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-   -   Punch 500a2 messed up channel (http://www.diyaudio.com/forums/car-audio/209279-punch-500a2-messed-up-channel.html)

brandes.cm 21st March 2012 04:39 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 07:02 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.

brandes.cm 21st March 2012 11:56 AM

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 02:38 PM

Does lifting D104 allow it to produce clean audio?

brandes.cm 21st March 2012 03:02 PM

Nope, tried that already.

Perry Babin 22nd March 2012 10:34 AM

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

seankane 23rd March 2012 02:47 AM

If the 20 ohm resitors were bad you would think there would be DC on the channel?

brandes.cm 23rd March 2012 03:14 AM

Those were replaced when the transistors were replaced. I will double check them.

brandes.cm 27th March 2012 02:07 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 11:21 AM

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


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