New A40 built but -30V in R. channel - diyAudio
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Old 24th October 2010, 10:39 PM   #1
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Default New A40 built but -30V in R. channel

I've built an A40 and on first power up I have -30V (full negative PS rail) measured at the output of the right channel (left is OK). I swapped all of the output devices between left and right and the problem remains at the right channel. I used colored wiring between driver board and output transistors to prevent confusion and checked the PCB well before soldering it up to all wiring. Can somebody with a close understanding of the driver circuit offer some suggestion of a place to start looking for trouble?
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Old 24th October 2010, 10:52 PM   #2
llwhtt is offline llwhtt  United States
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Since the PCBs are mono, check each against each other, then recheck. Make sure the transistors and diodes are in correctly, location and polarity. If that checks out check the resistors and capacitors, again location and polarity. I'm doing the same thing on mine as far the colored coded interconnect wiring. What did you do for the CCS? FET, resistor, or ?. Also make sure the power supply is connected with the correct polarity. If nothing found start checking for bad parts. If you have the AA article I think there are some troubleshooting tips in there too.

Craig
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Old 25th October 2010, 12:25 AM   #3
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Thanks Craig. I've been using the AA article since I started and it is limited.
I'm really trying to find some help in understanding how the circuit could go full negative in a failure. Certainly I could check everything as you suggest but since I did that before powering up I'm really looking for an understanding of the circuit for some depth and focus.
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Old 25th October 2010, 01:20 AM   #4
llwhtt is offline llwhtt  United States
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Troubleshooting a previously working amplifier is a little different than one that has never worked or something that you built. A previously working amp that went bad obviously would have a bad part(s). If one of your boards has a part(s) installed backwards that would not be an ordinary failure of a part(s) and give different results. Also to consider is trying to troubleshoot an energized circuit that may be ruining parts the longer you leave it on. Make sure your parts are correct, then make sure your wiring is correct, make sure there are no solder bridges, and last but not least are the PCBs you are using. If they are from Old Colony did you do the mod required to the traces? Did you mod them both the same? If they are from NP then they should be OK. If all that checks out you probably have a part(s) that went south. I just reread NP's article again and there is a lot of troubleshooting advise in the article didn't any of that help? Did you have any blown fuses? Did you have the correct voltages at the places NP talks about?

Craig
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