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Old 6th April 2004, 10:14 PM   #1
VictorG is offline VictorG  United States
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Default How bad did I kill my Leach amp?

Alright, I made a dumb mistake.

After initially gettting my Leach amp to work, I unplugged it to screw everything in to thier final positions on the chassis. Unfortuantly, I accidently crossed the unplugged centeral ground with the positve end of one of the charged filter caps!!!

Massive spark as you might expect, but even worse, its seems that since in effect I made ground suddenly become +, I reversed the current on two of my caps, blowing C22 and C21. I mean literally they have holes on the top of them now....

At any rate, I have replacments for these on the way, but is there any otehr part i need to be replacing? There appears to be no more damage on the board, and the cross only occured for about a second.

The wierd thing is, with both boards connecting to the same ground, I only lost the two caps on one of the boards, the others remain are undamagaed.

If there is anything else I need to be checking, please let me know!

Ashamed of my stupid mistake,
Brian
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Old 6th April 2004, 10:40 PM   #2
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You won't know until you power it up. Every polarized component is now suspect. I would not suggest a wholesale replacement though, maybe you got lucky.

You can start by checking all the semiconductors. If you have an incircuit transistor tester, use it on each one. If not, use the diode range on a DVM, and look for reasonable drops from base to collector and emitter on all devices. You may weed out the one or two (hopefully) that are shorted that way. Honestly, you are in the twilght zone --- if a fuse blew quickly, maybe you got lucky.

I always bring up a new amp on a variac and it has paid for itself a hundred times and more. You would have stopped right away if you had done that.

Don't feel too bad, because we've all done something equally bonehead. I remember a scope ground lead slipping and blowing an entire computer power supply on a consulting job I was doing. I had to pay to replace the supply because their accountants hemmed and hawed and I had to get the job done.
I still made money, but it bit into the profit.
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Old 6th April 2004, 10:44 PM   #3
VictorG is offline VictorG  United States
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slowhands- clapton fan? Just love the guy and just getting into his material.

Back to the amp,
with a DMV, how exatly do i measure for faulty transistors / what is an unresonable "drop"

Thanks,
Brian
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Old 7th April 2004, 04:41 AM   #4
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Most junctions are between .55 and .65 on my DVM. It puts a little power out to forward bias the junction, and at that current the Vbe is lower than in typical operation. If you see less, there may be a resistor in parallel. If it is rising, there is a cap in parallel. This is just a cheap and dirty quick test, but you find shorts and opens fast.

Consider picking up a used transistor tester on eBay, a Sencore Cricket or super Cricket might be $50 and worth it (or an old Heathkit).
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Old 7th April 2004, 05:17 AM   #5
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Poor man's variac...

Best thing I did was buy 4 alligator clips and a handfull of 100r 0.25W resistors. Clip them across the rail fuse holders when you 1st power up, and your amp will be almost 100% protected (Almost, not quite)

Adrian
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Old 7th April 2004, 10:35 AM   #6
DRC is offline DRC  United Kingdom
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Quote:
Back to the amp,
with a DMV, how exatly do i measure for faulty transistors / what is an unresonable "drop"
Assuming all bipolar transistors:
Use a "known good" spare transistor and your DVM on ohms range so that you know what to expect. Test b-e / e-b / b-c / c-b (2 will read open circuit and 2 will read the conducting voltage drop). The in-circuit mesurements will differ from this as described previously.

Happy hunting
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