I found a bad resistor! And that’s not all...
Fist of all, thanks everyone for the responses yesterday. I’m familiar with some electrical stuff, but a noob when it comes to fine electronics. I really appreciate your input.
Since this CCRM doesn’t have a traditional coil-type relay for the AC circuit, I started poking the other components on the board with my multimeter. One of the first resistors I tested (R9) came up bad, giving me a fussy reading of somewhere around half of its stated resistance.
But before getting too excited, I continued testing, and soon found another bad resistor (R11). And a bad diode (D4).
Since I was doing a round of individual component tests, I went ahead and started trying the relays one more time, just for kicks. Surprisingly, *pop* one let the smoke out! I have no idea why. All four of them were good just the other day. Alas, the low-speed fan relay now has zero resistance across its coil. Well, that sucks. I wonder if one of my test probes slipped earlier. Maybe I errantly discharged a capacitor or something.
I can see now why remanufactured modules are so expensive. Between the diagnosis, labor, and testing involved, a hundred bucks doesn’t sound so bad after all. But I like learning, and am not quite ready to give up on this board yet.
Speaking of learning, I hear that capacitors can’t be tested in-circuit. And I’m still unclear as to how to perform a complete test of a transistor, but a preliminary diode check between Base and Emitter terminals is showing 3 of them (Q4, Q5, Q6) to be bad.
The mosfet on the side, which appears to be an important part of the AC “solid state” relay circuit, also failed a diode check between the B and E terminals.
So it looks like this board will need two 100kΩ resistors, one relay, one diode, three transistors, a mosfet, and maybe (?) some capacitors. Is it worth it? Should I just trade it in (core) and get a remanufactured unit?
BTW, I’m having a hard time getting good close-up pics, but I stumbled across this TCCoA thread, which contains excellent photos of a board identical to mine. Check it out for a better view of what I’m looking at.