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Choosing a Radio to Convert
Bands and "splits"
For the purposes of this article, we will assume that there are only 4 bands of interest for conversion: 10m, 6m, 2m, and 70cm. A "split" is a region of a band that a radio built with certain parts will tune. It is the range of frequencies that the radio is capable of operating on. You will need to refer to the combination numbers to determine what radio you want to find to convert.

  • VHF Low Band: There are 4 "splits" available in this band. The only interesting splits are the "12" split (25-30 MHz, rare!), the "13" split (30-36) and the "33" split (42-50 MHz). The "23" split should be avoided at all costs; it is useless for amateur operation. The 12 & 13 split radios can be tuned into the amateur 10-meter band, and the 33 split radios can be tuned into the amateur 6-meter band.

  • VHF High Band: There are 2 splits available in the VHF high band. The "56" split is from 138-155 MHz. These are more desirable for amateur work, and much more difficult to find. (If you find some, get one for me!) The "66" split is from 150.8-174 MHz, and is more commonly available. The high split radios can be re-tuned into the amateur bands, typically without too much difficulty, but care must be taken that the transmitter does not have excessive spurious emissions. The 66 split exciters can be modified to be 56 split exciters by replacing a dozen or so disc capacitors. The 66 split radios are preferred for 222 MHz conversion.

  • UHF: There are five different splits for UHF MASTR IIs. Only three are interesting for amateur use: the "77" split (406-420 MHz), the "78" split (420-450 MHz, rare!), and the "88" split (450-470 MHz). The other two splits (89 & 91) are in the so-called "T-band" (UHF television spectrum) and are not useful to amateurs. The 77 split (406-420) radios can be retuned into the bottom segment of the 420-450 MHz U.S. amateur 70cm band, where they are useful for linking repeaters, remote receivers, etc. (See Crossbanding Applications for some ideas...) The 78 split (420-450) radios are already in the U.S. amateur band, but are extremely rare. Radios in the 88 split (450-470) split can be re-tuned into the 440-450 band and used as UHF repeaters.

Frequency Stability
MASTR IIs are available in 2 PPM (parts per million) or 5 PPM frequency stability. The difference between the 2 stabilities available is in the channel elements, which GE likes to call ICOMs, for Integrated Circuit Oscillator Modules. The ICOMs are available in 3 different styles, both in receive and transmit varieties, for a total of 6 different ICOMs for a given band.

  • 2C ICOMs: These are the 2 PPM oscillators. They are what you want if you are going to build a wide-area coverage repeater that will not be in a climate-controlled environment. The ICOMs themselves are more expensive, as are the crystals that go inside the ICOM.

  • 5C ICOMs: These are the 5 PPM oscillators. Most mobile radios will use 5 PPM oscillator stability and will have at least one 5C ICOM. These are reasonably stable, and can be used for repeater operation if the environmental temperature is relatively constant. 5C ICOMs provide "temperature compensation" to EC ICOMs.

  • EC ICOMs: These are used in conjunction with 5C ICOMs. DO NOT USE EC ICOMs IN A RADIO WITHOUT A 5C ICOM INSTALLED. The oscillator frequency will not be stable. These ICOMs provide 5 PPM stability when used with a 5C ICOM. They use a "temperature compensation" signal provided by a 5C ICOM. Typically, the receiver has a 5C ICOM and the transmitter a EC ICOM.

You can change the ICOM's frequency by replacing the crystal. However, if you change the crystal yourself, you risk upsetting the temperature compensation of the ICOM, which must be matched to the crystal. If you are going to pay for a 2C ICOM with a 2 PPM crystal, it is well worth the money to get a reputable crystal house (like International Crystal Manufacturing) to compensate the ICOM to match the crystal. This typically costs about $25.00.

The VHF high-band radio has two different exciters available: a "multiplier" exciter, and a "PLL" exciter. The PLL exciter is preferred for repeater use, it has a somewhat cleaner output (less spurs) than the 150.8-174 MHz multiplier exciter does when it is tuned into the amateur 2-meter band. The PLL exciter still requires a crystal in a special FM ICOM, it is not "synthesized" in the sense that you cannot change the operating frequency of the exciter without changing the ICOM crystal. The PLL exciter is true FM, which may make it preferable for packet work. (The multiplier exciters are phase-modulated.)

There is a true-FM exciter available for UHF MASTR IIs, but it is somewhat rare and the prices reflect that. You can recognize this exciter by the larger ICOM module, the FM ICOMs are about 50% wider and 50% thicker than the usual ones.

Power Amplifiers
In most cases, you will want the biggest PA you can find. However, there is one important exception to this rule: MASTR II PAs don't like to operate below about 40% of rated power output, especially the UHF ones. If you plan to drive an external amplifier with the transmitter output, check the input power level minimum and maximum of the external amplifier and choose your MASTR II power level accordingly.

The typical accessories for a MASTR II include a control head, microphone, microphone hanger, speaker, power cable (control head to battery) and control head cable (radio to control head). If you have no accessories, you will want to get the all of the above, with the possible exception of the microphone hanger. Lack of these accessories make tune-up and service ridiculously difficult. For building repeaters, you can get by with a control cable alone.

Handle Color and UHF MASTR IIs
Beware of "silver handle" UHF MASTR IIs. These typically have a VHF exciter and a "tripler PA" in them. These radios are notorious for spurs and general noise on the RF output. The "painted handle" UHF radios are definitely preferred. The silver handle radios are an older vintage MASTR II, and are perfectly ok for low band and VHF.

Many GE MASTR II stations have come on the market recently due to the fact that all Public Safety, Industrial, and Business users using 25 KHz channels are required to switch to 12.5 KHz channels by January 1, 2013. These surplus stations represent a tremendous opportunity for Amateur Radio repeater builders. The stations have significantly better isolation between transmit and receive, and many have continuous-duty power amplifiers with large heat sinks.
What to Pay
I have seen MASTR II mobiles sold at hamfests for as low as $15, with accessories, and for as much as $150, without accessories. $75-$100 is the typical price range. More than $100 is too much. Keep shopping.