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Old 29th January 2013, 07:51 AM   #1
ZeroG is offline ZeroG  United States
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Default Soundcraftsmen A200 Bad Channel

Recently I dragged a Soundcraftsmen A200 amp (125 wpc) home. It had one blown burnt channel. After testing the mosfets (hopefully got them all right) found and replaced two of the mosfets, several resistors, a diode and a couple of pre-drivers. Testing with dim-bulb tester shows no short.

The channel works now, but is noticeably thin sounding (and a little weaker) compared to the other channel. It also runs much hotter than the other channel.

What's next to check? It occurs to me to swap the two channels in order to see if it could be a bad filter cap, but considering that this channel was blown/burnt seems more likely that it's in the amp channel itself. Does that make sense?

Could more of the Mosfets be wounded but not bad? There are a total of 6 per channel - 3 each of the K1058 and the J162, and I replaced one of each. Unfortunately there's no service manual available, and there are no adjustments on the board.

Also odd, but the DC offset is 34mv on the cool , good sounding channel, and -1 on the hot, thin sounding one. (I would have suspected the reverse).

I'd appreciate any helpful guidance offered. It's a pretty good little amp and I'd like to rescue it.
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Last edited by ZeroG; 29th January 2013 at 03:30 PM. Reason: Revise part number prefix.
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Old 29th January 2013, 08:39 AM   #2
djk is offline djk
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Try replacing C6 and adjusting R9.
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Old 29th January 2013, 03:29 PM   #3
ZeroG is offline ZeroG  United States
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Quote:
Originally Posted by djk View Post
Try replacing C6 and adjusting R9.
Thanks for your response. I had already replaced all the electrolytics with the exception of C1 (22uF 35V nonpolar) which I did not have. R9 is non-adjustable, but I did lift one leg and it is right on spec.

FWIW, I've replaced R13, 22, 23; D8; and Q9 and Q6 (predrivers). I did replace one K1058 and one J162, but without a board diagram I can't say which ones they were.
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