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Re: IC-820H Mods



Les,
	Some time ago I downloaded this from some FTP and kept it in my HD, I
believe it is still around somewhere on the internet.
	This MOD unlike the 821's was not written by myself and I did not even
read it completely, so I cannot comment on it.
	I did read the section of X-band repeater enabling mod!, does anybody know
of something similar for the IC-821 ?.

	73 de Fernando, CX6DD. (text is below)

                               ICOM IC-820H
                   EXTENDED RECEIVE/TRANSMIT MODIFICATION
                   --------------------------------------

         This modification is to open the IC-820H for extended receive
    (and transmit) operation.  This is a two (four) step modification
    which will open the IC-820H to the following frequencies:

    VHF Band    136 - 174 MHz
    UHF Band    420 - 460 MHz

    These frequencies are not guaranteed for accuracy or sensitivity.

    1.  Remove Diode D25 on logic board for VHF RX Expansion.
    2.  Remove Diode D26 on logic board for UHF RX Expansion.

    3.  Remove Diode D27 on logic board for VHF TX Expansion. *
    4.  Remove Diode D28 on logic board for UHF TX Expansion. *

    * RX modification must be done before the TX modification will work. *

    ======================================================================


                               ICOM IC-820H
                      CROSS BAND REPEAT MODIFICATION
                      ------------------------------

    1.  Add a diode on the blank position between D27 & D28 on the
        Logic Board. The part is 1SS353  P/N 1750000390.

    AFTER HARDWARE MODIFICATION

    1.  Set lock mode.  Push [LOCK]

    2.  Turn Power off.

    3.  Hold [M/S] and turn radio on.

    4.  Turn repeater off.  Push [LOCK].


    Note:  If you removed Diode D25 or Diode D26 in the first modification
           then it can be used in the second modification as it is the
           same part.


                             IC-820H DISPLAY BOARD
                             ---------------------

    The following is a partial, not-to-scale, parts placement layout of
    the IC-820H DISPLAY UNIT circuit board (bottom view). The DISPLAY UNIT
    is the large circuit board that is located directly behind the front
    panel.  Locating the following four distinctive parts will ensure you
    have the right diodes: the main dial encoder, the microphone compression
    level variable resistor (R101), J10 a ten pin header and a row of surface
    mount resistors immediately above J10.

    CAUTION: These diodes are NOT labelled sequential! Ensure you are 
             certain of which diode you wish to remove.
                                                                       |
                                                                       |
                                                                       |
    UHF TX -->  -[  []-  D28                                           |
    CB RPT -->           BLANK                                         |
    VHF TX -->  -[  []-  D27                  -----                    |
                -[  []-  D24         [R106]   |1o |                    |
    UHF RX -->  -[  []-  D26         [R107]   | o |                    | B
                -[  []-  D23         [R108]   | o |           MIC      | O
    VHF RX -->  -[  []-  D25         [R90 ]   | o | J        COMP      | T
    NOTE 2 -->  -[  []-  D22         [R91 ]   | o | 1        ------    | T
                                     [R92 ]   | o | 0       |      |   | O
                                     [R93 ]   | o |         | R101 []- | M
                                     [R94 ]   | o |         |      |   |
                                     [R95 ]   | o |          ------    |
                                              | o |                    | E
                                              -----                    | D
                                                                       | G
                                                                       | E
                       /-------------------------------------|         |
                     /                                       |         |
                   /                                         |         | O
                 /                                           |         | F
               /                                             |         |
              |                  E N C O D E R               |         |
              |                                              |         | C
              |                  O P E N I N G               |         | A
               \                                             |         | R
                 \                                           |         | D
                   \                                         |         |
                     \                                       |         |
                       \-------------------------------------|         |
                                                                       |


    Note 1: The IC-820H Service Manual calls this board the "DISPLAY unit" 
            and the modifications call it the "logic board"; they are one
            and the same.

    Note 2: Diode D22 is listed as "[EUR],[AUS] only" in the IC-820H
            Service Manual.


                        QRP mod for ICOM transceivers
                        -----------------------------

                       /
  9-volt   - o-------/  ------/\/\/\/\/\---------
  alkaline            S1          R1            |
  battery  + o--                                |
                |                                >
                |                              <      shielded cable
             _____         variable  R2-->       > <------------------> ALC
              ---                              <    ------------------> JACK
               -                                 > |                 |
                                                |  |     ground      |  ON
                                                |__|                 |  RADIO
                                                |                    |
                                              _____                _____
                                               ---                  ---
                                                -                    -


       With this circuit connected to the radio, the power output
    is adjustable between 0 ---> 100W.  For best results. leave the
    RF power control at maximum and adjust for desired power using
    R2.

       S1 is used to turn off circuit when radio is not in use or
    QRP operation is not desired.


    Radio Model:                                      R1 value    R2 value
    IC-761, 765                                       47K         20K
    IC-720(A), 735, 740, 745, 751(A), 275A/H          2M          1M
    IC-725, 726, 728, 729, 736, 737(A)                220K        100K



                                                         06/01/94 CJR



    For the IC-820H use the values for the IC-761, 765.  The IC-781,
    IC-765 and the IC-820H all have the same ALC characteristics:

           Control voltage  : -4 to 0 volts
           Input impedance  : More than 10 K ohms



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