I CANNOT VERIFY THIS. However I do know the
aircraft (hold F down then LOW) toggle does work.

TH28A Mods:
 

1) Remove the 4 screws holding the radio together.
2) With your finger in the battery bay, separate the two parts.
3) The inside of the keypad case has a copper RF shield.  Move this shield
    out of the way by removing the 2 screws and bending it up and away from
    the PC board.  (There may be double sticky tape holding the shield to
    the IC.)  Under the shield you will find a large IC surrounded by
    an array of diodes, as follows:
 

                    +-------+
                    |       |
                    |   IC  |
                    |       |
           D D D D  +-------+
           1 1 1 1
           5 4 3 2
 

                   D D D D
                   1 1 9 8
                   1 0
 

    (The diode numbers are read vertically.)
 

    NOTE: Diodes D15 & D14 are NOT present in the American version.
 

4) Remove diodes D8 and D10.
5) Install one of these diodes in D14.
 

This enables the following three extended modes:
 

    1) Extended receive from 360 Mhz to 400 Mhz.
    2) Extended receive from 400 Mhz to 520 Mhz.
    3) Extended transmit/receive from 136 Mhz to 174 Mhz.
 

in addition to the existing aircraft frequencies of:
 

    4) Receive from 118 Mhz to 136 Mhz. (AM)
 

(AM and FM are switched automatically according to the frequency range.)
 

To switch between the modes, press the [F] key for one second, then
press the [LOW] key.  This will switch between:
        a) Amateur VHF band (136 Mhz to 174 Mhz).
        b) 340 Mhz band (340 Mhz to 400 Mhz).
        c) Aircraft band (118 Mhz to 136 Mhz).
 

To get the extended amateur UHF receive, press the [BAND] key while in the
136 Mhz to 174 Mhz mode.
--

  By just cutting diode 9, you enable the wide band receive on UHF
(from 400-520) and from 360-400 by hitting F and LOW.

  I would be careful when cutting diode D10 out though, I did it
(I've done surface mount mods before) and my row 3 keys stopped working
(MEM, 3, etc)  I am positive I didn't clip the wrong diode because I
clipped the connection leading out from the diode (you'll see what I
mean...)

  Just don't get carried away, that's all..

>This enables the following three extended modes:
> 
>    1) Extended receive from 360 Mhz to 400 Mhz.
>    2) Extended receive from 400 Mhz to 520 Mhz.

The above is available from the diode that I mentioned above.  By the way,
the mods presented here hold some validity as they come from a tech sheet
that I got from the dealer.. a certain big mail order company down the states)

>    3) Extended transmit/receive from 136 Mhz to 174 Mhz.
> 
>in addition to the existing aircraft frequencies of:
> 
>    4) Receive from 118 Mhz to 136 Mhz. (AM)

This mod comes from simply holding down F and hitting LOW.  If this is
all you want, don't cut anything.. I shudder at the thought at clipping
out surface mount diodes...

Rx     AM  118.000 - 135.995
Tx/Rx  FM  136.000 - 173.995
Rx     FM  340.000 - 399.995   
Rx     FM  400.000 - 519.995

Here are the diode settings 

             |
  o o o o    |
  | | | |    |   I.C.
  o o o o    |
             |_______________

             o o o o
                 |  
             o o o o

the above freq. ranges and the above diode 
settings definitely work !!

thanks to;
Iain Campbell                                        [email protected]
Radio Schweiz AG                                     [email protected]
Laupenstrasse 18a
3008 Bern                                            Tel: +41 (0)31 509 855
Switzerland                                          Fax: +41 (0)31 509 675
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