[sdiy] Unusual transistor failures

[Steve Lenham]

Thanks for your suggestions, chaps.

On this occasion I can more or less rule out fakes/cheapos - these are 
the original (TI branded) devices from 1980, with the dabs of factory 
screw-locking paint still intact on their fixings.

They are indeed TO3s - I'll get the hacksaw out later on and see what's 
inside.

Cheers,

Steve L.


On 11/09/2012 12:31, Tim Ressel wrote:
> 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
>
>
>
>
>> ________________________________ From: 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