F5: How much smoke did I let out? - diyAudio
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Old 11th November 2009, 11:52 PM   #1
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Default F5: How much smoke did I let out?

Ok, so I abandoned trying to do my own p2p as I want to first do some builds, and get them right, to increase my knowledge and then I'll have some more fun. (By the title, you can tell this was probably a good idea!)

I built the Pass style PS on some breadboard I had, used cviller pcb's for the channels and Tech-Diy Fairchild kits.

I have one channel built and working, it's a thing of beauty! However...I'm sure I let the smoke out of Q3, maybe Q4, and who knows what else. When I put power to it the first time I was measuring across R11 and R12. Very quickly R12 went up to something like 16v while R11 stayed at 0v. I removed power and checked my meters to make sure they were on the correct settings and I was on the right parts. After confirming I was, I applied power again and got the same thing. I became gun shy about applying any more power in troubleshooting. I would like to troubleshoot on the board without removing parts, if possible.

So I reset the trim pots on my working channel to 0 and figured I would try to just test out the resistance of every part. (Is that a valid troubleshooting method?) R11, R13, R17 and R21 were all quite far off what I would consider a margin of error. I don't know if it matters, but R14 was off a bit as well.

Part__GoodCh__BadCh
R11_____0.5_____27.5
R13____47.5_____27.2
R17___129.8____150.0
R21___199.6____150.0

Did I fry Q3? Q4? What others? What else could I do, without removing parts (and potentially damaging other good parts), for further troubleshooting?
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Old 12th November 2009, 12:29 AM   #2
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Probably time to go back to basics and check all the component values and orientation before powering up again.

I tend to not connect the output transistors to start with and feed the output back into the ltp. In this mode its highly unlikely to blow any fuses.
Only when I am convinced the driver circuit is working right do I even consider adding output pairs.

Sadly switching it on with a fault can damage components and leave youy in a bigger mess than you had to start with.
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Old 12th November 2009, 03:58 AM   #3
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Yes, I should have been more careful with the power up sequence. I have read various methods about how I should have done it, but it all seemed so simple and I didn't see how I could screw up basic soldering. I thought I had tested out the PS properly before I connected the channels, but obviously not properly enough. Oh well, fortunately it's a cheap learning experience!

All the individual part values are as they should be, as well as their orientation. I believe I had a bad ground/short in the PS that caused V+ to go over voltage thus causing the smokage. I went back and triple checked every single connection in the PS and then did the same on every part on both channels. I have no intention of providing power to this any time soon which is why I'm looking for assistance on any possible no-power-keep-parts-on-board diagnostic techniques. If there aren't other methods or it cannot be determined what the damage is in any other way, I'm probably just going to buy another board and parts kit so I can get this baby singing. Then I'll go back, do an autopsy by pulling all the parts off the board to do individual component testing, and keep the good parts as spares for some other project. I would like to see if I could salvage this channel though.
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Old 12th November 2009, 07:41 PM   #4
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If you de-solder parts from that board use care. You may only get one more run at it. It won't survive more than 2 rebuilds.

Is that 199 millivolts?
Did you try to test resistance with power to the amp? If you did...don't.
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Old 12th November 2009, 10:38 PM   #5
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Yes, de-soldering is not high on my wish list.

No, that's ohms.

No, it was power off.
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Old 15th November 2009, 01:10 AM   #6
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I think the easiest thing to do is remove the active components, check values of the resistors (if burned or not), check the Idss of the Jfets and test the mosfets out of the circuit. You can use one of the matching jigs on passdiy.com to test the mosfets.

Or, like you said, buy another board and start over. Do a post mortum later on the removed parts.

After all said and done it could be a cold solder joint somewhere. Troubleshooting stinks sometimes.
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Old 16th November 2009, 02:47 AM   #7
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Thank you for the follow up. Since I have not received any other ideas on how to do it on board, it looks like off board is the route to take.
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