Modification of MICOR Low-Band Receiver
for use in the 6 meter Ham Band

By Kevin K. Custer  W3KKC
with additions by Sam Kirby, WB4HAP

To modify a Motorola MICOR Low Band 42-50 Mc. split receiver for use on the 6 meter amateur band.

Modification Description:
This modification involves altering component values in the front-end and crystal oscillator injection circuitry.  This modification will provide original sensitivity. There are two conversion methods listed here. Both will work satisfactorily. The second "better" conversion came directly from the Motorola engineering department in the late 70s’ by hams that worked there.  Doing the second modification will produce a board that will meet or exceed design parameters for each individual circuit. (The values, after all, were determined by the origional Motorola engineers!)   Some of these cap values are hard to find.  They can possibly be found in other boards of different ranges. (Sometimes it good to have a pile of junk boards laying around.)

Important Note:
Be certain the receiver operates correctly on the origional frequency range before attempting to modify it for the new band.  You may want to document the sensitivity of your particular unit, using your test equipment, so you know what to expect after the conversion is done.  Please read through these instructions completely to become familiar with each procedure before attempting the conversion.

MICOR Receiver Modification - Simple Method
Capacitor Replacement:  Remove the aluminum shields from coils to gain access to the capacitors.
Remove and replace the following capacitors with the values indicated.
Replacement capacitors should be of the same type and style as the original.

Front End:
C113 - 18 pF
C116 - 18 pF
C118 - 18 pF
C120 - 18 pF
C122 - 3.3 pF

Crystal Oscillator Injection (LO):
C108 - 33 pF
C109 - 68 pF
C110 - 68 pF
C125 - 33 pF
C127 - 43 pF

Tune the receiver as per the Motorola manual.

MICOR Receiver Modification - Better Method
From the Motorola Engineering Labs

Component Replacement:
Remove the aluminum shields from coils to gain access to the components.
Remove and replace the following components with the values indicated.
Replacement capacitors should be of the same type and style as the original.

Reference: 42-50 MHz Receiver schematic (TLB6864A) design center freq. at 52.525.

Change listed components to the following values: (all caps in pF)

C107 - 1.2 FC -- can be found in a 36-42 split receiver
C108 - 36 disc
C109 - 68 disc
C110 - 68 disc
C113 - 18 disc
C114 - 100 disc
C115 - 0.82 FC -- can be removed from C121 location
C116 - 16 disc
C117 - 0.56 FC
C118 - 16 disc
C119 - 0.51 FC
C120 - 16 disc
C121 - 0.56 FC
C122 - 3 disc
C125 - 33 disc
C127 - 40 disc
C128 - 1.0 FC
R103 - 3.3K
R109 - 120

Tune the receiver as per the Motorola manual.

You should be able to obtain original sensitivity on the new band.  The most important adjustment is the discriminator center.  If you have no way of properly checking on the discriminator adjustment, don't touch it.

Better receiver method and exciter modification information was supplied by Sam Kirby, WB4HAP,  January 2003
Older (simpler) receiver mods added back in February 2007, W3KKC
Original Copyright 1-26-1998 Kevin K. Custer  W3KKC    Any Comments? Send them here  e-mail Kevin

Last Updated: 3/7/07 N3XCC

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.

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