As I remember, but could not find on the schematic, there are two transistors that receive the combined TTL RX and TX signal from the CAT jack and separate it into the TTL separate RX and TX into signals for the logic circuit. These two transistors are where I would look for the loss of signals. I have had a problem here before, but the details have faded into the lost memory fog...
IF someone can guide me to the proper page I can look again, but if you find the proper input I think you can fairly easily track the signal to these two SMD transistors.
JD
KE4MD
