Application notes on using the AP-50
in a GE MASTR II "PM", modified for "FM"

By Kevin K. Custer  W3KKC

To get real nice sounding audio from a GE MASTR II PM (phase modulated) exciter, simply bypass the stock processing and PM modulator and "FM" it.
If you have a VHF repeater, consider replacing it with a "PLL" exciter.  If you don't want to replace your PM exciter or have a UHF repeater, you can modify the PM exciter to use a FM ICOM.
The FM ICOM is the fat one about 1/3 larger than a standard PM or receive ICOM.

At Dayton 2005, Repeater Builder demonstrated the AP-50 in a phase-to-fm UHF MASTR II mobile repeater conversion.  The PM exciter was modified to eliminate the use of the stock "blob" and phase modulator, and use the AP-50 driving a FM PLL ICOM.  While 'our' modification goes undocumented on the Internet at this time (other than what is suggested in this article link below and paragraph following), the information in the article below gives an overview of what we do:

The basic difference between the article Jeff DePolo wrote and what we do is two fold:

First, we remove the original ICOM contact pins (all channel locations) and replace them with one set of mating pins for the FM ICOM (more pins).  This is done by selecting a place where one single FM ICOM can be physically accommodated, and where it isn't too difficult to complete the connections of the new ICOM mating pins to the existing circuitry.  We accomplish this by clearing a spot and removing any unneeded traces, then, drilling the circuit board and installing the proper pin header for the FM ICOM.  This allows one FM ICOM to be plugged right onto the exciter, like any other element; without the use of a dongle cord.

Secondly, we don't modify the FM ICOM internally as outlined in the article above by Jeff DePolo, rather we place the added components and 'jumps' for the bias, bypassing, etc., onto the exciter board allowing any single FM ICOM to be utilized without the need of any modification to it.  The image below will help with the added circuitry.  Note:  Pin 9 is synonymous with pin 8, as there isn't really 9 pins.  Pin 8 is actually skipped, and pin 9 becomes "offset".

Folks were simply amazed at the simplex sounding audio quality of this repeater utilizing the AP-50, FM ICOM, and RC-210 controller.

Regular de-emphasized audio was used to feed the controller audio input and the output was fed to the AP-50; where pre-emphasis was added.  In this example, no controller modifications were needed.  The AP-50 fed audio directly to pin 3 of the FM ICOM. This conversion is also beneficial when using the AP-50 in a Flat Audio modified repeater or remote satellite receiver site. Wide-band audio will make your voting experience more pleasurable.

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Last updated 12-November-2005 by Kevin Custer  W3KKC
(added FM ICOM interface image)

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