Some Background:
The Micor 150.8 to 162 MHz. high-band receiver is the most common unit you will find in the used/surplus market.  These receivers were built in the day when most radio systems had only one or two channels of operation.  They were designed to be highly selective from the standpoint of both the front-end and I-F stages, and were built to operate on a narrow portion of the spectrum.  Parts for this type of equipment were specified to purposely limit the tuning range.  This allowed for more precise tuning and better stability from both a mechanical and electrical standpoint.  Channels were usually paired close together so the receiver would operate without reduction in receiver sensitivity as it was greatly reduced when trying to receive more than a few hundred kilohertz from where the receiver was tuned.  Referring to the manual and parts list shows the intended tuning ranges and the parts associated with them.

Concept:
The cost of the factory 132 - 150.8 mHz. coils has increased drastically over the last few years and replacement by this method is expensive to say the least.  This prompted us to do this research and seek other methods of modification that would give the same results as the factory low split coils.

Research:
The factory Helical Resonator (HR) coils for the Micor high-band receiver come in two splits even though the receiver RF board is built in four splits.  The low and high splits for the HR coils are 132 - 150.8 and 150.8 - 174 Mhz.  It *seems* that Motorola had "sub-splits" within the low and high splits.  The difference in sub-splits is in the length of the tuning screws, as we found out that Motorola used several different length screws depending on sub-spilt and actual frequency the radio set was originally built for.  I always thought the screws were the same length, no matter the frequency, because there is no breakdown of the differing lengths in the parts list of the manual.  What we discovered is that if a casting was to tune from 150.8 to 162 mHz., Motorola placed 150.8 to 174 meg coils in the casting and used tuning screws that are 1.25" long.  If the casting was to be used above 162 mHz., the screws were either 3/4" or 1" in length depending on exact frequency.  *However*, in no case have we found a screw length longer than 1.25".  Why?  At the point where the 1.25" screw is fully inserted into the casting (just before the tensioning nut pops off) maximum capacitance on the end of the HR occurs at which point longer screws are of no benefit in making the resonator tune lower.  In other words the 1.25" screw tunes the resonator to just about its lowest frequency.  Placing longer tuning screws, or larger tuning screws into the casting creates two problems.  First, the width of the bandpass is greater due to "pulling" the high band coil to resonance.  Second, Insertion Loss (IL) is greater than if the correct coils were placed in the casting.   These two things are (at least to us) unacceptable in duplex repeater service, especially on a 600 kilohertz split.

  Scott recently obtained an HP 8920 service monitor equipped with a spectrum analyzer and tracking generator that allows us to do very accurate filter testing.  We swept some factory original high-band, high and low split helical resonator castings to get a benchmark on how far they'd actually tune and determine their bandwidth and insertion loss at several points across their intended tuning range.  We agreed to use the mixer FET on the RF board in our testing because the tap point on the output of the HR is not at the 50 ohm point, so connecting 50 ohm equipment to it won't show the filters exact response.  What we did was bolt the HR casting to an old 150.8 to 162 mHz. RF - I-F board and power it so the mixer FET was operational.  This allows the match on the output of the HR to be very close to what it is in actual operation.  We simply capacitor coupled RF from the output of the FET to the test equipment.  Using this method we were able to make several response and loss measurements with the factory original high and low split helical resonator castings to establish the benchmark.  The results were documented and used in comparison with several coil/screw modifications to be absolutely certain of measurement accuracy.

  In the past we have had some success in modifying the high split HR coils by adding 1/2 turn to the grounded end to make them resonate lower.  It was found that adding 1/2 turn and using the 1.25" tuning screws was acceptable down to about 144.5 mHz. or thereabouts, but with decreased sensitivity and less desirable pass band response below about 146.0 megahertz.  After some extensive evaluation of this and several other methods to tune the 150.8 - 174 mHz. castings into the 2 meter ham band (bigger screws, longer screws, adding 1/2 turn to the coil, etc.) we determined the only acceptable approach was to either use the original 132 - 150.8 megahertz helical coils or wind our own coils that would tune this band.  Since the original coils cost so much, Scott and I spent numerous hours trying to come up with an acceptable method for fabricating a hand-made coil for the 2 meter band that would allow any factory length screw to be used.  We now have a method that works out very well.  How?  We built a form that allows us to wind a replica of the low split coil.  Then we came up with a method of attaching the replica coil to the original coils pedestal (foot).  This allows us to bolt the replica coil into the casting which allows it to be secured at both ends as original.  This method was tested on several receivers and we are quite happy with the outcome as the new coils perform identically to the factory original 132 - 150.8 megahertz units with sensitivity and selectivity identical to that of a receiver using a factory original low split resonator casting.   We did several drop and vibration tests to insure the preselector won't go out of tune during shipping or mobile operation.

If you are interested in having us modify your Micor high-band preselector to work properly in the 2 meter ham band, Go Here.
 

Copyright 10-13-2002  Kevin K. Custer  W3KKC
All Rights Reserved.

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