Icom IC-746 PRO (IC 746 PRO IC746PRO) transceiver mods reviews software and diagrams

   
 
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11 May 2021, 10:36 UTC 


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Propagation:
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Icom IC-746 PRO (IC 746 PRO IC746PRO)
IC-746 PRO
Programming interface schematics for Icom:
Programming software for Icom IC-746 PRO (IC 746 PRO IC746PRO) :
PDF User Manual for Icom IC-746 PRO (IC 746 PRO IC746PRO) : Icom IC-746 PRO Instruction Manual
PDF Adjusments Procedures Manual with schematics for Icom IC-746 PRO (IC 746 PRO IC746PRO) :
Schematics for Icom IC-746 PRO (IC 746 PRO IC746PRO) :

Mods for Icom IC-746 PRO (IC 746 PRO IC746PRO) :
    Icom IC-746 PRO (IC 746 PRO IC746PRO) Heatsink for PA Driver
    Icom IC-746 PRO (IC 746 PRO IC746PRO) backlight problem
    Icom IC-746 PRO (IC 746 PRO IC746PRO) Power adjust
    Icom IC-746 PRO (IC 746 PRO IC746PRO) AM tx improvements
    Icom IC-746 PRO (IC 746 PRO IC746PRO) Transmit Problem
    Icom IC-746 PRO (IC 746 PRO IC746PRO) SSB Monitor Level Fix
    Icom IC-746 PRO (IC 746 PRO IC746PRO) EU version
    Icom IC-746 PRO (IC 746 PRO IC746PRO) TX modification

Icom IC-746 PRO (IC 746 PRO IC746PRO) technical specifications :  Icom IC-746 PRO (IC 746 PRO IC746PRO) technical specifications



Icom IC-746 PRO (IC 746 PRO IC746PRO) AM tx improvements

This rig is a lousy performer on AM: Low modulation level (at 30w carrier, peaks never reach 70w), with muffled sound.

There are three serious design flaws who affects mostly AM mod.

  1. The ALC circuit loop in SSB mode keeps the RF peak at presetted value. In AM, a capacitor is connected as a low pass filter, to keep average RF value constant. This is done by a saturated bipolar transistor, who behave as a diode, not actually connecting the capacitor to ground.

    Correction:

    1. Replace C305 (10uF) by a 100uF one.
    2. Replace Q306 (DTC114EU) by a FDN339AN mosfet


  2. The first tx mixer (IC400) has gain in excess leading for IMD distortion in SSB and low peak level in AM.

    Correction:

    1. Replace R407 (220ohm) by 680 ohm. Re-adjust R331 to a higher RF level.


  3. There is a thick felt muffler in front of microphone electret unit. Remove it.

After modification my rig presents excellent AM, and the worst case IMD distortion in SSB 2-tone test dropped from -24 to -32 dB.

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