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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
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.
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.
The receiver channel element (K1005) still operates at the crystals 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.
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.
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 order altered coils from the author.
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.
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.
Remove all five aluminum shields from the oscillator end of the receiver circuit board as shown in the following image.
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: C-113. 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.
C 107: 0.75 pF (yes, under 1pf) (this one may already be a 0.75 pF, if
not, replace with one. Violet, Green, Gray, Silver)
C 108: 27 pF. (will already be a 27 pF if a high split receiver is being converted)
C 109 - 39 pF.
C 110 - 100 pF. (depending on the original split, this may already be a 100 pF, if not, replace with one)
C 113 - 4.0 pF for 220-223.5 MHz, or 3.0 pF for 223.5-225 MHz.
C 116 - 3.0 pF.
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 12 K 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...
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 is 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.
The crystal for the receive channel element (K1005) for use in the modified receiver can be purchased from Bomar or ICM. If ordering from BOMAR, specify the crystal is for a "KC1005/12" channel element and simply give them the desired operating frequency. They will build a 220 crystal and mark it accordingly. This specific number - "KC1005/12" - is logged into both Bomar's computer system and there will be no confusion as to how the crystal is to be built for this modification. Crystals may also be available from other vendors.
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.
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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.