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Basic Info on 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 and Kevin Custer W3KKC

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 !

Five different types of ICOMs are available for use in the Mastr II radio.   Each has it's uses.   Each of the ICOMs contains a crystal-controlled Colpitts oscillator, and three of the ICOMs contain compensator ICs.

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 which looks just like the EC PLL element but marked "5C".

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"Flip Top" ICOMs Later ICOMs have a black plastic "flip-top" that hides the frequency netting adjustment. Just slide a knife blade under the right edge and pry up. If you are gentle, 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.

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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 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", or the "2C modification".

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 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° degrees F.
EC ICOM - contains an oscillator only.   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. This small sized EC element is used in the PM exciter and in the receiver.
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. The photo is of a receive element.

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. We don't have a photo of the guts of a large 5C element (yet - anybody want to provide one?) but it has a daughter board in the top left corner (similar to the narrow 5C element above and mounted similarly to the 1C below).

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.

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 ICOM's will be stable over the temperature range from zero degrees C to 55 degrees C, or from 32 degrees F (freezing) to over 100 degrees F, and if in a controlled environment, will do well enough by themselves.   This 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 degrees C to just over +70 degrees 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 stand alone, the 5C is a master element, and the EC is a slave element.

In a station (i.e. a base or a repeater) the compensation pins within the RX chassis and the TX chassis are tied together into two seperate strings, but are generally NOT jumpered between the RX and TX chassis.   In a mobile all the ICOM compensation pins in the receiver AND the transmitter are tied together.   Adding that jumper to a base is a common ham modification - but GE left it out on purpose - in some cases of repeater operation that jumper caused desense.

In other words, in a stock Mastr II factory base or repeater you have to provide a separate compensation voltage generator for the RX and another one for the TX.   GE simply used two internally compensated ICOM's in a station... one 5C or 2C in the receiver and a second 5C or 2C in the transmitter.   In a mobile you would generally find one 5C and the rest would be EC's.   In a normal station or repeater you'd generally find one 5C in the RX and the rest ECs, and one 5C in the TX and the rest ECs.   If the application required 0.002%, for example in a GMRS environment, then the radio would be filled with 2Cs.

For highest stability and best performance in your repeater, leave the compensation jumper between the RX and 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 at 5C parameters 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 compensation - with only one exception that is listed below, and that trick is NOT type accepted and therefore not legal on commercial frequencies.

There is one way to cheat if all you have is ECs.   An EC will maintain better than 5ppm from +32 degrees F to above 120 degrees F only if you have a stable +5v DC voltage on the compensation line.   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 1.0K, 2.2K, 3.3K or 4.7K 1/4w or 1/2w 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 must be the same value, and they and the bypass cap can be soldered onto a base connector salvaged from a dead element - this 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 (or replace the ECs with 2Cs).   Until then, use the ECs, install them in the radio, and set the elements on channel with the radio in the 60 to 75 degrees F temperature range.   It will work well enough while you locate a couple of good 2Cs and have them rocked up on your frequency pair.

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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 LBI's titled "ICOM FREQUENCY ADJUSTMENT".   It contains a temperature graph and instructions on the proper setting of the ICOM.
For an example, look at lbi-38505a.pdf, then scroll down to page 5.   I suggest that you print the entire page 5 and add it to your radio notebook.

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 thermal compensation that was done at the time the crystal was installed in the element.   Raw quartz has a grain, just like wood, and the angle of the cut relative to the grain has a lot to do with the performance of the finished crystal.   There are several preferred angles to cut quartz for crystals, and the companies that do communications crystals use the cuts that are optimized for frequency stability.   Companies that make crystals for digital circuit clock oscillators make them to a price target and rely on the fact that any frequency error is reduced by the same divisor as the crystal frequency is divided down to a target frequency.   This is the exact opposite of the communications world where crystal frequencies are multiplied up to a operational frequency and every error is multiplied as well.   This is how the modulator in an FM ICOM works - a UHF transmitter with a multiplier of 36 only has to wiggle the crystal frequency about 150hz to get a full 5khz deviation.   In short, you need to get your crystals from a company that understands the technical requirements of the communications crystal as opposed to a clock crystal, and from one that operates to a quality target, not a price target.

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 ICOMs back to the crystal manufacturer and pay to have them compensate the ICOM to the new crystal.   The ICOMs will hold 2ppm or 5ppm only if the individual ICOM is matched to the crystal - i.e. if the crystal tends to swing upwards in frequency as the temperature rises, the ICOM has to swing lower, at the same time and the same amount.   Likewise, if it swings down in frequency, then naturally the ICOM has to swing upwards.   Another way to think of this behavior is "frequency tracking with temperature".

Note that the temperature 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.   Many of the cheaper suppliers simply install the new crystal into the ICOM and ship it back.   This can result in real problems if the new crystal swings the opposite of the old one, and as the temperature causes the crystal to drift off frequency the compensation components push it even further off frequency.

Note that there is a difference between a receive crystal and a transmit crystal.   A crystal with the *wrong* cut may not exhibit enough *rubberiness* for full deviation to occur.   This situation happens when crystal vendors cut a crystal for absolute stability and don't know that it will be used in an FM transmit ICOM.   This is especially true of low band transmit crystals/ICOMs where the multipler may only be a factor of 3.

International Crystal Manufacturing (ICM) and Bomar are the only companies that I know of that actually test the modulation capability, modulation linearity and modulation sensitivity of the transmit crystals they make (and if you purchase the installation and compensation service they test again after they are installed in the ICOMs), and they also insure modulation symmetry in the ICOM's as part of the final inspection.   Those two are also the only companies that I know of that actually change the temperature compensation components (capacitors and resistors) to *properly* temperature compensate the ICOM.

When you are ready to recrystal an ICOM and are calling around to get prices, you need to ASK what each company consider "compensating".   Considering what you are going to spend on the entire repeater paying a little extra for a professional top quality rock house to manufacture the crystals and do the compensation is definitely worth it.   See The Suppliers Page and scroll down to the Crystals section.

Note that new crystals tend to drift downward during the first few months of operation - this is very visible on 900 MHz and can been seen even on 28 MHz.   This drift is due to the crystal "aging", and IS QUITE NORMAL.   Even if your brand new UHF ICOM drifts as much as 7 KHz (on the UHF frequency, not the fundamental) during the first 5 to 6 months of operation, it does NOT mean that the crystal is defective.   Just plan on resetting the frequency a few times in the first 6 months of continuous operation (which may take a lot longer on a transmit crystal since it doesn't get as much "run time" as a receive crystal).   Once the crystal ages it will settle down.

Personal experiences from WA6ILQ:
As a personal example of the above aging, one of my UHF radios drifted 11khz in the first 5,000 hours of operation (four adjustments of about 5khz, 4khz, 2khz and 400hz across roughly 6.5 months), and hasn't budged 800hz in the 20-25 years since.   Interestingly enough, when the crystal was placed in service the adjustment was almost at one end, and when the crystal stabilized the adjustment was very close to right in the middle of it's range.   And note that you may need to have a "test exciter" on your bench operating 24x7 to "burn in" the transmit crystals/elements. The "test exciter" can consist of just a ICOM socket, a power supply (which can be a "wall wart" transformer) and a 1w resistor as an output load. All you are after is a way to run the ICOM full time.

My personal method is to set a brand new crystal/ICOM/channel element on channel and wait a few days and see which direction it heads as it ages (90% have gone downward in frequency).   I then set it back on frequency and a a little further in the same direction so the next bit of aging brings it back towards center of channel.   I then check it at a week, two weeks later, a month later, at 2 months and at 6 months, plus any other opportunities when the service monitor is on-site.   And I'll recrystal my own base station ICOMs or mobile ICOMs, but if it's going on any hilltop, or into any non-air-conditioned/heated building, I send the ICOM/channel element to International Crystal.
End of note from WA6ILQ

In the mid 1980s crystal manufacturers adopted a technique called "pre-aging", where they "age" the quartz crystal material in a power oscillator circuit under elevated temperatures for several weeks.   Then they make the crystals using these pre-aged blanks.   This technique produces a crystal that might drift 3 to 5 khz total.   In closing, after it has been in service for a while a properly compensated ICOM will normally drift no more than a couple hundred Hertz.

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 24 or even 48 hours of operation in the heated environment to stabilize.   Once the initial "heat soak" is complete and the frequency set the new crystal will hold frequency.   Then later on when you need to reset the frequency you need to let the radio "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
Photos copyright © M. Scott Zimmerman N3XCC and Kevin Custer W3KKC 2002, 2004
This page first posted 2002-Jan-11