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  An Overview of the Various Types of G.E. MASTR II ICOMs
by Kevin Custer W3KKC, Scott Zimmerman N3XCC, and Mike Morris WA6ILQ
Photos by Scott Zimmerman N3XCC, Kevin Custer W3KKC and Terry Dalpoas KM5UQ
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The topic of the differences between the various types of MASTR II ICOMs (official name is "Integrated Circuit Oscillator Module", but see below) comes up on the GE mailing lists on a regular basis. I've put together this page to summarize the repeatedly posted information... Now I can just point the folks with the questions here !

There are a number of different types of ICOMs are available for use in the MASTR II series radio. Each has it's uses. Each of the ICOMs contains a crystal-controlled Colpitts oscillator, and several of the ICOMs contain compensator ICs. GE's own technical literature claimed that the ICOMs were not repairable, and GE would void the warranty if the case was opened.

ICOM Pictures

Front view



Top view

From left to right they are the EC PLL (the FM wide element), EC (PM), 5C (PM), 2C (PM) and 1C PLL (FM)
Not shown is the wide 5C PLL element and wide 2C PLL element, both of which looks just like the EC PLL element but labeled "5C" and "2C".



"Flip Top" ICOMs


Later ICOMs have a black plastic "flip-top" that hides the frequency netting adjustment.
Just slide the edge of a knife blade under the right edge of the cover and gently lift it up.
If you are very careful the plastic tab can also be used to pull the ICOM out of the radio. If you are too rough, or if the plastic has aged, become brittle or lost its flexibility and strength the tab will come off in your hand.



ICOMs and the Exec Series




If you have one of these, it's a crystal element for a MASTR Exec II and it WILL NOT WORK in a MASTR II. These plug into the oscillator board in an Exec, as opposed to the ICOMs above which contain the crystal and the oscillator circuitry in one module. The Exec II crystal module contains only the crystal and the components needed for frequency netting and temperature compensation. They were designed to be open, there never was a cover for them. These crystal elements can hold 0.0005% just fine, but can not do 0.0002%.

When the Exec was used in a 0.0002% environment (like GMRS) they swapped the crystal element motherboard for one that held 0.0002% ICOMS. You may hear it called the "2C board", the "2C modification", or the "2C mod".


The rogues gallery - the small ICOMs are on the top, the big ones at the bottom:

5C ICOM - This one contains an oscillator and a 5 part-per-million (±0.0005%) compensator IC (on the small daughter board at the top left). This ICOM compensates itself and provides the compensation voltage to the compensation line in the radio for any large or small EC-ICOMs.

The 5C (large or small) is the only element that drives the compensation voltage line in the radio.

The 5C is 5 ppm from -40° to +158° F, and 2 ppm in the narrower segment from +32° to +131° F.
EC ICOM - contains an oscillator only. This ICOM requires external compensation from a 5C-ICOM for full range performance. Note the space on the top left of the PC board, and compare it to a 5C or a 2C.

The small sized EC elements are used in the PM exciter and in the receiver. The particular one in the photo was used in a receiver (note the "R" at the front of the second line from the bottom).

EC ICOMs by themselves are stable over the temperature range from zero° C to 55° C, or from 32° F (freezing) to over 130° F - but they are not type accepted for commercial use without a 5C.
2C ICOM - contains an oscillator and a 2 PPM (±0.0002%) compensator IC (on the small daughter board at the top left).

This ICOM is a self-contained oscillator module and will not provide compensation for EC-ICOMs. In fact the compensation pin isn't even hooked up.

The 2C ICOM is 2 ppm over a range of -40° to +158° F.

The large ICOMs below are used in the VHF phase-locked-loop (PLL), and non-PLL UHF FM and 800 MHz. FM exciters. The exception is that the 1C comes in receive and transmit versions.

1C FM ICOM - contains an oscillator and a 1 PPM (±0.0001%) compensator IC (on the small daughter board at the top left). Like a 2C, this one will not provide compensation for an EC-ICOM.   This unit is most commonly found in 800 MHz and 900 MHz radios.

The photo is of a receive element.
2C FM ICOM - contains an oscillator and a 2 PPM (±0.0002%) compensator IC in the top left corner. Like the narrow 2C, this one will not provide compensation for an EC series ICOM.
5C FM ICOM - contains an oscillator and a 5 PPM (±0.0005%) compensator IC in the top left corner. Like the narrow 5C this element has a compensator IC mounted similarly to the wide 1C ICOM above.
EC FM ICOM - contains an oscillator only. Requires external compensation from a separate 5C-ICOM for full temperature range performance.

The EC ICOM, large or small, will maintain 5 ppm from -40° to +158° F only if compensated by a 5C.


One of the pins on the ICOM - the crystal oscillator module - is labeled on the schematic as "compensation", short for "temperature compensation voltage". The compensation voltage is a DC voltage level that varies with temperature and keeps the crystal oscillator on frequency. By the way, the compensation generally comes into play only on severe temperature extremes. By themselves EC ICOMs will be stable over the temperature range from zero° C to 55° C, or from 32° F (freezing) to over 130° F, and if in a controlled environment, will do well enough for test applications. This temperature range is documented in the LBIs for the MASTR II transmitter - see LBI-4622, for example.

An EC (standing for "External Compensation") ICOM uses the voltage generated by a 5C, and only a 5C. With a 5C in the radio the EC will maintain 5ppm from -40° C to just over +70° C. Note that you could have an EC on your repeater frequency (in the F1 slot) and add a 5C on ANY frequency - even a commercial channel - in ANY other slot of the same chassis and the compensation would work. It could even be a 5C from a different band (for example, a highband 5C in a UHF repeater) as it's only being used to generate the compensation voltage.

Another way of looking at things is that the 1C and 2C elements are totally stand alone units, the 5C is a master element, and the EC is a slave element. I repeat - the 1C and 2C elements are NOT master elements! And while the ECs will be stable from zero° C to 55° they are NOT type accepted as stand-alone elements. In other words, you can use them that way in amateur service but not in business, commercial or public safety.

Any voltage change on the +10VDC power supply line will change the frequency on the ICOM, so make sure the +10V is rock steady. The DC voltage (generated by the 5C ICOM) on the compensation line directly controls the frequency of any EC ICOM, or the 5C ICOM itself! Any disturbance (even a cracked solder joint causing an intermittent connection) on the compensation line will cause a change in frequency on any EC or potentially the 5C ICOM. Make sure the line is "clean" - nothing wired to it, no corrosion (green mold / green glop) or shunt resistors to voltage or to ground as a change in the DC voltage will change the frequency. To be more precise, a decrease in the DC voltage on the compensation line increases the capacitance of the varactor inside the ICOM and lowers the output frequency. Years ago I saw a radio where someone tried to "push" an ICOM down to the adjacent channel - from 462.650 MHz (GMRS) down to 462.625 MHz). He did it by connecting a resistor from the compensation line to ground, and bypassing it with a capacitor (I don't know why as there is a 0.01 µF cap from the line to ground in the exciter). Unfortunately the added cap was temperature sensitive and leaky - and with a known good element installed the radio randomly drifted about 2 kHz in a half an hour. Removing the resistor and capacitor (they were hidden on the back side of the exciter board) restored the radio to rock-solid stability.

A quick test: Unplug the multipin cable feeding the main exciter connection, the coax cable, and any ICOMs. Put an ohmmeter on the lowent (X1) ohms scale from pin 2 of any ICOM socket to pin 4 or pin 5 of any ICOM socket. You should see no needle movement (infinite ohms). Change to the highest ohms scale on the meter. It should stay at infinity. Now move the probe from pin 4 or 5 to pin 1 of the ICOM socket. You should again see infinity on both scales. If either test shows a reading you need to remove the board from the chassis and carefully trace the compensation line.

In other words, in a stock MASTR II factory base or repeater you have to provide a separate compensation voltage generator for the receiver and another one for the TX. In a single frequency base station GE simply used two internally compensated ICOMs... one 5C or 2C in the receiver and a second 5C or 2C in the trransmitter. In a multichannel station you'd generally find one 5C in the receiver and the rest of the receive ICOMs would be ECs, and one 5C in the transmitter and the rest of the transmitter ICOMs would be ECs. In a single frequency mobile you would generally find one 5C and an EC, or in a 0.0002% environment it would have a pair of 2Cs. In a repeater (i.e. single channel) you'd usually find a pair of 5C ICOMs, one in the receiver and the other in the transmitter. If the application required 0.0002%, for example in a GMRS environment, then you'd find 2C ICOMs everywhere.

In a station (i.e. a base or a repeater) the compensation pins within the receiver chassis and the transmitter chassis are tied together into two separate strings of elements, but the compensation lines in the receiver and transmitter chassis are generally NOT jumpered together. In a mobile all the ICOM compensation pins in the receiver AND the transmitter are tied together into one long compensation line. Adding that jumper to a fixed station or repeater is a common ham modification - but GE left it out on purpose - in some cases of repeater operation that jumper acted like an antenna and the RF pickup created a problem that resembled desense. If you absolutely have to go this way, and have a high RF level at your site then use a small diameter coax like RG-174 as the jumper and ground the shield to a nearby ground pin. But the right way is to use a pair of 5Cs or 2Cs on high band, 2Cs on UHF, and 1Cs on 900 MHz.

For highest stability and best performance in your high band or UHF repeater, leave the compensation jumper between the receiver and transmitter chassis out, (or if you find one, remove it) and install one 2C in the receiver and a second 2C in the transmitter.

Despite what you may have read elsewhere, an EC in a radio with a 2C is NOT going to work because from the point of view of the EC the 2C isn't even there - the compensation in a 2C ICOM is completely internal and the compensation pin isn't even hooked up. An EC needs a 5C to have ANY compensation - with only one exception that is listed below, and that trick is NOT type accepted and therefore not legal on commercial or GMRS frequencies.

There is one way to cheat if all you have is ECs and your repeater site is human-comfortable year-around. An EC will maintain 2ppm from +32° F to around 130° F only if you have a rock stable DC bias voltage on the compensation line at about 1/2 the 10V supply voltage (i.e. about 5vDC). This is plenty good for a garage repeater (i.e. your test bed system), while you finish it and burn it in. A voltage divider made from a pair of identical 1/4w or 1/2w 5% metal film resistors in series from the +10VDC supply to ground, with the center point tied to the compensation line is all that is needed (with suitable RF bypassing). The two resistors can be anything from 10K to 22K as long as they are the same value, and you could probably go as low as 4.7K. Solder the resistors and the bypass cap onto a base connector salvaged from a dead element, or if you have to you can solder them to the board. If you use an element connector that allows you to leave the exciter or receiver circuit board unmodified and the voltage divider plug can be removed easily when you find a 5C element to serve as a master element or replace the ECs with 2Cs. Until then, use the ECs, install them in the radio, and set the elements on channel. It will work well enough while you locate a couple of good 2Cs and have them rocked up on your frequency pair.   Just remember - this resistor trick is not any kind of (extra) temperature compensation, is not type accepted, and is not a good long term solution!   It is a only a test bed or workbench solution!


ICOM Schematics

You should never need them, but if you are interested on knowing what is inside an ICOM, go to this page from Hall Electronics and scroll down to find schematic diagrams of the ICOMs. Note that the exact value and presence or absence of some of the components are determined during temperature compensation of the installed crystal (or as GE says, "value determined in test").


Notes on Temperature Compensation:

Lastly, realize that the frequency stability of the ICOM is only as good as the crystal installed in it, and the testing and temperature compensation that was done at the time the crystal was installed in the element.

There is no point in repeating information that is on another web page at this site. I suggest that you go read this web page on ICOM / Channel Element recrystalling and temperature compensation.

Note that the compensation components in the ICOM are dependent on the characteristics of the individual crystal which have to be measured after the crystal is made, and the components selected by hand to match, and then installed in the ICOM. Hence, this has to be redone whenever the ICOM is recrystaled. The same process also tests the functionality of the remanufactured element.

If you are serious about your repeater or link, especially if it is going to be located in a mountaintop or elevator building that has no environmental controls, do yourself a favor and when you purchase the crystals for your repeater send the 2C or 5C ICOMs back to the crystal manufacturer and pay to have them manufacture the crystals, then install the crystals, compensate them to the ICOM and test the final result. The 2C or 5C ICOMs will hold 2ppm or 5ppm only if the individual ICOM is matched to the crystal, and you can see from the above writeup, there is a good reason to be within 5ppm of channel on VHF and within 2ppm on UHF.


ICOM Frequency Adjustment

Proper frequency setting procedure of the ICOMs is also very important. Folks have set ICOMs "on frequency" in freezing temperatures and then wonder why they are off frequency in hot weather (or vice versa). GE provides a write up in most MASTR II receiver LBIs titled "ICOM FREQUENCY ADJUSTMENT". It contains a temperature graph and instructions on the proper setting of the ICOM - and the frequency characteristics versus temperature graph zeroes out at 80°F / 26.5°C. Yes, I've known people to take a heater (or at least a hair dryer or heat gun) and a non-contact IR temperature sensor with them on a winter trip to a repeater site just to warm up the exciter (or receiver) and the ICOM to about 70-85 degrees before they set the radio on frequency.

For more info, look at LBI-4561C, then scroll down to page 11 or page 14 or LBI-38505A, then scroll down to page 5. I suggest that you print the entire of one of those pages and add it to your radio notebook.


A comment on oven or heated crystals:
A number of older Motorola, GE, RCA, Uniden and other brands of radios used heater elements of one form or another to operate the crystals at elevated temperatures to maintain stability. These heated crystals have completely different type of drift. Brand new crystals need an uninterrupted (at least) 24 or even 48 hours (preferred) of operation in the heated environment to stabilize. Once the initial "heat soak" is complete you can set the frequency. And if you need to reset frequency at a future date you need to power the radio up and let it "cook" for at least 3 or 4 hours before you do any frequency setting.


Lastly, while GE calls the module an ICOM, with the name being derived from "Integrated Circuit Oscillator Module", other names have been derived over the years by various 2-way techs...
From an old email:
> >What does ICOM stand for?
> >
> >I Can't Open Mine
> >International Crystal Overcharged Me
> >I Could've Owned (a) Motorola
> >It's Copied Off (a) Motorola
> >Improper Clone Of (a) Motorola


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Text copyright © Kevin Custer W3KKC, Scott Zimmerman N3XCC, and Mike Morris WA6ILQ 2002, 2004, 2005
Doug Marston WB6JCD, Jeff Kincaid W6JK, and Bob Dengler NO6B contributed to the text.
Photos copyright © M. Scott Zimmerman N3XCC and Kevin Custer W3KKC 2002, 2004
Large 5C icom photo courtesy of Harold Read K3FG
This page first posted 2002-Jan-11

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.