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Old 22nd July 2010, 06:50 PM   #1
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Default Musical Fidelity A100 problem.

Hi. I've read loads on here about my amp and as a result, collected all the parts I needed to overhaul it. But then it started misbehaving. First it cut out but then was okay again after I left it to cool down. Then it made a horrible load hum - so loud I instantly turned it off. I decided to replace all the electrolytics, including the smoothing caps, with 105 degree versions. After this I briefly powered it up with the lid off and the hum remained. Not only that, R2 (0.47ohms, 3w) glowed red hot.

So I replaced al four power transistors and the four drivers too. Plus the four zeners. But the problem remains. Any ideas what I should try next?

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Old 22nd July 2010, 09:26 PM   #2
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Try the amplifier without the power transistors installed, nothing should get hot. If it's fine without them, check the insulating washers and shims of the power transistors and replace if needed (my best guess is that this is the problem, transistors shorting to heatsink). Also check any bias controls.
"Audio grade" components simply means that they failed at a more critical job.
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Old 23rd July 2010, 07:08 AM   #3
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Thanks, but I checked for shorts between the heatsink and PTs when I replaced them and all was well.
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