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By Kevin K. Custer W3KKC
Original Concept by David A. Cooley N5XMT
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Comprehensive Conversion of the Motorola® MICOR® "Sensitron™" High-Band
receiver for use in the 222 MHz band By Kevin K. Custer W3KKC Original Concept by David A. Cooley N5XMT |
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Background and Concept:
There are no high quality 220 MHz. repeater-grade radios at a decent price, so hams have
to modify other equipment to work at 222 MHz. The Motorola "MICOR" line is plentiful and
not hard to convert. The high band covers 132-174 MHz, and to provide the optimum
performance, Motorola makes their equipment to cover specific frequency ranges, called
"splits" (and the receivers on the two low splits are rare). In this writeup we will show
how to modify any of the following MICOR highband receiver boards for use in the 222 MHz
amateur band.
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Description:
I have tried several approaches to modify the MICOR receiver for 1¼
meter (222 MHz.) use. This modification is the result of approximately 40 hours
of engineering and testing, using an old mobile frame for a test bench. The
only respectable solution I found was to alter the preselector coils in a way
that allowed maximum coupling while retaining the use of the original tuning
screws. Placing longer or larger tuning screws into the casting creates two
problems. First, the desired passband is wider due to "pulling"
the coil to resonance. Second, insertion loss is greater when this or any
other procedure was tried. Wider front end bandwidth and excessive
insertion loss are two things that are usually unacceptable in repeater
service.
This modification will allow any of the above receivers to be converted to the 222 MHz amateur band while retaining original receiver specifications. The original 150.8 - 174 MHz. coils will be altered for use in the ham band. Capacitor replacement in the oscillator multiplier stages are offered in 2 splits to allow better tuning and stability. From the factory, some preselectors used a tuning screw that is ¾" long while some were 1" or 1-1/4" long. This modification will allow any factory screw length to be used in the 222 - 225 MHz ham band.
Crystal Information:
The receiver channel element (K1005) still operates at the crystal's third
harmonic as original, however the multiplier is now operating on a factor of 4
times instead of 3. No modification to the K1005 channel element is required.
The multiplier provides Low Side Injection, and the crystal formula
is Fcr = (Fo - 11.7) divided by 12, where Fcr = the
crystal frequency, and Fo = the operating frequency.
The local oscillator provides 12 times the crystal frequency to the mixer,
instead of the original 9 times.
This brings up a neat situation. For instance, a crystal cut for a Highband MICOR on 146.280 MHz will also work on 222.340. This happens because the 142-150.8 MHz MICOR operates on High Side Injection and the second multiplier operates at a factor of 3. If you have any 2 meter elements for a MICOR, they will work for testing the 220 receiver. The corresponding range is 144.525 to 148.275 for 220 to 225 MHz respectively. I have set up a specific reference number with one crystal vendor to eliminate any confusion as to building the correct crystal. See below for crystal ordering information.
Before Modification:
Be certain the receiver operates correctly on the original operating
frequency before attempting to modify it for the new frequency range.
The author suggests that you document the performance of your particular
receiver, using your test equipment, so you know what to expect after the
conversion is done. This includes logging the reading, in microamps, of
each metering point with a known level of signal from a calibrated
generator, perhaps at levels of 0.2, 0.5, 1.0 and 2.0 uv. This will allow
you to verify the performance before and after conversion. Please read
through these instructions, in their entirety, to become familiar with each
procedure before attempting the conversion. You may also be interested in
reading another background information article at this website:
A Comprehensive Description of the MICOR Sensitron
highband receiver before conversion.
The Modification:
Remove the receiver "RF & I-F" board from the radio set. Remove
the RF preselector casting by unplugging the small black wire on the output
end, and removing the 12 screws securing the casting to the circuit board.
Remove the 12 screws that hold the cover on the preselector. Now remove
the 5 helical coils, one at a time, by taking out the 2 screws holding each
resonator coil, and by unsoldering the input connection to the input resonator
(L-101). Leave the wire connected to the input connector in the casting. The use
of a quality 7/32" nut driver will make removal of the screws less difficult.
Do not use a power screwdriver here. Unsolder or break the coil ground
end connections with a pair of good needle nose pliers. The output coil has a
small wire connected to it that passes through the casting. This is the output
connection of the filter assembly that goes to the mixer. Simply unthread the
wire out of the casting. Do not remove the mixer wire connected to the
output coil (L-105). Label each coil as to which position it came from (1-5)
and how it was oriented to aid in remembering where the coils came from and
how the input connection was soldered so you can reinstall the coils into
their original locations and solder the input connection similarly.
Alter the helical preselector coils exactly as suggested below or get your casting(s) modified by the author.
Alteration Notes:
There are similar conversions elsewhere on the Internet for altering
the coils, however none of these take full advantage of the original coupling
scheme used in the preselector casting. These coupling iris openings determine
the bandwidth and insertion loss of the preselector casting. Most other coil
mods remove turns that are exposed to the iris openings between the sections
of the resonators. I have tried these other methods, and even some of my own
that change the iris opening. Of all of the methods I have tried, the one
presented here seems to work best.
Casting Reassembly:
Install the altered resonator coils into their corresponding locations
by inserting the two screws into the coil base and then carefully dropping
the coil, with screws down, into the casting. Stick your finger in the
plastic former to guide it into its location. This makes it easier to start
the screws. Install the output coil first by threading the wire back through
the hole in the side of the casting and securing the two hold down screws.
Use a thin shaft #1 phillips screwdriver so you don't break the solder
connection where the two parts of the coil are mated. Install the rest of
the coils by securing the screws. Now solder the grounded ends of all of the
coils to the casting. A small torch is handy when soldering the grounded ends
of the resonators to the casting. I use a butane pencil torch as it works
well for this. Last, solder the input wire to the input coil at the same
distance from the ground (cold) end as it was connected to the original coil
(approximately 1/4 turn.) Let the casting cool and finish by reinstalling the
cover. Set the casting aside for later installation back onto the circuit board,
as it will be easier to change the capacitors, and inductor in the oscillator
multiplier without the added weight and mass of the casting.
Oscillator Multiplier:
Remove all five aluminum shields from the oscillator end of the receiver
circuit board as shown in the following image.
Capacitor Replacement:
Remove and replace the following capacitors with the values indicated. There
are two tuning ranges for the multiplier depending on the desired new operating
frequency - one component determines the range: C113. Replacement
capacitors should be of the same type and style as the original. The use of
a desoldering tool or solder wick is recommended to open up the holes in the
circuit board for the capacitors and the shields.
C107 - 0.75 pF (yes, under 1pf) (this one may already be a 0.75 pF, if
not, replace with one. Violet, Green, Gray, Silver)
C108 - 27 pF. (will already be a 27 pF if a high split receiver is being converted)
C109 - 39 pF.
C110 - 100 pF. (depending on the original split and revision, this may already be a
100 pF, if not, replace it with one)
C113 - 4.0 pF for 220-223.5 MHz, or 3.0 pF for 223.5-225 MHz.
C116 - 3.0 pF.
Inductor Replacement:
The small inductor near the connection of the output side of the helical
resonator casting - L112 - needs to be checked for the proper turns count.
The factory used two different coils here and you can identify which you
have by the color of the coil form. If it is a white or clear coil form,
the coil has 13 and one-half turns, on the other hand if it is a yellowish
color form then it has 14 and one-half turns and you will need to remove
the inductor from the circuit board and carefully remove one complete turn,
and reinstall.
Additional Part: Insure that R107 is a 12k ohm, if not, replace it with one.
At this point you are finished with the conversion and all that is left is reassembly and tuneup. I suggest that you take a Sharpie marking pen and write on the side of the preselector something like "Modified for 222 MHz (date) (your callsign)" so that the next guy knows what is going on...
Reassembly:
Place the preselector casting back onto the receiver printed circuit
board, secure all screws and reinstall the aluminum shields over the coils.
Be careful not to over tighten the casting screws since the printed circuit
board is easily damaged. Do not use a power screwdriver.
Tune the receiver per the procedure in the Motorola manual. You should be able to obtain original sensitivity on the new frequency. Use your notes from the original testing to verify the tuning of each stage. The most important adjustment is the discriminator center. If you have no way to properly check the low IF stages or the discriminator adjustment, don't touch them. It is best to tune the MICOR receiver during the day. What?.....Yes, the receiver is so sensitive at the I-F frequency (11.7 MHz.) that it is difficult to tune the receiver with the covers removed during night hours due to the pickup of short-wave broadcasts in that range. This problem makes it difficult to determine the peak in tuning. I have been able to obtain original sensitivity on the new band from several receivers converted for the purpose of testing and proving this modification. My test setup includes an IFR FM/AM 1200A service monitor capable of performing SINAD measurements. I tested several unmodified MICOR high band receivers on my test bench to determine a baseline sensitivity using my equipment. The average sensitivity of the unmodified receivers was -116 dBm for 12 dB SINAD, this relates to the book specification of .35 uV. The sensitivity of the converted receivers ranged from -115 to -117 dBm, practically the same as before. It is not a necessity to use a preamp with this conversion, however one may provide extra sensitivity if needed or desired.
I have tested the Ramsey PR-20 preamplifier with the converted receiver. The Ramsey PR-20 will provide an additional 5 to 10 dB of useable sensitivity, nearly the same as that provided on high band with an original MICOR preselector preamp. The preamp actually has more rated gain than this, however this is the amount of improvement gained by adding the preamp to this receiver.
Purchasing Crystals:
The crystal for the receive channel element (K1005) for use in the
modified receiver can be purchased from Bomar.
Specify the crystal is for a "K1005" channel element and simply give them
the desired crystal and operating frequencies. They will make a 220 crystal
and mark it accordingly.
Final Notes:
Interesting pins on the receiver connector (P904) are:
Pins 1 and 16 = Ground
Pin 9 = Positive 13.8v
Pin 6 and 13 = positive 9.6v
Pin 15 = discriminator audio output to audio squelch card input
Connect a jumper between pin 7 and pin 14 to enable metering of L108
and L109 adjustments.
Ground pin 2 to enable the channel element in position number 1.
Up one level (MICOR index)
Up two levels (Motorola index)
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Revision history:
Copyright January 1, 2000 Kevin K. Custer W3KKC
Questions or Comments? Send them to kuggie /at/ kuggie /dot/ com with a subject line
mentioning the MICOR 220 receiver conversion
This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.