Modifying the MASTR II System Board for Full Duplex In my conversions, I used the MASTR II control head and control cable. I found that it was nice to have a control point at the repeater site. If you choose not to use the control head, you will need to build the volume and squelch circuits for the radio. Above is the solder side of the system board which is located at the front of the radio. P901 is the control/power cable connector. The H6-H73 jumper assures that channel one is always selected. H89-H90 applies A+ to the radio circuits. H79-H81 forces the RX oscillator to always be on. Cut the trace between H95-H96, this prevents muting of the receiver during transmit. That is it, your done. Your MASTR II is now fully duplexed. If you have any questions please drop me an Email.