First I opened up what turned out to be the source module - the one that provides the infrared light "beam" for the sensor to pick up. Not visible signs of problems there. Then I opened up the other one, the sensor module. After a few minutes of tracing out the circuit, I ran across the obvious. Can you spot it (click the pic to expand it)?
In case you missed it, there's a pin near the upper left corner of the board that isn't soldered in. That is the collector pin on a transistor. Pretty important connection to be made in most circuits. I guess this one was bent over far enough that it was contacting the pad on the PCB and had worked that way for untold years. ICs on various boards in the system have batch numbers from late 1997, the house was built in 1998 so the silly thing may have been working for as many as 12 or 13 years with this flaw. I figure either I, the cat or the kids wacked into it one time too many and jostled that delicate connection loose. Anyway, after a bout with the soldering iron, I put it back together and it works fine again.
This experience was similar to another one recently when the blower fan for the air handler in the central heat and air system stopped running. In that case, the fan control PCB's faulty connection *had* been soldered in, but the pad on the PCB apparently was not designed to handle the current. Over time, it melted away the solder first, then arced away at the remnants of the pad until it finally had nothing to arc to. Soldering a piece of 14 AWG wire across the right points took care of that one.
Funny thing, there - I knew exactly what to look for because precisely the same thing had happened at the "old house" a couple of years earlier. Took me about an hour the find and fix the first time and only 15 minutes the second time.
