DC offset in my p3a- need help! - diyAudio
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Old 12th January 2005, 10:44 PM   #1
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Unhappy DC offset in my p3a- need help!

Hi,

I just finished an ESP p3a amp, and I can't get it to work. When I turn it on, the LED's light up and the amp remains cool, but I'm measuring huge DC offset in the output. I am getting +25v in one channel, and -42v in the other. None of the transistors look burned out, and no fuses blew. I am not using Rod's PCB's, but put the components on a breadboard and connected them with short lengths of hookup wire. Also, I am using the maximum allowable voltage (42v) in the supply, but I didn't think this would be a problem, since I have 8ohm speakers. Again, nothing appears to have burnt out, but it is unusable. If anyone knows some points on the schematic to take measurements from, I would really like to see them. I have a digital camera, and can post pictures if anyone would like to see them.
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Old 12th January 2005, 10:54 PM   #2
Stocker is offline Stocker  United States
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first thing is to check all your transistors are in the right way round
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Old 12th January 2005, 11:04 PM   #3
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Did that with the outputs, and with the others a little while earlier. The problem might be with the inputs (BC546). different manufacturers use different pin arrangements for them, e.g. Fairchild and OnSemi have C,B,E from left to right, Philips has E,B,C. But I probably would have burnt them by hooking them up the wrong way around. What perplexes me is that I have the supply voltage as the DC offset in one channel.
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Old 13th January 2005, 04:44 AM   #4
Stocker is offline Stocker  United States
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break the schematic in half and check it there is the general principle. do you have the offset at the input to the output stage? At the input to the VAS? Is your preamp shot?

Must
Make
Measurements

oh yes... no solder bridges?
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Old 13th January 2005, 08:11 PM   #5
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I will check those out, and post measurements as soon as I will get around to making them; RealLife(TM) has taken over until Friday evening.
BTW, I didn't adjust the quiescent current to 75mA yet, but it doesn't make a difference as far as the offset goes.
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Old 14th January 2005, 09:30 AM   #6
MikeB is offline MikeB  Germany
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Quote:
Originally posted by dmitriy167
different manufacturers use different pin arrangements for them, e.g. Fairchild and OnSemi have C,B,E from left to right, Philips has E,B,C. But I probably would have burnt them by hooking them up the wrong way around..
Exchanging C & E seems only to degrade quality, i tried once, and
the only thing measured was a drop of hfe down to 10.
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Old 14th January 2005, 12:14 PM   #7
paulb is offline paulb  Canada
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It's almost certainly a wiring error or a bad solder connection. It could also be a bad device, but that's not as likely.
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