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Enable AM transmit on Icom IC-745

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Author Topic: Enable AM transmit on Icom IC-745  (Read 14798 times)
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Posts: 25

« on: February 27, 2006, 11:37:18 AM »


This is a modification written by Paul N2FAN that I had conducted on my own IC-745 a few years ago. It works well and allows for very good quality AM mode operation with the stock filters.  A simple diode switching arrangement can allow for wider 455 kHz IF filters for high fidelity on AM.

I use the IC-745 with 6, 8 and 12 kHz IF filters as a backup to my modfied Kenwood TS-850 for AM mode operation.

The marked up schematic is available at http://www.qrz.com/download/mods-i-k/ic745diag.zip


** Updates and Fixes  Submitted 05/2000 **
   Info: Some miss labeling and updates.
** Schematic Scan and Labeling  05/2000 **
 PJH 5/2000
----- MODIFY FORUM appended at 23:17:57 on 92/04/27 GMT (by PAULH at POKADD6) -
Subject: /********* AM TRANSMITTER ENABLE FOR THE ICOM IC-745 **********/


 After returning home from purchasing a used (but in good condition)
ICOM IC-745, initial check out revealed the transmitter did not function
in AM transmit mode. A look and the owners manual revealed:
 "AM Receive Only"  What!!!??? This is supposed to be an ALL MODE RADIO!
In reality the words used were..."ALL BAND HF RADIO"

Below is the Mod of Mods for IC-745 owners:

 (All Modifications administered to the "MAIN UNIT" board located under
  the top cover of the radio.)
 (Except for R21 Removal on the logic board for the Power Mod.)

Parts List:
8 pin diodes. Factory Diodes are P/N 1SS53. 1N914's are fine.
3 10k resistors 1/4 watt.
1 NPN TO-92 pkg. switching TX, 2N3904 or equivalent is fine.

Source Voltage Terminology and Locations:
R8   = 8 Volt Receive Mode Source
AM8  = 8 Volt AM Mode Source
FM8  = 8 Volt FM Mode Source
CW8  = 8 Volt CW Mode Source
RY8  = 8 Volt RTTY Mode Source
USB8 = 8 Volt USB Mode Source
LSB8 = 8 Volt LSB Mode Source

                         |                     |
        "J7"      Pin #1 | O O O O O O O O O O | Pin #10
     Main Board          |_____________________|
                           | | | | | | | | | |
                           --- --- | | --- ---
                            A   R  U L  C   F
                            M   Y  S S  W   M
                            8   8  B B  8   8
                                   8 8

        "J21"           Pin #1 | O O O O | Pin #4
       Main Board              -----------
                                 | | | |

/***************************** THE MOD *********************************/

1) Pull up the anode end of Diode D44. (located by TX Q39)
   Build the additional circuit below. This will enable the B+ to the
   product detector and 9 Mhz BFO oscillator.
   The 9Mhz osc will now be working for AM transmit as well as BFO Rec.

                  o to AM8  (Where the Anode of D44 was)
                    \| B       10K      1N914
                     |------\/\/\/\/-----|<-----o to R8
to R239 o----|<----/    2N3904 or Equivalent
            D44    E

2) Remove R66.  (Located by TX Q13)
   Relocate and solder one end of R66 to the bottom of the board.
   (The end that goes to R23 and R30)

    (The schematic shows a "BREAK" in the land. By removing the
    resistor from the PC board this simulates the break.)

    Solder two 1N914's to the other end of R66. (Cathode ends of course)

Note:  All pin assemblies will be connected on the underside of the Main
       Unit PC board.

   This is part of the Mode selection process, and turns off the
   mike shunt in AM mode. (So audio can pass to the Balanced Modulator.)

to R23 & R30   o-----\/\/\/--:----|<----o   Anode to CW8
                      R66    :----|<----o   Anode to RY8
                      10K     pair 1N914's (add)

3) Solder one 1N914 diode (Cathode end) to the cathode connection of D15
   (by TX Q19) under the board. Connect the anode to AM8.
   This will enable the B+ to the Mic Amp stages and VOX ckts.

        to R31 (10K)
         |           D15
         |------------|<----------------o Goes to 8 Volt LSB Source
         |------------|<----------------o Connect to AM8 Source
         |           1N914 (add)

        to R100 (100 Ohms)

4) Remove R31 (Located by TX Q8)
   Relocate and solder one end of R31 to the bottom of the board.
   (The end that goes to R30 and the base of Q8)

    (The schematic shows a "BREAK" in the land. By removing the
    resistor from the PC board this simulates the break.)

    Solder Three 1N914's to the other end of R31. (Cathode ends)
    Tie the Anode ends to LSB8, USB8, FM8.
    Function: This portion of the mod will turn Q8 off in the AM mode
    to unbalance the balanced Modulator (IC1) for AM full carrier
                  o                      3 1N914's (add)
          (Q8)      \| B         R31     |--|<-----o to  LSB8
        Bal/^Bal     |---------\/\/\/\/--|--|<-----o to  USB8
        switch      /|  |   |            |--|<-----o to  FM8
          for      /    |   |
          IC1     o     \   \
                E       /   /
                    R29 \   \ R30
                        /   /
                        |   |     R's=10K's
                       ___  |
                        _   o

5) Remove R24 (Located by TX Q7, "Mike Shunt Ckt")
   Solder a 1N914 Diode (Cathode end) into one of the pc holes
   that R24 came out of. (The one end that connects to R65 by Xtal X1.)
   Tie the anode to AM8.
   This will enable the 9 Mhz CW xtal to generate the correct center
   frequency for AM.

    --  |
    X1  \
    R64 \                         1N914 (add)
    10K /       R65
        |-----\/\/\/\--------------|<------o AM8
       ___      220     |
   C44 ___              ---|<---o to CW8
   47pf |
       ---                D13

6) AM Power Level Adjust (optional)
   Due to the duty cycle requirements of AM, output power of 40/50 Watts
   RMS should be the limit. Use the Front panel RF adj to reduce the
   output from 100 Watts or do the following:

 A) Connect a small 22 gauge (or so) wire from AM8 to the anode of
    Diode D26 by TX Q20. Add a series 10K resistor.

    AM8 o------\/\/\/\---------- o-->|-----o to Q20
                 10K             |  D26
   |________________________|    o
            Add this             o S1

 B) Set the 50/100 Watt power switch (S1) in the 100 Watt
    Position if you desire 100 watt operation of SSB,CW,RTTY.

 C) Adjust R116 (in AM xmit mode) for a power output of 40 Watts RMS.

 D) What were doing here is using the 50 Watt low power ckt as the AM
    power adjustment control.
    Paralleling another connection to FM8 will allow FM to be reduced in
    similar fashion (as well as AM) instead of the factory 100% duty

    FM8 o---->|-------/\/\/\/----o
            1N914    |  10K      |
    AM8 o---->|-------           o-->|-----o to Q20
            1N914                |  D26
                                 o S1

 Note: When 10 Meters is selected port 28M (pin 47,IC3 on the logic
       unit) goes high, this turns on pin diode D27 on the main unit.
       This ultimately turns on the 50 watt (low Power) ckt allowing only
       low power operation from 28-30 Mhz?
       I don't know why Icom did this? My guess is for 10 Meter FM
       overlooking the fact SSB/CW will be running reduced also.
       You may want to remove R21 (10K) on the logic board to remedy this
       situation. After R21 is removed you can limit the duty cycle on
       AM & FM as follows:

    FM8 o----\/\/\/\----->|----
               10K   |   D27   |
                     |         |
    AM8 o----\/\/\/\----->|-------o to Q20
               10K   |   D26
                     o  S1

 Low power Adjustment R116 will now adjust output power for FM as well
 as AM.


   The above AM/FM power mod is recommended. Higher power levels
  (Greater then 70/80 Watts RMS) result in severe distortion of
  the AM output signal.
    Surprisingly the audio quality is quite good on AM xmit. (40/50 Watts)
  Several on the air discerning ears concluded the fidelity was quite
  acceptable. (Even from the Johnson Viking Guys on 3.880 (+-))
  For you skeptics wondering how an AM signal can sound acceptable
  through a 3Khz wide 455Khz SSB filter....................Huh
  A quick turn to the schematic on the IF board depicts filter F-I1.
  (Ceramic filter CFW-455IT) This filter is a 6Khz wide ceramic filter
  used for AM receive mode. After the completion of the Mod this filter
  will be used for AM xmit also, by default! >>> It's in there!! <<<
  (Wasn't that a commercial once?)
  Filter F-I2 (ceramic Filter CFT-455K14) or the optional FL-44A
  (SSB filter) is in operation for all other modes except FM.
  The optional FM board employs it's own filter arrangement.

    That's it. Your all mode radio is now all mode!

73 Paul N2FAN

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