Very interesting read and well done for seeing the problem solved.
My own experience of the P.C board was after a bird strike which broke the fan blades.
Before I refitted the fan assembly I tested the relays by putting 12 volts across their coils, and then refitted the fan assembly, only to find that the slow speed did not work.
Upon further investigation one of the relay coil switching transistors had failed (surface mount PNP device had gone base/emitter short circuit). I had a quantity of BC327 transistors laying around and mounted one of these in place of the surface mount device. This solved the problem.
I think that testing the relays while still in circuit damaged this transistor due to the back EMF of the coil, and point this out as warning to anybody testing the relays in this way
Cheers, Pete.
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