Modifying the MASTR II System Board for Full Duplex

Modifying the MASTR II System Board
for full duplex operation.
By Kevin Custer W3KKC

Concept: Modification of the "System Board" to allow a GE MASTR II Mobile radio to operate full duplex. There are at least two different types of System Boards and information is provided in this article for both the Common and Special System Board.

It is necessary to activate the receiver circuits continuously by applying power to the "IGN" and "A+" leads. In the past, many builders have chosen to power these circuits directly with jumpers on the System Board. I feel a better method of supplying power to these circuits is through fuses located in holders. Since there are two methods of supplying the "IGN" lead with power, you can choose the option that best suits you. This option is explained in the text below.

Common System Board

Common System Board Modification

Pictured above is the solder side of the common 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 1 is always selected.
H89-H90 applies A+ to the radio circuits continuously no matter if the transmitter is on or not; see 'Option' below.
H79-H81 forces the RX oscillator to be activated continuously.
Cut the trace between H95-H96, which prevents muting of the receiver during transmit.
Go to Powering Procedure below.

Special System Board

Special System Board Modification
Pictured above is the solder side of the special system board which is located at the front of the radio.
P901 is the control/power cable connector.

H79-H81 forces the RX oscillator to be activated continuously.
Cut the trace between H95-H96, which prevents muting of the receiver during transmit.
The other jumper straps F1 channel select to A- enabling both RX and TX F1 positions.
Go to Powering Procedure below.

Powering Procedure

Option: There are two methods of supplying power to the receiver circuits continuously, you can jumper H89-H90 as in the common System Board outline, or you can supply A+ to these circuits via a fuse (preferred method) as indicated below in Step 1. Steps 2 - 4 need to be done no matter which method of powering the IGN is used.

Power connections:
Connections must be from the large power leads to the System Board in order to supply power to the radio circuits. The control head and cable, which we are not using, originally supplied this power. I run wires, which are a few inches long, for these connections, on the back side of the control head connector J901. Inline fuse holders mounted in a convenient place make installing and replacing the fuse for the IGN and A+ power paths easy. I have found the best place to connect these leads is to the back of the control head connector as the points are plainly marked with the pin numbers. You may need to remove heat shrink from the large power connections where they solder to the back of the control head connector to gain access to them.

As pictured above, the fuse holders can be mounted to the plastic separator wall in the space previously occupied by channel ICOM's 3 - 8 on the receiver board.

If not already done, as per the option, wire control head pin 24 (IGN) to the large TX A+ lead (pin 39) (large red) through a 3 to 5 amp fuse in a holder.
Wire control head pin 29 (A+) to the large TX A+ lead (pin 39) (large red) through a 3 to 5 amp fuse in a holder.
Wire control head pin 21 (Control A-) to the large TX A- lead (pin 40) (large black) with a wire.
Wire control head pin 30 (A-) to the large TX A- lead (pin 40) (large black) with a wire.

NHRC MASTR II (type) Controller Notes:
If you are planning on using a NHRC/M2 controller you may elect to do these modifications to the System Board. This will allow the controller to plug in, and supply paths, to the CAS/RUS and MIC HI connections so an interface cable is not necessary. By cutting one trace and adding two jumper wires to the System Board, these paths to MIC HI and CAS/RUS will be completed. This is necessary because these signals do not appear on the Channel Guard plug from original. Some people prefer to use CAS, and some RUS, for supplying the receiver activity signal to the controller. Wire either one or the other (not both) as indicated below.

Wire a 1 uF 16 volt (or higher voltage) blocking capacitor from control head pin 4 (or Mic Hi - P902 pin 6) to pin 2 of P908 at any convenient place along the circuit trace. A blocking capacitor is necessary to keep voltage from reaching the NHRC/M2 controller. I prefer to use a 1 uF metalized film non polarized capacitor, for this purpose, like the Radio Shack 272-1055.
Cut trace leading from P908 pin 3 on the top side of the circuit board.
Wire P908 pin 3 to either RUS or CAS found on P904 pins 8 or 9 respectively.

Here are images for reference to the NHRC/M2 Special System Board modification.
Bottom view

Top view

Additional Note: In most cases selecting the F1 channel position as above will enable both the receiver and exciter F1 position. Sometimes the F1 channel select line is severed between the exciter and receiver. I check this line by doing a continuity check between P902 pin 8 and P903 pin 1 on the System Board. If these two points read shorted, the procedure outlined above will enable both the receiver and exciter F1 positions. If the two points don't read continuity, either fix the severed trace or tie both P902 pin 8 and P903 pin 1 to A-.

Go to Step 6.