Subject: ICOM IC-745 extend xmit



Submitted to QRZ.COM on 06-1999     7 years, better late then never!



73 Paul N2FAN       (n2fan




   REVISIONS:

---------------

** 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 **********/



Intro:



 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"



  After analyzing this perplex situation I came to the conclusion

ICOM must have been doing their part in trying to save precious spectral

bandwidth by not incorporating AM transmit in this particular model.

 No one really utilizes AM mode these days (including myself) but this

radio has every mode ever conceived and to leave AM transmit out really

aggravated me!



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              -----------

                                 | | | |

                                 -------

                                   R8



/***************************** 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.



                C

                  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)

         ^

         |

         |

        ___

       break

        ___

         |           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

    operation.

                 C

                  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.



    --

 o-||||--->|-o

    --  |

    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

    cycle.



    FM8 o---->|-------/\/\/\/----o

            1N914    |  10K      |

    AM8 o---->|-------           o-->|-----o to Q20

            1N914                |  D26

                                 o

                                 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

                     o  S1

                     |



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

 as AM.



OPERATION/CONCLUSION

---------------------



   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....................???

  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



*05/2000

I have scanned the Main Board and Logic Board to depict the electrical

changes made to the circuits. I tried to nicely re-label all the changes

using VISIO technical, but I ran into some offset problems.

The hand written changes will have to do for now until I can get Visio to

work right.

If you have any questions about this Mod feel free to ask via e-mail

only (I'm hard to get a hold of)  e-mail = n2fan


This is not an easy cut the wire mod, you will need to have some good

electronic technician skills. PhD's forget it! ...............

This Mod works, I still have my IC-745 after 8 years!

73 and Good Luck!