ST60 Tridata no SeaTalk output? (Very Technical)

[nigelmercier]

I have an ST60 Tridata with a faulty Seatalk output. The output is generally at 0V, but with both positive and negative glitches which bizarrely coincide with the frequency of the input signal from the Paddle Wheel on the green pin (simulated in my case).

I've traced the Seatalk signal internally, and it appears to look like this on the TX output of the microcontroller. I'm inclined to think that the fault lies with a discrete component, perhaps a clock generator for Seatalk. 

Although I have a schematic diagram for this device, I don't have any details of the various signals around the uC. Can anyone help?