Sylvia Else wrote:

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.
To me it looks like a safety circuit that is designed that requires you
to pull the plug , so that Q1 being a thyristor in my case of thinking
will clamp down on the circuit from an over voltage, and remain that way
until you pull the plug and wait. The 8.2R could just be burnt out or
it maybe located in a thermo area intentionally to burn out (thermo Fuse)
in which case, Q1 would also clamp.

Either scenario will prevent the coil to be energized, if that is what
T1 is?

Since you haven't supplied any component part numbers, it's hard to say
really, if that is what's going on.

--
"I'd rather have a bottle in front of me than a frontal lobotomy"

http://webpages.charter.net/jamie_5"