As you found.
The XM19A has the power line transmitter in it.
The signals are common ground, DC+, keying pulse from the SC1200 to key the X10 transmission on and off and the Zero Crossing pulses for the SC1200 to time the transmitter keying pulse.
You may want to look at the DC+ and Common to see if it is excessively noisy or the Zero Crossing signal is messed up. My usual suspects would be electrolytic capacitors drying out.
Another possibility would be the power line transmitter circuit. Getting the proper keying signal but not putting a signal on the power line.
I have not seen any schematics or service tips for it. Maybe a similar function supply may have information on it you could use as a rough schematic of the XM19A.
I did roughly tune one with my XTBM showing signal strength and trim it closer to the 120KHz.
If worse comes to worse. X10 sells a replacement XM19A. Though they show it out of stock.