wrote in message
ups.com...
Well, D2 is toast. Shorted. I'm hoping that's all that's gone. The
transistors close by (TS1 & TS2) are all surface mount, so I'm not
planning on ripping them out just to test them. TS3 seems ok as LED1
still lights.

I'll get another zener diode in the morning, and keep my fingers
crossed.

Just out of interest - is there a quick way I can test TS1 & 2 whilst
still in-circuit ?

Cheers,

Kev.


R4, R5, R7 and R20 should have protected the four transistors. 10k should be
enough to prevent enough base current flowing and causing damage with 9v
applied.

I have to say though, it's a poor design that allows a fault condition to be
produced, when a ( comparitively ) small voltage gets accidentally applied
to an external user input. This sort of thing is bound to happen on a
regular basis with this type of equipment.

A 1k resistor in series with the tip connection, would prevent the zener
from failing, whilst still allowing it to clamp to 3.9v

Arfa