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
Concept:
To modify a Motorola MICOR Low Band 42-50 MHz. 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.