These instructions will work for MVP's on any band. Before you start, test the radio completely to establish a benchmark for post conversion testing. It is better to find any problems that exist in the basic radio and fix them now. After the conversion you only need to find the problems you created. MVP C.G. encoder/decoders are pretty useless after the radio has been duplexed so now is a good time to get rid of it. Don't forget to replace the resistor between H1 and H2 on the System-Audio-Squelch board (The SAS Board). Most of the duplexing is done on the SAS board.
Remove the SAS board from the radio.
Cut the printed circuit land that connects J904 pin 2 to U902 pin 7. The best place I've found to do this is right near U902 pin 7 on the solder side of the board. There is a land that goes from U902 pin 7 to a via pad. This trace is only about 1/8" long and is shown in gray in the MVP manual Outline Diagram. I use an Exacto knife to make these cuts. Make two cuts perpendicular to the run of the land about 1/16" apart then remove the land in between by prying up one end and pealing the copper away. This disconnects the receiver oscillator control line so that the U902 no longer can turn the oscillator off during transmit.
Place a jumper between J904 pin 1 and pin 2. I do that right at J904 on the solder side of the board. This puts regulated 10V back to the receiver osc. control line so that the oscillator is always enabled.
The next cut is a little harder to find. The line on the schematic that connects the junction of CR901, U902 pin 6, and J906 pin 5 to the junction of R905, R906 CR905, C906 and U901 pin 7. On the schematic it's a nice straight vertical line just above U902 but on the board it runs all over the place in and out of via's, from the solder side to the component side and back again. The place I cut it is on the component side of the board near J905. Set the SAS board on the bench with the component side up and the board oriented as shown in the MVP Manual Outline Diagram. On the Outline diagram you can see two component side lands that run parallel along the bottom edge of the board the whole length of J905. The bottom most land connects H16 to J906 pin 4. Don't cut this one... The one you want is the next one up. It kind of looks like and S on the board and runs between J906 pin 5 and a via just below U902 (next to H5). I make the cut near the end (pin 7 end) of J905 before it makes a turn and goes up toward U902. Make the cut as described in 2) above.
All of the I/O lines to the radio can be found on the SAS board and should be connected before you put the board back in the radio. PTT is at H17. This is a ground to transmit line. COS is at J912. This line is ~0.1V when the radio is squelched and ~9.0V unsquelched. Don't try to run a relay from this COS line. I'd use a CMOS gate to buffer it. If you're using a modern controller chances are that it uses COMS logic and the buffer isn't required. Mike in is at J913. This line has mic bias on it and should be capacitor isolated. It takes about 2V P-P to drive the radio to 4.5 Khz deviation. Receiver audio is found at H16. This is unprocessed audio. It is not squelch gated and is not de-emphasized. There is ~4V P-P with 4 Khz deviation on an on frequency signal applied to the receiver. This should not be loaded by less than a 10K input.
Put the SAS board back into the radio.
Unscrew and unsolder the SO-239 antenna connector and remove it. (If this is a UHF radio I'd throw it away and get an N connector but it's up to you). After the conversion this will be the transmit antenna connector.
Unsolder and disconnect the coaxial jumper at H2 on the Filter board. Disconnect P1 (this is the antenna relay control and can be cut off and pulled out of the harness or left alone. It won't do much when you're through).
Remove the filter board from the radio.
Using solder wick remove as much solder from around the antenna relay can on the component side of the board as possible. Also use solder wick to remove as much solder as possible from the relay pins on the solder side of the board. If you can remove enough solder from around the pins you can break them lose with a small screwdriver of knife when the solder is cold. This will make the relay easier to remove. With a small pair of diagonal cutters grab a corner of the relay can. Using the soldering iron, heat whatever is stuck and remove the relay.
Now you need to remake the connection between the output filter and the antenna connector. I use a piece of center conductor from a piece of RG-58 to do this in the hopes that the dielectric and the conductor diameter will keep the impedance about right. This jumper should be placed as close as possible to the circuit board in the holes left by pin 4 and pin 7 of the relay.
Now you need to rig up an antenna connector for the receiver. THE MOST IMPORTANT THING HERE IS TO HAVE A CABLE WITH A CONTINUOUS SHIELD from the receiver connector on the front end casting to the antenna connector. This means no voids in the shield around the connectors at each end. If there are unshielded parts of this assembly the receiver will hear the transmitter and the resulting desense will be unacceptable. Use a good quality connector like a BCN or TNC (or type N if you can make it fit...). I like all of the connectors to come out the back of the radio. This is real handy from the point of view of working on the radio later and looks real nice but is a pain to do. If you chose to do this (or put an accessory connector on the back panel of the radio) you need to take all the boards out of the radio and mask off the back of the radio from the rest with newspaper and masking tape to keep drill filings from getting into every nook in the sheet metal and causing problems later. Mount the connector as close to the edge of the cover opening as you can or it will hit the filter board when you put it back in. Make sure whatever connector and coaxial you use for this will make the turn to miss the filter can. You could mount all this stuff on the front (plastic panel) and save yourself a lot of work but it wouldn't look as nice and it only takes time to do it right.
If you choose to put an accessory connector on the back panel I suggest you put it near all the other connectors. To make room for this you will need to remove the little sheet metal doodad G.E. decided to put on the back of the radio. Don't forget to mask off the power connector especially around the base where it meets the back panel of the radio. Metal filings like this way of getting into the radio. The doodad is held by two spot welds. If you look carefully you can see where they are. Drill these down to the point where the drill is just getting into the back panel. Be careful not to screw up the power connector in the process. I put the accessory connector next to the power connector. I like 9 pin D type connectors. They are widely available and of good quality. The D shape makes them a pain to mount without a punch (the Greenlee punch for this is >$280.00). Get the connector as close to the edge of the cover cutout as you can. This keeps it up high in the exciter area and out of the way. You are working through two layers of sheet metal so be careful when (if) you tap the mounting holes as the tap tends to bind between the layers. Put the radio all back together now.
Put the output filter board back in and solder the transmitter antenna connector back to the tab on the filter board.
Retest the radio to make sure it survived the operation.
That's all it takes to make a $100-$200 MVP nearly as good a duplexed radio as a $1,000-$2,000 MSTR II base station!