Phil Allison wrote:
"Sylvia Else"

The main switching transistor appears to be intact. I haven't removed it
from the circuit, but the voltage/current plot given by a component
tester feature on my oscilloscope gives the expected traces for the base
collector and base emitter junctions, allowing for the presence of the
diode across the base emitter junction.

I further surmise that the failure of Q1 doesn't explain the failure of
the 8.2 Ohm resistor, because even with its specified maximum beta of 40,
the switching transistor wouldn't pass enough current to burn out the
resistor.

The implication is that some other event has done for the both of them,
but surprisingly not destroyed the switching transistor.

sigh I thought I'd understood the sequence of events, but clearly not


** In an earlier post you state the PCB had been subjected to water ingress
and insect attack - so almost anything is possible.

Odds are, the switching transistor was forced hard on by the above, taking
out the 8.2 ohms and Q1 in quick succession.

Relace whatever parts you find are damaged, clean the PCB carefully with
detergent and a brush (as you would washing glass ware etc in the sink )
rinse with clean water and dry thoroughly with lotsa hot air ( ie a hair
dryer).

Then cross you fingers and try it again.

If all is well, coat the PCB is clear acrylic lacquer to help it survive.


It's true that I mentioned brushing away a dead spider.

I wish I'd noted exactly where the spider was. It conceivably was around
the 200K resistor, which could explain both the failures, and the demise
of the spider.

Anyone know the resistance of a small spider before it dies from shock?

Sylvia.