copied from the Internet: 
From: mark@ve6mgs.uucp (Mark G. Salyzyn VE6MGS) 
Newsgroups: rec.radio.amateur.misc 
Subject: IC-751 split mode mod 
 
GOAL OF MODIFICATION: 
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. 
 
Enjoy, 73 de VE6MGS/Mark -sk- 
 
 
 
 
