Maybe IC3001 or IC3002 defective ?
Or the transistor is open-circuit?
The transistor (tested with multimeter) seem to be good (the two junctions
are ohmnicalli corrected)
I can try to substitute it. Now I haven't the board near me, I not remember
if is a standard NPN
I think it is just a standard NPN transistor. From what I remember, it
had a non-standard 'house number', but something like a 2N3904 will
probably work.
Still, you're now down to one simple-ish
stage that presumably you can
debug.
Yes , the only question is : what is the tolerance of these protections ?
As yuo know, there are 8 comparators on this board, 4 for overvoltage and
4 for undervoltage (no, it's not, IIRC, one chip for overvoltage, etc).
You can trace back the sircuit from the base of the each of the 2
transistors to find out which comparators are for each function.
I think what I might do next is get a spare comparactor chip of the same
type (LM339) and connect the power and ground pins to the power and
ground pins of the chip on the board (I am not _sure_ they both have the
same power and ground connections, check this!). Then link the inputs --
right way round -- to each of the 4 comparators in the undervoltage
circuit in turn. Put a suitable pull-up resistor (to the comparator power
connection on the output). See if any one of the 4 undervoltage
comparators appears to have an output that's always low (remember, a low
output from the comparator will cut off the transistor and therefore
trigger the SCR).
You may find one of the output voltages is not coming up properly.
-tony
