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  Modification of the MICOR "Sensitron" 450-470MHz Receiver RF & IF Board for use in the UHF ham band
By Kevin K. Custer W3KKC
Some notes added by Mike WA6ILQ
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Background:
The UHF MICOR Receiver RF boards that were manufactured are:
Early ID Late ID Frequency
TRE1201 TLE8031 406-420 MHz
TRE1202? TRE1201BA-SP7 420-450 MHz
TRE1203 TLE8032 450-470 MHz
TRE1204 TLE8033 470-494 MHz
TRE1205 TLE8034 494-512 MHz
The TRE1202 is very, very rare, and only found in European systems (the business / commercial band starts at 440MHz in Europe and the UK), or where a MICOR was factory ordered on amateur channels in the USA. The 420-450 range is skipped over (in the manual) in the factory stock receiver boards.

Concept:
The 450-470MHz TLE8032 A or B boards are the most common UHF receivers, however to operate properly in the 440-445 range they require modification.   This modification described below is not necessary if the radio will be receiving above about 445 MHz.

Description:
Some of the stages in the receiver don't tune well if the receiver is to operate below 445 MHz.  Doing the following modification will alleviate the tuning problem.

You may also be interested in reading a A Comprehensive Description of the MICOR Sensitron hi-band receiver.
Although written for the MICOR Highband receiver, most of the information in the article is pertinent.

Modification:
If your radio will be receiving below 445 MHz, change C117 to 24 or 27 pF,  C119  to 36 or 39 pF, C136  to a 47 pF, and C125  to 12 pF. Also change R118  to a 1.5 k Ohm on the receiver board.  Again, this modification is not necessary if the radio will be receiving above 445 MHz.

The factory made 420 - 450 MHz helical resonators for the UHF MICOR. The part number is 2484731E12, but they are no longer available from Motorola (as of June 2006). Ultimately, all of these parts are needed to have a "real" 420 - 450 MHz MICOR receiver, but simply changing the caps and resistor will help a lot.

Tune the receiver as per the Motorola manual. No other modifications should be necessary for the above receiver to perform as specified in the manual, when tuned to between 440 and 450 MHz.

Notes:
When using a converted mobile for repeater, you must defeat the AFC otherwise an off frequency user will force the repeater transmitter off frequency.  These days,  it's not likely that a users radio will be off frequency so AFC isn't as important as it was 20 years ago.
If you have reversed the injection in a Station configuration for some reason, you'll need to reverse all four discriminator diodes (CR105 through CR108).  They are very fragile, so be extremely careful or you will be needing new ones.  It's been my experience that one good practice is not to unsolder them, but to clip them off the board and then tack solder them back into place after reversing them.  Motorola used hollow rivets in the construction of the circuit board and this makes it difficult to remove a part without destroying it.

The easiest way to defeat the AFC is to ground the AFC line by shorting across C115, a 10uf cap, which is located in one corner of the receiver board near the channel element selection diode matrix. On some receiver boards it's labeled as C116, the easy way to locate it is to trace P904 pin 7 from the connector across the board past the channel elements and the first cap you come to is it.

The UHF MICOR receiver boards are interchangeable between mobile and station and use either a KXN1024A (0.0005% AFC) or a KXN1029 (0.0002% accuracy, non-AFC) channel element.

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This page originally posted on 30-Jan-1997
Last modified 12-June 2006

Copyright © 01-30-1997 Kevin K. Custer W3KKC
Italic notes by Mike WA6ILQ

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.