Concept:
Are you trying to use that commercial split MICOR receiver in the ham band?  Having some trouble getting it to tune and have good sensitivity?  You have come to the right place!  The idea here is to modify a TLD4073, TLD5273, TLD5773, TLD8273 or just about any other model number MICOR high-band commercial receiver for use in the 2 meter ham band.  Please read through the entire article before attempting the conversion.
You may also be interested in reading a  Comprehensive Description of the MICOR Sensitron high-band receiver.

Background Theory:
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 made 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 view.  Channels were usually paired close together so the receiver would operate without reduction in 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.  The "Comprehensive Description" link above provides more in-depth theory on how the receiver operates, and its tuning ranges.

There are several different approaches of modifying the MICOR receivers named above for ham band use.  One acceptable solution I found is to change the 150.8 to 174 MHz. preselector coils to the ones that are intended to tune the ham band.  Placing longer or larger tuning screws into the helical casting creates two problems.  First, the width of the bandpass is greater due to 'pulling' the high band coil to resonance.  Second, insertion loss is greater than if the correct coils were placed in the casting.  These two things are (in my opinion) unacceptable in duplex repeater service, especially on two meters.  Research was done on pulling and other methods of making the 150.8 to 174 MHz casting tune the ham band.  Go Here to view the research.

Modification Description:
You'll need to replace the old 150.8 - 174 MHz. helical coils with ones that tune the 132  - 150.8 MHz. range.  Re-done front-ends are available from me, just click on the helical resonator ordering instructions link below.  From the factory, all 5 coils are identical except for the two end coils that have wires soldered to them.  Since you are converting an existing casting, just salvage the wires from the old end coils and solder them to the new ones at exactly the same place from the grounded end.   This ensures the correct impedance point for the coils.

Helical Resonator Casting/Coil Ordering Instructions   Click to purchase a remanufactured 132 - 150.8 MHz. preselector casting for two meters.
There are some options to choose from....

Modification of Helical Resonator Casting:
Remove the "sensitron" receiver "RF & IF" 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 by taking out the 2 screws holding each resonator coil, and by unsoldering the input connection from the input coil [leave the little wire connected to the input connector, just unsolder it form the coil.]  The use of a quality thin wall 7/32" nut driver will make removal of the screws less difficult.  Unsolder or break the coil ground end connections with a pair of needle nose pliers.  The output coil has a small wire connected to it that passes through a hole in the casting.  This is the output connection of the filter assembly that goes to the mixer.  Simply unthread the wire out of the casting then unsolder the wire from the coil and save it for installation on one of the new coils.

Some capacitors and one resistor in the oscillator multiplier need to be changed to insure these circuits properly tune the 2 meter ham band. Instructions are given below for their replacement.

Modification of RF - IF board:
150.8 - 162 model receivers will require the changing of some capacitors and possibly a resistor in the oscillator/multiplier section.  Change C108 to 27 pF, C109 to 47 pF, C110 to 80 pF, and C113 and C116 to 7.5 pF.  If 7.5 pF capacitors cannot be located, 7 pF or 8 pF will work just fine in these two locations.  It is helpful to do these changes with the helical casting and all 5 aluminum coil shields in the area removed to gain access to the capacitors.

The text below was offered by Robert E. Swoger, Engineer at Motorola:  It is noted by its designer, Joe Sallak that the mixer transistor is starved for LO injection when the LO was switched to low side injection. The change of R107 to 82K is to give adequate injection to the mixer; an improvement to make the sensitivity spec in the factory.  The R107 change in the HB receiver board is very important when converting to the ham band. This is a swamping resistor at the output of the source follower. When inverting the injection side, there is a reduction in LO drive for the mixer. The resistor must be changed from 12 K to 82 K. Anyone who ever told me they were getting marginal quieting were found to have failed to change that resistor.  The design of that section of the receiver was Joe Sallak, N9TI, Engineer, Motorola Inc.

Authors Note:  I have converted approximately 300, 150.8 to 162 MHz receivers to the 142 to 150.8 split.  I have never had a problem with LO injection (lack of sensitivity) after changing the 5 ranging capacitors in the LO chain, and the Helical Resonators, but, the note by the designer was offered by Robert Swoger, K9WVY, Engineer at Motorola for the MICOR, and I agreed to note it in the text.  I find that the MICOR receiver has between -117 and 118.5 dBm sensitivity (for 12 dB SINAD) in the ham band when properly converted.  Of course, YMMV.

Refer to the images below:

Here are images of coil shields and component locations.

Reassembly:
Start the reassembly of the casting by installing the salvaged wire from the old output coil by soldering it to one of the new ones at exactly the same point from the grounded end with a soldering gun or large enough pencil.  This ensures the correct impedance point for the coil.  Install the new resonator coils by inserting the two screws into the coil base and then carefully dropping the coil, with screws down, into the casting.  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.  Install the rest of the coils by securing the screws first then make sure the coils are centered inside each chamber and solder the grounded end of all of the coils to the casting.   A propane torch is handy when soldering the grounded end of the resonators to the casting.  I use a pencil torch as it works well for this.  Let the casting cool so you can handle it.  Next, solder the input wire to the input coil at exactly the same point it was attached to the old coil, insuring that the little wire doesn't short out against the casting.  Finish by reinstalling the tin cover and place the completed casting back onto the receiver printed circuit board by securing all screws.  Be careful not to over tighten the casting screws as the printed circuit board is easily damaged.

Tune the receiver as per the Motorola manual.  No other modifications will be necessary for the above receivers to perform as specified in the manual when tuned to the 2 meter ham band unless you are using the optional AFC plug-in module.  If using the AFC plug-in module with the appropriate channel element, reverse the polarity of diodes CR103 and CR104 in the crystal discriminator portion of the demodulator.  These diodes are very fragile and it is difficult to remove them without damaging them.  Some people have had success in simply cutting the diodes from the board, turning them and tack soldering then back in place.  If you are not using the AFC option (most likely) there is no need to reverse the diodes as this would be as useless as reversing the polarity of the speaker leads on a monophonic radio.

Crystals, for the receive channel element (K1005), can be purchased for the ham band from Bomar.
Complete channel elements on your frequency can be obtained from Bomar Crystals.
 
 

Copyright 1-30-1997  Kevin K. Custer  W3KKC    Any Comments? Send them here >  e-mail Kevin

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