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Old 22nd August 2013, 06:14 PM   #1
davym is offline davym  Scotland
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Default sos magic smoke ..... help please!

I finished installing and connecting up the boards etc inside the case today, wish i had tested it better but as it had seemed fine outside it's case i didn't bother, first time too, doh!

Anyway the PSU took a hit, seems to have fried a couple of diodes at least but i'm stumped as to how it happened. The wiring checked out, i checked it over at least 8 times before i powered it up.

The affected diodes are clearly shorted and cooked but i dont see any other visible damage.

I have tested the obvious and now i'm stumped!

I measure a dead short between PG+ and PG- on both amp boards with the PSU totally disconnected, shurely this is not right! does this mean i have also somehow fried both chips? I've taken some pic's, can some kind soul please have a look and give me some pointers as to what went wrong.
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Old 22nd August 2013, 06:18 PM   #2
davym is offline davym  Scotland
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Old 22nd August 2013, 06:20 PM   #3
davym is offline davym  Scotland
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Old 22nd August 2013, 06:22 PM   #4
AndrewT is offline AndrewT  Scotland
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Use a mains bulb tester every time you modify your project.

Could the underside of any of the PCBs be shorting to the Chassis?
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Old 22nd August 2013, 06:26 PM   #5
AndrewT is offline AndrewT  Scotland
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Are any of your brass washers or metal standoffs shorting traces?
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Old 22nd August 2013, 06:30 PM   #6
AndrewT is offline AndrewT  Scotland
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I suggest that for safety, you double insulate your mains wiring.
And add a rubber boot around the IEC socket. Plenty of well secured insulating tape is nearly as good. I use a tyrap to secure it.
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Old 22nd August 2013, 10:20 PM   #7
davym is offline davym  Scotland
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Hi Andrew, I know I should have used the tester again, couldn't find it at the time, took a chance, kicking myself now!

Had visitors and a couple of drams which DID help me, they just left so i'll have a go at the amp tomorrow, probably/hopefully it's a short as you suggest caused by the standoff washers or some such issue. Have to get me some new diodes though at the very least.

I was going to wrap the mains socket in self amalgamating tape (great stuff that)before the lid went on, that'll have to wait a bit now.
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Old 22nd August 2013, 10:25 PM   #8
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The washers that you have got under your standoffs look dangerously close to the PCB traces to me. You should be using insulating washers there.
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Old 22nd August 2013, 11:31 PM   #9
davym is offline davym  Scotland
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It annoyed me enough to have another look, it appears that I bridged a track on the left amp board to ground with the brass washer which is a tiny bit wider than the screw head, enough to just land on the very edge of the nearest + track. This is an issue with this board, there is a small copper ring where the screw head lands but it is separated from the + track by a gap of slightly less than 1mm. If I had not bothered fitting washers or used nylon standoffs, it wouldn't have happened!

Anyway with the boards now unscrewed I get the following readings from both amp boards.

PG+ to ground = total short
PG- to ground = total short
The left channel did read a short between V- and ground before I unscrewed it's board, it now seems ok, well open circuit anyway?

Pity I did not think to take these reading before I messed up, doh!

Anyway I can't test the chips so i'll have to get a set of PSU diodes and replace them then start testing all over again. Unless anyone can verify, going by my readings, if the chips are damaged. The amp board readings were taken with the PSU totally disconnected.
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Old 23rd August 2013, 12:53 AM   #10
davym is offline davym  Scotland
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This is a case of my paying attention to detail like using a magnifier to check the tracks and joints etc (my eyes are not as good as they used to be) I checked all connections many times over, then I go and slap some screws with washers on right where they would do damage.

Worst of all, since it seemed fine out of the case and I couldn't find my tester rig I went and plugged it in. An amateur mistake from someone who should and does (normally) know better! Guess how I feel now
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