SM7OVK RX/TX split


In the first modification I put an extra input N-connector on the rear panel and a switch. With this switch it is possible to change between the original connector, then using the original relay, or to the new rear connector and then using a outside coaxial relay. This N-connector was mounted instead of a small cover-plate. The switch is a small commonly available switch, something you normally would not see on VHF but since the path to the front-end is already far from perfect I have not really noticed any difference, (at least not with a pre-amp at the antenna).
The arrangement is made by feeding the mid-position of the switch to the front-end board, PB1746 pin5, or the MuTek board if you have one. The other two positions of the switch are connected to the new N-connector and the normal feed of the front-end pcb. Use only coaxial cable and as short pieces as possible.
I decided not to have any DC current on this receiving-input. If I put any filtering before the FT225 it's often DC-grounded, so I stick with my bias-Tee.

Transverter output

As a complement I placed one more switch, same type as mentioned above, where it is possible to choose if the drive should be forwarded to the power-amplifier, ie the power-amplifier of the FT225, or get out on a BNC-connector on the rear panel. This is superb for transverter use. This cost some of the phono-connectors on the rear-panel but I think it was worth it.
With this and the extra RX-input it is possible to connect most transverters directly without any extra coaxial-relays.
The mid-position of this switch is connected to the Exciter-unit, PB1762 - pin 17. The other two positions of the switch are connected to the Booster-unit, PB1744 coaxinput, and the new rear BNC-connector. Use only as short pieces of coaxial cable as possible and connect only the inner conductor to the switch.
The type of cable isn't important, as long it's 50ohm. I have used a RG174 since this is a thin cable and easy to handle. A cable with Teflon isolation would be preferable since it will be much more easy to work with. Double-shielded cable is always to prefer since it reduces the risk for RF-coupling between different stages.
I now have the possibility to choose if the drive power from the Exciter-unit should feed the Booster-unit or the transverter.
The output-level on my FT225 is 28dBm, but slightly higher levels are reported, see measurements. If this level is too high for your transverter it'll be easy to attenuate. You can build this attenuator by yourself, it's not critical and you'll find the resistor values in any ARRL handbook or similar publication.

Real RX/TX split

One explanation of the high total noise-factor of the receiver is the antenna changeover relay. The path from this relay to the front-end is far from perfect. An important first step is therefore to improve this path. My solution is to have two connectors on the rear-panel, as described earlier, one for receive and one for transmit. I have chosen N-connectors since they are very reliable but it would be possible to use BNC-connectors.
The received signal is fed from the N-connector with a thin coaxial cable to an SMB-connector mounted on the new front-end board. In this way one can be sure that the feed is 50 ohm all the way. With a connector on the front-end it is also easier to run the board on the test-bench. The original front-end uses the PCB as a connector and this is not ideal, not even for VHF.
It's still possible to use the original front-end board since the original RF-path is still there, but I don't use the receiver switch after this modification. (My idea is to keep the design in a way where it's possible to run the originals and therefore it's much easier to compare the new board with the old.)
After these modifications my rear panel looks like this. (On this picture there is an N to BNC adaptor on the TX-output.)