Pearl shorted...please help me out of this! - diyAudio
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Old 28th May 2007, 06:00 PM   #1
h_a is offline h_a  Europe
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Angry Pearl shorted...please help me out of this!

First channel of the pearl soldered today. I have a nice short somewhere, would be great if somebody could shortly take the time and help me a bit!

Being a genius, I left out R8 connecting the regulator with the actual amplifying stage to test first the regulator.

Surpassing myself I also forgot to solder Q2 and Q12.

The result was that R2 and R3 start to get cookingly hot (the 3 Watters of the regulator).

After disconnection and soldering in Q2 and Q12: no change (now circuit fully populated).

I don't even get proper voltage of the toroid, seems the voltage completely breaks down because of a short.

Measuring the resistance between PAD1 and PAD2 (where the psu-board leads connect to the pearl) I get a resistance of 9.5 Ohms indicating that the short is after the 2 power resistors (being 4.7Ohms each).

So what's wrong?

First I thought that I soldered the snapin elcos with far too much solder so that it made a short with the above lying ground plane (btw I failed desoldering them ARRGG), however this shouldn't be possible due to the laquer on the pcb.

Shorted diode also nay, since there would be still the 3k32 resistor R5 giving a higher resistance than I measured.

Any tips greatly appreciated! Thanks a lot guys!

Cheers, Hannes

PS: btw the rail (coming from psu-board) is unloaded 39.9V DC.
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Old 28th May 2007, 06:03 PM   #2
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Probably shot the IRF9610? Maybe exceed the 20V gate source voltage? But how should this be possible with the 10k R6 in front...

Hannes
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Old 29th May 2007, 09:00 AM   #3
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Nobody keen and clever seing my obvious mistake instantly?

Naaah, I think you guys want me to learn? Ok so there will be a analyze/desolder session in the evening...

However if somebody wants to give me a hint... then give it!

Thanks! Cheers, Hannes
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Old 29th May 2007, 03:31 PM   #4
cviller is offline cviller  Denmark
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Have you checked the polarity of the 10mf capacitors?
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Old 30th May 2007, 08:51 AM   #5
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Polarity is ok.

Well, what I know now is that the short is after the 2nd 4.7Ohm resistor (R3). After this R3 I measure zero resistance between signal and ground. Resistors R5, R6, R7 measure ok. The IRF has no short (and does not have a connection to ground anyway).

I think one of the big 10mF caps is shorted. I don't know wether this happens now and then, but I see at the moment no other possibility.

I desoldered the R3 and the voltage after the 1st 10mF cap is fine (40V DC).

At least I learned now to measure also the big caps before soldering them in.

Desoldering snap-ins is really a pain.

Cheers, Hannes
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Old 30th May 2007, 12:12 PM   #6
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Feed the scientists! My pearl does not obey Kirchhoff!

The remaining 2x 10mF elcos are now desoldered. Both are perfectly ok!

Short still there! All resistors connecting in some way to ground are also ok! Fet is ok!

Everything is ok! Though it is shorted!

Cool

Hannes
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Old 30th May 2007, 12:16 PM   #7
cviller is offline cviller  Denmark
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Post pictures and schematics with measured voltages, then I or someone else might be able to help.
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Old 30th May 2007, 12:57 PM   #8
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Oh it's really easy! After R3 I get zero resistance between signal and ground. Both elcos after R3 are now desoldered, R5, R6, R7, R8 measure ok! Fet measures no short.

I can post a pic a bit later! Thanks a lot, Hannes
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Old 30th May 2007, 01:07 PM   #9
cviller is offline cviller  Denmark
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Zero resistance is not much - if C2 and C3 are out of th e picture, then only C4 has direct contact to gnd and I doubt it is shorted. Sounds like you have a "mechanical" (not component) short somewhere...
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Old 30th May 2007, 01:08 PM   #10
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Is your heatsink by any chance grounded?
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