Warren:
I haven't a clue what a Zener Diode is
I had one good transistor left, which I removed and used for the 'critical' one listed in the instructions. All of the other transistors were replaced with new equivalents as recommended.
Don't know how to test the unit, but the microswitch and solinoid check out ok, so I either blew it on the rebuild or maybe there's something in the wiring that I missed.
Thanks for any advice you can provide!