
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Looks like a very clean amp Ed...happy listening!
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