Full duplex instructions for MASTR-II

  1. Jumper 10 V to the receiver oscillator --- Pin 11 on the oscillator board. On the system board, center group of pins (P903), count from the left.

  2. Make a "miniature control head" and attach to pins on the receiver board volume and squelch with 10k pots and a capacitor (see control head schematic).

  3. Cut receiver mute trace on the system board. Note that this trace runs vertically (front-to-back) on the bottom of the system board. You will cut between two "vias" (small dots on the system board where the circuit traces connect from top to bottom or vice-versa).

    To get to this trace, you must remove the radio's bottom cover. On each front corner you will see three Phillips head screws --- two in-line and a third to the inside. The "inside" screw threads into a long hex spacer. Remove this "inside" screw from each side. Remove the bottom panel.

    The receiver mute trace starts from Pin 6 (count from the right!) of the only IC on the system board. This is a SIP-style IC, just to the right of center. From pin 6, it connects with a small glass diode, which goes to the left. The diode is soldered to the system board through the first via (H96). Find this trace on the bottom of the board. About 1/2" above that via you will see a second via (H95). Cut the trace between these two vias.

    NOTE1: Confirm that you have the correct trace by checking continuity with the receiver mute pin --- Pin 7 on the receiver board. On the system board, right group of pins (P904), count from the left.

    NOTE2: Do not cut the trace between the IC and the diode!

  4. Run a new receiver antenna lead --- an SO-239 fits nicely into the hole where chassis lock is.

    The receiver normally gets its RF from the T/R relay through a small grey coax jumper with RCA plugs on both ends. Remove this jumper and connect the new antenna lead to the RCA jack on the receiver (or preamp, if present).