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Author Topic: Metal baked transistors electrically conductive?  (Read 374 times)

erokit

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Metal baked transistors electrically conductive?
« on: April 28, 2012, 02:31:11 PM »

I blew up my power section after testing successfully without the heat sink attached. The new trans used metal back packaging. Is that why I had a short when I put it back together? Do I want to use mica insulators?
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Minion

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Re: Metal baked transistors electrically conductive?
« Reply #1 on: April 28, 2012, 03:39:08 PM »

All non-insulated transistors should be insulated with a mica or sil pad and an isolation washer .... You can get away without one if there is no way the heatsink will come in contact with ground ......
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J M Fahey

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Re: Metal baked transistors electrically conductive?
« Reply #2 on: April 28, 2012, 09:24:57 PM »

Agree and add: the metal back is normally connected to the collector or drain terminal, so they need to be insulated.
A *few* amps (some Peavey, and QSC big amps) use grounded collectors, so they don't use micas ... but it's something very unusual.
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erokit

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Re: Metal baked transistors electrically conductive?
« Reply #3 on: May 14, 2012, 10:11:53 PM »

Taking this slowly, as my enthusiasm has burned out the amp twice.

1) how do you fix broken eyelets and/or solder to the remaining lead on a pc board? My desoldering bulb just doesn't have enough vacuum.

So I caused damage shorting the power transistors. Likely any damage in the preamp? Or just the power section transistors including the phase splitter. How about the bridge rectifier. I have checked these myself and am replacing. However I have somewhat a confidence problem after my rush to success  :'(
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J M Fahey

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Re: Metal baked transistors electrically conductive?
« Reply #4 on: May 14, 2012, 11:55:28 PM »

If you "lost" a couple pads, you do nothing too special, either bend the part leg towards the remaining track and solder it there (you may have to scratch the anti soldering paint to leave copper visible) or you make a small wire loop which you solder to the remaining track , the end ring becomes the new "pad".
if you destroyed more than 5 or 6, the bpard can be considered "lost" because even repairing all, it will be too weak and easy to crack again , simply by vibration.
Not reliable.
To avoid losing the amp, maybe you can buy a power amp kit and mount it there, using the original power transformer, plus the preamp , cqbinet and chassis, of course.
If the original PCB is too damaged, it may be the best option.
What amp is it?
Post a couple gut pictures.
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