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Old 21st March 2012, 05:39 AM   #1
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Default Punch 500a2 messed up channel

Power was applied backwards to this amp and it took out one channel. I found Q122 and Q123 shorted, those were replaced. I still can't track down the culprit. Here are a couple wave forms. One is from the collector of Q103 and the other is from output (2012-03-20-22-55-30_126).
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File Type: jpg 2012-03-20_22-56-58_150.jpg (479.4 KB, 85 views)
File Type: jpg 2012-03-20_22-55-30_126.jpg (301.8 KB, 80 views)
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Old 21st March 2012, 08:02 AM   #2
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If there is no DC on the output, connect a dummy load to the channel, drive a sine wave into it and post the waveform on the output.
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Old 21st March 2012, 12:56 PM   #3
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No dc on the output.

Waveform is missing the lower half. The signal looks ok until Q102 and Q103, it gets squared off.
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Old 21st March 2012, 03:38 PM   #4
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Does lifting D104 allow it to produce clean audio?
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Old 21st March 2012, 04:02 PM   #5
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Nope, tried that already.
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Old 22nd March 2012, 11:34 AM   #6
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Are the 20 ohm resistors connected to the emitters of the drivers within tolerance?
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Old 23rd March 2012, 03:47 AM   #7
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If the 20 ohm resitors were bad you would think there would be DC on the channel?
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Old 23rd March 2012, 04:14 AM   #8
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Those were replaced when the transistors were replaced. I will double check them.
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Old 27th March 2012, 03:07 AM   #9
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Back to fumbling on this amp. Q122, Q123, R174 and R175 replaced. Still no lower half of the sign wave with D104 lifted. D104 cathode -37.14Vdc and anode -36.74Vdc. I have -3.8Vdc on the cathode of D204 in the good channel.
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Old 27th March 2012, 12:21 PM   #10
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Are those voltages for D104 on the solder pads or have you reinstalled D104?
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