Icom IC-751 (IC 751 IC751) transceiver mods reviews software and diagrams

   
 
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26 February 2021, 5:02 UTC 


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Icom IC-751 (IC 751 IC751)
IC-751
Programming interface schematics for Icom:
Programming software for Icom IC-751 (IC 751 IC751) :
PDF User Manual for Icom IC-751 (IC 751 IC751) :
PDF Adjusments Procedures Manual with schematics for Icom IC-751 (IC 751 IC751) :
Schematics for Icom IC-751 (IC 751 IC751) :

Mods for Icom IC-751 (IC 751 IC751) :
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    Icom IC-751 (IC 751 IC751) Dual RAM back-up Battery Modification
    Icom IC-751 (IC 751 IC751) RAM Card Backup Battery Replacement Instructions
    Icom IC-751 (IC 751 IC751) Allow CW Xmit & USB recieve split mode
    Icom IC-751 (IC 751 IC751) to 37 MHz
    Icom IC-751 (IC 751 IC751) Bad Cap Problems
    Icom IC-751 (IC 751 IC751) General coverage mod
    Icom IC-751 (IC 751 IC751) Switch Matrix
    Icom IC-751 (IC 751 IC751) How to replace the lithium battery in your ICOM radio

Icom IC-751 (IC 751 IC751) technical specifications :  Icom IC-751 (IC 751 IC751) technical specifications



Icom IC-751 (IC 751 IC751) Allow CW Xmit & USB recieve split mode

Allow CW Xmit & USB recieve split mode to still use the CW VOX. This allows one to operate with limited privaledges when communicating with SSB stations above 10MHz. Below 10MHz, the CW receiver is actually LSB so split operation in that case is effortless and does not require the split oepration to be invoked. The original bogus operation can be confirmed by placing the rig into CW Tx and USB Rx split operation and operate the key. A more `graphic' example of this operation is CW Rx and USB Tx split and operate the key (scary eh? :-).

PARTS REQUIRED:
1 4.7Kohm resistor and 2 1N914/1N4148/1N4448 diodes
SCHEMATIC:         +8V (Pin 14 of IC1)
                     |
                      >
                     <
                      > 4.7Kohm
                     <
                      >
                     |
                     +-------------//-----------------+
                     |                                |
      +-------+------(----------+                     o
      |       |      |          |                    /
      |       |1N914 | 1N914    |                   /
      |      _|__   _|__        |               +--o   Split Switch
      |        /     /        |            ___|___
 D11  |     __/__ __/__       |            / / / /  o-----> to `split' display
 | /| |       |      |          |     |                       segment driver
 |/ |_|_/____|______|______/__|____ |
 | |   /    |      |      /       |
 | |         >      >                |
             < R29  < R26
              >5.6K  > 4.7K
             <      <
             |      |


DESCRIPTION:

On the Main PC board, there is only one cut trace because R29 & R26 are both fed from 1 trace and isolated from the rest of the circuitry. One of the diodes is surface mounted on the solder side of the board over the cut. R29 is unsoldered so that a thin wire (from w/w or pulled from a ribbon cable) is then subsequently inserted with the R29 lead back into the hole to bring R29+R26 common point up to the top of the main board.
The 4.7Kohm resistor is soldered to pin 14 of IC1, the other end of the resistor is soldered to the anode of the other diode's anode and another long wire to be routed to the split switch. The cathode of the diode is attached to the wire inserted earlier with R29.
All the components may be surface mounted if desired on the bottom of the main board, but I chose to perform this mod on the top of the board to allow easier removal of the main board for servicing. The long wire that was attached to the second diode and the resistor is routed to the empty pin on the split switch that is driven to ground when the switch is in the OFF position.

BUGS:

The CW VOX will be active when using split mode, even if neither the transmit or receive mode are the CW mode.

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