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Full Duplex Conversion
  1. Before You Begin

    You will want to have the MASTR II fully tuned up and operational on your repeater frequency before you begin...

  2. Add Receiver Antenna Jack

    I have seen many MASTR IIs modified by drilling a hole in the cover over the receiver input jack for the receiver's input connector. I hate this; it is ugly and limits where you can put the radio and what you can put on top of it. I prefer adding a jack for the receiver antenna on the front panel of the radio, in the hole where the mounting lock used to be.

    Remove the lock on the front panel of the radio. Remove the jumper from the T/R switch on the power amplifier to the receiver front end. This is a short jumper with RCA plugs on either end.

    Prepare a length of RG142 or RG400 that is long enough to go from the hole where the lock was removed to the receiver input jack. If you like, you can route this cable next to the existing antenna connector cable, by removing the side panel of the radio chassis, or you can use the hole through the plastic support to route the cable into the receiver oscillator area, and then over the mixer/preselector casting and into the antenna jack. Make sure that this cable is long enough so the channel-guard board will still fit in the radio when the cable is installed. Do not install the cable at this time.

    Install a chassis mount N female connector on one end of this cable. You can also use a long, threaded SO-239 barrel connector, with a PL-259 on the inside. Whatever connector you choose, it will be mounted in the hole that was formerly occupied by the lock. Depending on the connector you choose, you may need to drill holes in the front panel to mount the connector.

    Install a male RCA-type plug on the other end of the cable. I typically try to reuse one of the plugs from the T/R switch jumper; they seem to be quite resistant to soldering heat. The molded plastic on these plugs can be cut away with big diagonal cutters, and the plastic inside the connector body melted with a soldering iron and pulled out with a dental pick. Whatever RCA plug you use, make sure the shield coverage is as close to 100% as you can get. The main goal here is to make sure that the antenna jack jumper is shielded extremely well. Do not cut corners here!

    After the cable is assembled, install the jack in the lock hole, route the cable, and plug the RCA plug into the receiver input jack.

  3. Deal with T/R Relay (or not...)

    Once the radio is duplexed, there is no need for the T/R relay. There are three things that can be done with it:

    1. Nothing. You can leave the T/R relay alone, clicking in every time the transmitter is keyed. The problem with leaving the relay alone is that the relay can fail, or make a bad connection.

    2. Jumper it in Transmit. You can jumper the radio's system board to keep the relay pulled in all the time. This eliminates the clicking, but the relay can still fail. jumper instructions go here...

    3. Remove the Relay. On some power amplifiers, there are two or three separate PC boards. One PC board comprises the harmonic filter and the T/R relay. If you have one of these PAs, you can remove the screws hold down the cover for the harmonic filter, and the screws that hold down the harmonic filter / T/R relay assembly, then unsolder the jumper that couples the PA output into the harmonic filter. Remove the relay (this is a bit of work!) and jumper the harmonic filter output into the antenna jack on the PC board. I typically cut away some of the foil on the bottom of the board, and use a bit of flattened braid for this jumper. Verify that you have not shorted out the PA's output, and reassemble the PA. It is possible to remove the T/R relay on PAs that are made of a single board, but extreme care must be exercised or you will destroy the expensive RF transistors. When loosening or tightening the transistor mounting nuts, you must hold the end of the stud with pliers or a small ignition wrench to prevent the transistor from rotating and breaking. When reinstalling the PA to the heat sink do not overtighten the transistor stud or you will break it right off. (Some things are better left alone...)

  4. System Board Modifications

    The modifications to the system board disable receiver muting during transmit, leave the receiver's power on all the time, and optionally, select the F1 channel element (ICOM) for the transmitter and receiver.

    These modifications will require access to the top and bottom of the system board. You will need to remove the bottom cover on the radio. This cover is held on by two screws on the top side of the radio, on either side of the system board.

    To disable the transmit receiver muting, cut the trace between H95 and H96. This trace starts at J904 pin 7 (IF/Audio/Squelch module RX MUTE) and goes to pin 6 of U901 (the 10 volt regulator IC). This cut disables receiver muting during transmit.

    Install a jumper from the trace leading to J903 pin 11 and the trace leading to J903 pin 12. J903 is the system board connector for the oscillator/multiplier board. This jumper supplies 10V to the oscillator all the time, regardless of whether the transmitter in on or not.

    You may or may not wish to install a jumper on the system board to select F1 (channel 1) all the time. I always install this jumper; I have never made a two-channel repeater. J902 pin 8 is the exciter's F1 select, and J903 pin 1 is the oscillator/multiplier's F1 select. Normally these two pins are connected together, through a trace between H2 and H5 on the systems board, but sometimes this trace is cut. Both of these pins need to be grounded to select F1. Convenient places to get ground for the F1 selects include J902 pin 4 and J903 pin 10.

  5. Powering the MASTR II

    Since the Mastr II was designed to be used in either a positive or negative ground vehicle, the A- is NOT connected to the chassis ground, but it is usually desired to strap them together when building a repeater.

    There are several power leads for the Mastr II. There are the large RED lead that powers up the PA (TX A+), as well as the A+ (16 Ga yellow) and IGN SWITCH (red). All of those need +13.8 Volts DC.

    Then are the grounds... The large black wire (TX A-) and the A- (16 ga black), these can all be tied together.

    If you don't have voltage connected to all of the points necessary (common when you don't use a control head and cable) the radio won't operate correctly or at all.

    If you are or aren't using a cable, you need to insure all of the voltages and grounds are being supplied as connecting voltage to the two large terminals only supplies power to the Transmitter PA, again, as the radio was designed to operate either positive or negative ground and the chassis is not connected to A- (power ground), but it needs to be, and also A+ needs supplied to the IGN lead and A+ lead as this supplies voltage to the rest of the radios circuitry.

    Install a 3 to 5 amp fuse in a holder from the large red wire to both the IGN and A+. Then install a wire from the large black lead to both A- pins. (Thanks to W3KKC for this section.)

  6. Controller Interfacing

    There are at several different schools of thought on interfacing MASTR II radios to repeater controllers.

    • An interface cable for the repeater controller can be wired to the control head.
    • An interface cable for the repeater controller can be wired directly into the radio's system board.
    • A control cable can be "sacrificed" by cutting off the control head plugs and installing a small box with volume and squelch controls and an interface cable for the repeater controller.
    • Some combination of the above.
    In any event, consultation of the system board or control head schematics are a must.


    Be very careful of the CAS signal, it cannot source much current at all, and replacing the hybrid on the IFAS board is very expensive. You can convert the CAS signal to open-collector by using a 2N2222, with the emitter grounded and the base connected to the CAS signal through a 10K resistor. The collector of the 2N2222 will be pulled to ground when the squelch is open (signal being received).


    Push To Talk requires a closure to ground to transmit. You can use an "open-collector" or "open-drain" signal from your repeater controller to supply the PTT signal.

    Transmit Audio

    Inject the transmit audio into the microphone input of the radio. Be aware that this input has a +10V bias applied, which is intended to power the microphone preamp. You might need to insert a DC blocking capacitor here, probably an electrolytic in the range of 1-10µF (watch the polarity!). If you are planning to use a local microphone on the control head, you might want to insert a resistor in series with the controller's audio output to prevent the controller from loading down the microphone's output. 5K would be a good value to start with.

    Receiver Audio

    Receiver audio should be taken from "Volume/Squelch Hi". This signal is unsquelched and not de-emphasized, and is not subject to adjustment by the volume control. The controller should provide muting so unsquelched audio does not play over the repeater during the repeater's tail (hang time).

    This source of receiver audio must be de-emphasized. The deemphasis filter should have a -6dB/octave slope for proper frequency response. If the receiver audio is not de-emphasized, the repeater will sound really "tinny". Many controllers have built-in deemphasis filters. If yours does not, you can make your own with a 15K resistor and a .22µF capacitor. Wire one side of the 15K resistor to the volume/squelch hi signal, and the other side to one side of the .22µF capacitor. Ground the other leg of the capacitor. Take the receiver audio from the capacitor-resistor junction.


    We prefer the Comm-Spec TS-64 CTCSS board for use with these radios, rather than the GE unit. The TS-64 can simultaneously encode and decode, and the CTCSS frequency can be changed by simply moving some DIP switches. Use volume/squelch hi for the TS-64's input (for decode), and TX CG HI for the CTCSS encode. You may or may not want to use the TS-64's high-pass filter to remove the CTCSS tone from the received audio. You will still need to deemphasize the audio, regardless of whether you use the TS-64's low pass filter. The CTCSS detect signal from the TS-64 gets wired to your controller. Consult the controller's and TS-64's manuals for this wiring.