Sir Brett . . . . .
DAMN ! . . .Sam . . . . . do I see lamb stew coming up as entree, on the immediate menu ?
O.K. . . . . . I do see the proper ? SCR and a replaced 22 ohm film resistor nearby.
So is where I see the cleaned up spot, being for a poly cap ? . . . which is probably being the C element of an R/C snubber pair, which are shunting the SCR. , OR, its C element could be, that also nearby disc ceramic capacitor.
Now, in looking at that line up of conformally coated components across the bottom row.
Don't you see:
On row 1 :
Item 1 as being a resistor, then a space then possibly a hefty diode ( ZENER ? ) then a space and then a resistor and then a glass diode ( ZENER ? ) and then a vert mount poly cap and then a TO-92 cased transistor and then a film resistor and then a small diode (750 mw Zener ? ) and then a small axial leaded electrolytic and then 3 more resistors on the extreme end
Next row up:
A glass diode( ZENER ? ) to the right of the SCR lead and then a resistor
Top row:
Change mounting plane to hoz and find another glass diode ( ZENER ? ) and then a metal film resistor and a final resistor on top.
I would be testing to confirm if those mentioned locations are diodes and testing for being conventional diodes initially.
Then a power supply and a 1K current limiting resistor should let you bias them to test as being zeners.
Or, if being a 'po boy, with no adj power supply, one or two series 9 V batteries would suffice for up to 8 and then up to 17 volts, Zener threshold, test levels.
I don't know the original SCR you found being used, I am unfamiliar with the one now being used.
NXP BT151-500C . . . .as a 500V 12 A unit.
The mentioned TO-92 cased transistor may actually be a 2 lead Diac trigger diode for the SCR gate.
Does the low voltage output relate to there also sounding like there being a loading down of the engine ?
Thassssssssit . . .
73's de Edd
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Doing a good job at some places is like peeing your pants in a dark suit; you get a warm feeling, but nobody notices it.