- Before You Begin
You will want to have the MASTR II fully tuned up and
operational on your repeater frequency before you begin...
- 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
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.
- 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:
- 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.
- 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...
- 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...)
- 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
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.
- 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.)
- Controller Interfacing
There are at several different schools of thought on interfacing
MASTR II radios to repeater controllers.