Today was another R&D day at Repeater Builder (the company). Scott and I successfully built an interface module that allowed us to use a GE MASTR II PLL ICOM in a Motorola Micor VHF phase exciter. This research is part of development we are doing to produce a plug-n-play interface to allow easy installation of a Mastr II FM transmit element in several different radios that were originally phase only modulated. We hope to make a series of professionally built interface modules that will make it easy to "FM" radios like the phase modulated VHF Micor, UHF Mastr II, VHF and UHF EXEC II, and the Hamtronics TA series exciters.
We found that the PLL FM ICOM wouldn't drive the Micor exciter directly for several reasons. The biggest reason was the very low impedance of the input of the exciter because of the 150 ohm return resistor for Meter 2. The FM ICOM's out is somewhat lower in amplitude than the stock K1007 channel element. After looking at the schematic of a KXN-1019B channel element, it was decided that we needed some gain and buffering between the GE ICOM and the exciter. A single stage NPN transistor amplifier stage was added between the FM ICOM and the exciter; which alleviated all of the interface issues we discovered. The 'interface module' PC board provides the 6.1 volt bias for the varactor, as well as the 5 volt bias for the compensation line. The interface allows an unmodified FM ICOM to be used; it only needs recrystaled (and properly compensated) for your operating frequency. We are looking into putting temperature compensation components on the interface to allow the use of more common EC ICOM's to be used, but have the same compensation as if it were a 5C unit; or was driven with a 5C ICOM. We have successfully used PLL ICOM's in UHF phase Mastr II, VHF phase Mastr II, and now the VHF phase Micor. Why are we doing this? 1, True FM UHF Mastr II's are VERY scarce. This interface allows you to have the same desired FM audio, without paying the BIG price for the real FM UHF Mastr II exciter. 2, While you don't have the better phase noise advantage with a FM'ed VHF Mastr II phase exciter compared to a real PLL Mastr II VHF exciter, the phase exciter is much more common. 3, In the Micor VHF, the KXN-1019B's are getting hard to come by. This interface allows you to use a FM ICOM even in a 3 pin phase exciter. The interface will also allow easy connection, and possibly mounting area, for the RB AP-50 audio processor; which will be great for those wanting to do away with the stock audio path totally. Comments - Questions are welcomed... And, a question for you folks... Should we consider this retro-fit in any other equipment? Thanks, Kevin Custer
