Should I remove the MASTR II mobile TX - RX antenna relay?

Concept:  Once the radio is duplexed, there is no need for the T/R relay since both the receiver and transmitter have their own antenna connections on the front of the radio set.  The power for the T/R relay is supplied by the 2 pin connector directly to the right of P904 on the right edge of the Systems Board.  This connector is referred to as P910 and mates with J910 on the board.  Originally, power (A+) is constantly applied to the positive lead (red), and the black lead (ground) is switched by the PTT signal to actuate the relay.

There are four things that can be done, as the transmitters RF signal will still pass through the relay if it is not removed.  The new receiver connection that will be added to the front of the radio will bypass the relay, so, as for the receiver, the relay is of no concern.

Procedure:  Here are the 4 options:

  1. You can choose to do nothing with the relay.

  2. 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.
  3. You can jumper power to the relay coil so it stays in the transmit position.

  4. Applying power the relay coil at all times will keep the relay pulled in. This eliminates the clicking, but the relay can still fail.  To jumper the relay into the transmit position there are two options;  One is to cut the trace leading directly from P910 pin 2 on the top side of the board and grounding this pin.  This option separates the PTT signal from getting to the relay and provides a constant ground for the relay.  Pin 2 of P910 is the pin closest to the front of the radio and is the one that the black lead goes to.  Alternately, simply unplug J910, cut the connector off, and apply 12 volts to the red and black leads observing polarity.
  5. Remove the Relay.

  6. 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 that 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 over tighten the transistor stud or you will break it right off.
  7. Jumper around the relay with a piece of coax.

  8. Cut the RF traces at the relay (the one to the relay, and the one from the relay) with a sharp knife like an X-Acto and install a short length of RG-174 or RG-400 coax.  This effectively jumpers around the relay.  The shields are connected to the ground plane.  Simply snip off the leads that power the relay.
Note:  If you are uncomfortable with modifying the board, it may be best to do nothing.  Some things are better left alone...  In most cases I do nothing with the relay as they seem to last even in repeater duty.

This site, its contents, and look & feel are Copyrighted©
Original author Jeff Otterson N1KDO
Text additions 25-Sept-2001 to present, W3KKC
Option # 4 was suggested by Tracy B. Hooker  KA5ECS, Nov 2002.
Copyright 2001 Kevin Custer W3KKC
All Rights Reserved.