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Old 5th May 2004, 05:00 PM   #1
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Default p3a bias trouble, quick question :)

One channel of my p3a biases fine... can adjust it as I need... the other channel bias cannot go below about 300mV at the emitter resistors... I can bring it higher, but not lower than 300mV across the emitters. Any ideas?

-Matthew K. Olson
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Old 5th May 2004, 08:05 PM   #2
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Hey Matthew, I saw that exact same problem in my friend's P3A, one channel worked fine and the other channel drew 200mA all the time. It turned out nothing was really wrong and there was a bad solder joint somewhere. However, it was pretty clear the problem was in the CFP output stage. You can verify the Vbe multiplier is working by checking that the base voltages of the output followers are near ground (1V and -1V, approximately). And of course check the input stage current but it better not be that high.
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Old 5th May 2004, 08:42 PM   #3
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See, this is where my ignorance kicks in, what's an output follower? Someone else stuffed and soldered these boards for the most part, and has claimed that they work fine. I don't mind checking voltages, but beyond telling me ie between r9 and r10 or something like that, i'm lost. Sorry

-Matthew K. Olson
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Old 6th May 2004, 06:49 AM   #4
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I don't know a huge amount about this amp, but if you have one channel that works, you need to start checking voltages on the working unit, and compare with the one that doesn't.

Ya oughtta be able to handle that
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Old 6th May 2004, 11:03 AM   #5
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Tis a good thought... thanks... keep those ideas comin!

-Matthew K. Olson
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Old 6th May 2004, 06:32 PM   #6
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Kilentra, i checked Q5 and Q6 base voltages on both left and right channels. The left channel (the one that works) is about .7V+- the right channel is about -.6v and +.3v does that sound like something's wrong? I'd just like to know where i need to focus, especially if i'm going to be takin the board off the heatsink. Later!

-Matthew K. Olson
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Old 6th May 2004, 06:40 PM   #7
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http://www.instantboard.com/users/ro...light=p3a+bias

Almost the exact same problem here, only mine is reverse in polarity, i get -.6v and +.3 instead of the other guy's problem which was .6v and -.4 ????

-Matthew K. Olson
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Old 6th May 2004, 08:10 PM   #8
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Haha, that post is about my friend's amplifier. Anyway, as you can see in the diagrams posted on that page, something in the circuit of Q5/Q7/R11 or Q6/Q8/R12 (those are the CFP followers) is messed up. Though it is really strange that the voltage across the bases of those is negative. Definitely check that Q9's in correctly and its base-collector resistance is 1k and base-emitter is 2k. And also, see if all the other voltages line up similarly to the working channel, or the first diagram in that other post.
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Old 6th May 2004, 09:07 PM   #9
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Ok, i reheated the solder joints...no help. I checked the 1k and 2k etc....thats fine. Turns out my problem is slightly different from your friend's problem. The voltage at r9 is sorta ok. its about 600-700 mV. The voltage at r16 is the problem. I measured on one side of r16... .35mV and on the opposite side, .08mV BUT...heres the catch, r16 itself measures fine. On the good board, r16 measures 600-700 mV on one side and .22mV on the other.

Also, between r9 and r10 on both boards measure fine...14.7V which is fine because i'm running 29v rails. Now i'm really stumped. If i thought it was r16...i'd replace it. No problem. But thats not it, the pot is fine apparently, etc. ??????????

-Matthew K. Olson
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Old 7th May 2004, 09:56 PM   #10
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Ok, upon further inspection of the schematic, i figured Q4 was a more likely culprit. Instead of replacing everything, i just replaced Q4 to start. Sure enough, i replaced it...everything's fine now. I still don't know if the thing plays music, but it biases just fine. Thanks guys!

-Matthew K. Olson
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