[sdiy] Unusual transistor failures

[Steve Lenham]

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