Assumptions - I have made are that you have a M2 which properly operates, you have your crystals and they are mounted in the ICOMs.  ICM is a good source for M2 crystals but there are other sources.

CTCSS
The GE Channel Guard boards will not work as a full duplex encoder/decoder.  I like most others have a budget which puts encoder/decoders out of reach.  Lets face it, every time we add a repeater or a repeater link it is another $65 for a CTCSS encoder/decoder.  MX-COM offers a small inexpensive encode/decoder (MX465) for less than $8 in signal lot quantities.  Although this device does not work as a full duplex encoder/decoder, I simply use two of them.  Total cost is a few hours of construction time and less than $20 for a full duplexed CTCSS encode/decoder.  I would like to share this link with you, it is a link to all of the Semiconductor Manufacturers on the Internet.

Heating and cooling
issues are always a problem, depending on the power output ability of your MASTR II will dictate how much cooling is required. I have a 110-watt MASTR II powering a 2 meter repeater, I’m sending 54 watts in to the duplexer.  I added a fan on the top cover of the radio over the PA deck and an air inlet slot to the left of the PA deck on the top. I also added a few fans to blow cool air over the heat sink fins.

I also have a 65 watt MASTR II powering our 70-cm repeater which is putting about 25 watts in to the antenna, this installation required no additional cooling.

Don’t push your MASTR II’s PA deck to the limit. A MASTR II’s PA system will deliver more power than the RF parts can handle, too much current will pass and you will lose the PA. Also, the removal of heat from the RF parts mounted in the heat sink are an issue.

We all want to push as much power as we can and I'm no different than you.  You can push the PA deck to maximum power (110 watt VHF deck will do 140 to 160 watts) but the heat sink can not take the heat away fast enough.  Let's look at GE's continuos duty rating for each of the three PA decks:

Series 5, 35 watts  -  Series 6, 45 watts    -  Series 7, 55 watts

M2 PA decks should not be adjusted to less than 40% of their rating.  My suggestion is to add a cooling system to your PA deck as described above and keep you power output with in reason.  Remember, 6 dB equals one S unit, so a few extra watts will mean nothing.

Desense
is another one of those "I’d rather not talk about it" things we amateurs have. Well, any desense at all will degrade the receivers sensitivity. Depending on your installation and MASTR II model will determine if you will have to separate the PA from the receiver. Neither of our GE MASTR II's (2 meter and 70 cm) have been separated. The more critical 2 meter repeater (600 kHz split) has 10db of quieting with a .32 uVolt signal while the transmitter is wide open (160 watts). What I did was to add a BNC connector to the side of the radio by the receiver's antenna input jack. I then added a piece of copper pipe from the side panel of the radio into the receiver compartment. With out this shielded tunnel, I could not get the sensitivity that I stated above.

Of course, always use double shielded cable in you installation.   I use 214 for the transmit side and 142 for the receiver signal path.  If you are going to use a pre-amp, wire it in as close to the duplex as possible.  This will eliminate any amplification of noise that may have found it's way in to the coax.

ICOMs
are the crystal holders used in the M2 radio.  The come in three versions, 2C, 5C and a EC.  EC ICOMs are designed to work with 5C's.  You should have one 5C installed which can temperature compensate many EC ICOMs.  The 2C ICOM can not temperature compensate an EC.

M2 crystal frequency
calculations.  The IF frequency for both the UHF and VHF M2 is 11.2 MHz, CF equals the crystal frequency.

GE Channel Guard
If the M2 you have had/has a GE Channel Guard (CTCSS) board installed, or you can not get the transmitter to fire up (the transmit relay does not click) your radio may have been equipped with GE's Channel Guard board.  Look at the system board, to the left you will see the exciter connector, just below that you will see a 9 pin connector, just below this connector and below the right 3 pins of the this 9 pin connector and just above a delta symbol you will see a solder pad to either side of the delta symbol.  There should be a jumper installed on these solder pads, if there is not then install one to get your transmitter to work with out GE's Channel Guard board.

Henry, KC4KZT adds: If you are using the factory Channel Guard board for tone decode in a Mastr II, you cannot use CAS. You need to use RUS.  If in tone decode mode using the factory CG board, only the audio is muted. The CAS (or COS as some refer to it as) will still go high without the tone.  RUS line will go high only if the tone is decoded.  RUS line is not on the front connector. It is found on the audio/squelch board pins next to the CAS.

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