shorted ppa?
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groovy2

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hi all, this is a repost. My last post was in the wrong thread i believe. i finished building a PPAv2 and it sounded great, but overnight it has stopped working. I'm not sure what happened to it, as i live with several other people who use it but when i turned it on I was not getting sound from right channel. Checking the offset yielded a +6V! Q3G+ was getting to hot to touch so thinking maybe this was the issue i replaced it and new offsets are: L-1.5mV, R-1.5mV, G-.44mV. Anyone have any tips on how to find the shorted part. Isolating the rails might help a little and if i'm reading them right (ahem..i've no idea wt_ i'm doing) removing R1 will take a rail out of the loop. Any suggestions/help with troubleshooting are greatly appreciated. I've been spoiled by this amp; my music just isn't the same anymore


EDIT: getting .73V across D1???
 
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bhjazz

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Okay first off, I have not built a PPA, so take this advice with some salt. With previous projects, I have tried to run down similar problems. The two things you can do that come to mind are: 1) shut the unit off and visually inspect your work. Use a low powered magnifier if necessary 2) trace the circuit from top to bottom while it is running. Be careful of the voltages, obviously, but trace the channels and take some notes then compare.
 
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n_maher

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It'd also probably be a good idea to take some clear, detailed pictures of both the top and bottom of the board and post links to them. That'll help us understand exactly what we're dealing with.
 
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tangent

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If you have spares for that transistor, I'd replace it. Don't bother to hFE match or anything like that. If that fixes it, great. If not, then there could be something rather deeply wrong with the broken channel which is killing output transistors only as a secondary effect.
 
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groovy2

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I spent a couple hours last night combing thru the parts none of the readings have stuck out yet. Before i replaced Q3g+ I was getting sound out of the left channel, after replacing I've gotten nothing but the pop when the amp is turned off. So while the offsets seem to be closer to 0(although negative) I now have no sound from left channel. So i do think the problem is earleir in the circuit than Q3. Ok here are 2 pics bottom/top, I couldn't identify any burnt out parts, i cleaned the bottom of the board before taking these pics so forgive the slimy appearance:

http://photobucket.com/albums/a287/g...t=P1010003.jpg
http://photobucket.com/albums/a287/g...t=P1010403.jpg
 
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steinchen

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are the DC offsets -1.5V or -1.5mV ? Hope you didn't connect your headphones to an amp with an offset of several volts.

to what currents did you dial the classA bias and the output transistor bias ?
 
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groovy2

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they're at -1.5mV right now. Before replacing Q3 the right channel was a whooping 6Vs! My headphones are ok tho, using a pair of cheapies for testing.


edited 1st post w/correct offests
 
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