Hi
Sorry to be out of the loop. Unfortunately my job takes me away periodically.
I have supplied loads of interfaces for JR without any problem so I am sure we can solve this one.
Please do the following checks to help us focus where to look:
1] measure the voltage on the lead from your transmiter, first with your meter on a DC range and then on an AC range. DC should read '0' and AC around 1v.
2] with FMS running, the interface connected, but not the transmitter measure the voltage across the 5k6 resistor on the 9 pin D (R5 on my circuit diag). You should get around 13v. Now connect your TX. This should now drop to 8v or so.
If these tests work focus on the FMS setup - see detail on my website. If they do not let us know what you see and we will take a closer look at you interface.
Cheers
Mike