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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

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 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. 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.

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 - We don't have a photo of the guts of a large 5C element (yet - anybody want to provide one?) It contains an oscillator and a 5 PPM (±0.0005%) compensator IC in the top left corner (similar to the narrow 5C element above and mounted similarly to the wide 1C ICOM above).	Anybody have a photo ?
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 ICOM's will be stable over the temperature range from zero° C to 55° C, or from 32° F (freezing) to over 100° 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° 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 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 receiver chassis and the transmitter chassis are tied together into two seperate strings, but are generally NOT jumpered between the receiver and transmitter chassis.   In a mobile all the ICOM compensation pins in the receiver AND the transmitter are tied together.   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 resembling desense.

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.   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 receiver and the rest ECs, and one 5C in the transmitter 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 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 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 and your repeater site is human-comfortable year-around.   An EC will maintain better than 5ppm from +32° F to above 120° F only if you have a stable +5v DC bias 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 1/4w 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 coulc 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 can find 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 with the radio in the 60° to 75° 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.

ICOM Schematics