[Motorola] "New" Motorola Micor helical resonators for 2 meters.

[Kevin Custer]

Hi All,

  After a recent inquiry from an individual to modify several high-band
Motorola Micor receivers for use in the two meter band, Scott Zimmerman
and I have come up with a method of making new (cheaper) Helical
Resonator Coils for use in the 2 meter ham band.

  As you may know - modification of the Micor high-band receiver from
Scott or I for use in the 2 meter ham band included obtaining factory
132 - 150.8 mHz.  Helical Resonator (HR) coils from Motorola and
replacing the 150.8 to 174 mHz. units with them.  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 seek other methods of modification that would give
the same results as the factory low split coils.

  The factory helical resonator 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.  The bandwidth of the desired frequency is wider due to
"pulling" the high band coil to resonance.  Also, 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.

  Scott recently obtained an HP 8920 service monitor with 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 its exact response.  What we did was bolt the
HR casting to an old 150.8 to 162 mHz RF 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 coil/screw modifications and coils we would wind on our
own 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 poor pass band response below about 146.000.
After some extensive evaluation of this method and several others 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
meg. 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 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 new coil to the original coils pedestal
that bolts into the casting which allows the new coil 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 Scott or I modify your Micor high-band
preselector to work properly in the 2 meter ham band, email either of us
directly for more information and options.  Also, the website has been
updated to reflect this method which is ultimately cheaper, but without
any sacrifice of original performance.

http://www.repeater-builder.com/rbtip/coilinstructions.html

--
Regards,

Kevin K. Custer  W3KKC
kuggie@kuggie.com
Senior RF Engineer, Masters Communications, Inc.

M. Scott Zimmerman
n3xcc@peoplepc.com
Zimmerman Electronics - Repeater Builder

All of kuggies messages scanned with NAV 2002.