OK. Test results are in. When I swapped out the entire RX unit (everything else the same) the problem disappears. Next I swapped out just the crystals (from working set to potentially damaged set) and the problem is there. Thus it isn't the crystal but does appear to be a problem with the Rx unit.

Bummer. I suppose all I can realistically do at this point is bring it with me on my Xmas trip to the southern Cal (just a couple of miles away from the Airtronics headquarters) and see if they can bring it back to life. I suppose I could try to ship it off to Sanwa here in Japan but who knows how long that would take with all the confusion inevitably involved in these sorts of "cross-cultural" encounters.

As far as what specifically is wrong with the Rx, DrI is certainly right: nothing an amateur can fix. But it is curious that I get a certain type of cut-off. The Tx antenna needs to be extended even very close. Makes me wonder if there hasn't been some damage to the Rx antenna.

Guess I can always fly it with the other transmitter until I can get a replacement.