[sdiy] Unusual transistor failures

[Tim Ressel]

Perhaps vibration? Open in all directions suggests you lost the bonding wires. To lose both bonding wires in two transistors? Vibration and/or cheap transistors. Or like the man said, fakes.



Tim Ressel
Circuit Abbey
503-750-9331
timr at circuitabbey.com




>________________________________
> From: Steve Lenham <steve at bendentech.co.uk>
>To: SDIY list <synth-diy at dropmix.xs4all.nl> 
>Sent: Tuesday, September 11, 2012 2:33 AM
>Subject: [sdiy] Unusual transistor failures
> 
>Hi,
>
>I'm restoring a unit with a large linear +5V supply using a pretty standard uA723 circuit. There are two parallelled pass transistors rated at 60V, 25A and 200W each.
>
>The +5V rail was dead (no volts) but there was plenty of base drive to the pass transistors. Removing them for a test revealed that they were both open-circuit in all directions.
>
>The unregulated side is all OK - correct volts, no blown fuses, etc. Installing a temporary pass transistor restores a perfect +5V output, indicating not much wrong with the 723 and drive (though I will check more carefully). The unit has not been previously tinkered with - everything looks original.
>
>I can obviously repair this, but in my experience that is a most unusual failure mode for bipolar transistors - I would expect them to go short B-E or C-E under fault conditions, blowing at least the fuse and possibly other stuff too. I feel a bit uncomfortable not knowing what went on.
>
>Has anyone else ever encountered a similar failure mode, or can anyone suggest an explanation for the why the transistors died this way with everything else seemingly untouched?
>
>Cheers,
>
>Steve L.
>
>Benden Sound Technology
>www.bendensound.co.uk
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