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-   -   Punch 500a2 messed up channel ( 21st March 2012 05:39 AM

Punch 500a2 messed up channel
2 Attachment(s)
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).

Perry Babin 21st March 2012 08:02 AM

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. 21st March 2012 12:56 PM

1 Attachment(s)
No dc on the output.

Waveform is missing the lower half. The signal looks ok until Q102 and Q103, it gets squared off.

Perry Babin 21st March 2012 03:38 PM

Does lifting D104 allow it to produce clean audio? 21st March 2012 04:02 PM

Nope, tried that already.

Perry Babin 22nd March 2012 11:34 AM

Are the 20 ohm resistors connected to the emitters of the drivers within tolerance?

seankane 23rd March 2012 03:47 AM

If the 20 ohm resitors were bad you would think there would be DC on the channel? 23rd March 2012 04:14 AM

Those were replaced when the transistors were replaced. I will double check them. 27th March 2012 03:07 AM

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.

Perry Babin 27th March 2012 12:21 PM

Are those voltages for D104 on the solder pads or have you reinstalled D104?

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