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Burning Amp BA-3
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Old 21st July 2013, 06:30 PM   #1071
bcmbob is offline bcmbob  United States
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Wish this was my first build so I could claim nubee status, but I found an unsoldered lead on the bias board at R213. Probably took out Q204. I'll get a new one and try again. Do you think the caps should also be replaced?
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File Type: jpg Bias Sch.JPG (111.3 KB, 306 views)
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Old 21st July 2013, 06:31 PM   #1072
buzzforb is offline buzzforb  United States
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Use schematic and put voltages you have in important places. You can start at FE and work back. You do not have to have FE hooked up to bias/output combo, and vice versa. Perhaps you can isolate the problem this way.
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Old 21st July 2013, 06:33 PM   #1073
bcmbob is offline bcmbob  United States
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Will do. I'll publish when completed.
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Old 21st July 2013, 07:40 PM   #1074
buzzforb is offline buzzforb  United States
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BTW, the unsldered resistor was essential part of bias net, so I can see having some issue.
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Old 21st July 2013, 07:42 PM   #1075
bcmbob is offline bcmbob  United States
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Sorry, took a little "old mans nap". Just starting readings now.
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Old 21st July 2013, 08:49 PM   #1076
william2001 is offline william2001  United States
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I wouldn't assume Q204 is bad. Before a re-attempt, I would make sure the front end and bias circuit were functioning normally without the output stage. If the front end voltages are normal, and DC offset is zero and stable at R215, and that top bias circuit is set to output ~4.5 volts or so at R214... then I would start thinking about re-connecting the output stage. And with a bunch of unmatched output devices you could have some doing all the work and some that are just "loafing".
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Old 21st July 2013, 08:55 PM   #1077
bcmbob is offline bcmbob  United States
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Mouser order for sure. I had five measurements done and went for the big blue circle. Board looks OK, but I'd rather start over.

Thanks and I'll be back probably Tuesday.
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File Type: jpg B1.jpg (137.2 KB, 285 views)
File Type: jpg B2.jpg (95.8 KB, 251 views)
File Type: jpg Bias Sch1.JPG (76.4 KB, 269 views)
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Old 21st July 2013, 09:05 PM   #1078
buzzforb is offline buzzforb  United States
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Just remember you can do it in segments.
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Old 21st July 2013, 09:17 PM   #1079
bcmbob is offline bcmbob  United States
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I did. Removed the bias board and attached the PS directly. I wrongly read the schematic as 5 volts coming from the negative rail to Q204, but either hit the wrong pin or shorted two and sent the full load to the transistor. I have a tester function on my meter that I have never used, so I'll dig out the user docs and check it out. Not too hopeful though.
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File Type: jpg 20130721_171001.jpg (81.2 KB, 135 views)
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Old 21st July 2013, 09:30 PM   #1080
william2001 is offline william2001  United States
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I find that this schematic is a bit easier to follow than the one from the store.
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