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From: mark@ve6mgs.uucp (Mark G. Salyzyn VE6MGS)
Subject: IC-751 split mode mod
Message-ID: <1991Nov10.195437.10480@ve6mgs.uucp>
Date: 10 Nov 91 19:54:37 GMT
Organization: Amateur Packet Radio E-mail Gateway  VE6MGS
Lines: 60

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? :-).

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
             <      <
             |      |
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.

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-

From: (Dugal James P.)
#Subject: IC751 extension to 37MHz
Message-ID: <>
Date: 12 Nov 91 20:52:46 GMT
Sender: (Anonymous NNTP Posting)
Organization: Univ. of Southwestern La., Lafayette
Lines: 97

[I'm posting this for Tom N5OFF -- n5knx, ed.]

IC-751 to 37 MHz de N5OFF@W5DDL.AARA.ORG

With a great amount of help from Mark, VE6MGS, I was able to
modify my Icom 751 to operate at an extended frequency range of
up to 37 MHz.  This is how it was done. 

Mark developed a clever way to read and interpret the contents of
the Icom RAM chip (the old battery backed-up one) and alter its
frequency limits in the GENeral coverage and HAM modes.  This was
done in conjunction with the modification of his 751A to operate
six meters.  My interests are in listening to the low band skip
present above the ten meter band during band openings, so I asked
Mark for simply a new upper frequency limit, leaving the six
meter mods to the very ambitious. 

First, I changed the battery on my RAM board since mine was eight
years old, and I didn't want Icom messing up my custom program if
my battery died later. 

I then shipped the board off to Mark, and he changed my GEN mode
limits from .10-30 MHz, to .01 to 60 MHz (subject to VCO and RF
limits, naturally).  He also customized my HAM mode limits as

Before                  After
1.8-2.0               27.98-30.62   Default HAM mode selection
                                    changed from 160M
3.45-4.1              31.98-33.02
6.95-7.5              33.98-35.02
9.95-10.5             1.78- 2.02
13.95-14.5            3.48- 4.02
17.95-18.5            6.98- 7.32
20.95-21.5            9.98-10.17
24.45-25.1            13.98-14.37
27.95-30.0            18.05-18.18
none                  20.98-21.47
none                  24.86-25.01

These new HAM mode limits provide tighter control while hamming. 
Note the tight 12M and 17M limits.  Much more realistic. 

One problem encountered during the mod was that the rig's
processor (IC-14) instructs the rig to select the proper VCO and
bandpass filter for a given frequency, and it uses the highest
BPF up until 31 MHz, then must be further instructed as to what
to do at frequencies > 31 MHz. 

IC-14 has several pins which turn on and off to do this
instructing.  Mark found that pin 18 (originally not used) of
IC-14 turns on at >31 MHz.  This could be used to turn on the
highest BPF and VCO, thus allowing operation to higher
frequencies.  This was done physically by cutting the trace from
pin 17's normal route, adding a diode to each pin 17 and 18 of
IC-14, and joining the anodes of the two diodes to the circuit
originally driven by pin 17, but now isolated by the trace cut. 
The effect is the use of the highest BPF and VCO until 31 MHz
(pin 17) , and then the same BPF and VCO from 31 MHz and beyond
(pin 18). 

As expected, VCO4, as originally adjusted, dropped out at a low
frequency (32.33 MHZ).  Since the VCO has a range of about 15 MHz
(and was originally employed to cover only 8 MHz with 3.5 MHz of
lagniappe on each end), and kicks in at 22 MHz, I adjusted it for
a comfortable low limit of 22 MHz (with little margin), which
lifted the upper range to a maximum observed limit of 37.175 MHz. 
This was done by giving C107 on the VCO unit 1/4 turn CW. 

I haven't yet done any sensitivity measurements, as I'll have to
borrow the equipment from my employer when its convenient for
him.  I don't know where, or to what extent the bandpass filter
will cut off on the way to 37 MHz, if it does at all. 

I believe the rig "hears itself" on a few certain frequencies,
with strong signals at 30.715, 31.33, and 30.95.  I believe this
is normal as Icom probably moved these birdies to >30 MHz
purposely.  There are few other spurs. 

CONCLUSIONS: I borrowed an R-7000 from a friend some months ago,
and found that my favorite listening was in the lower 30's.  Now,
my 751 is as good to me as having my own R-7000 for what I would
use it for.  In the ham mode, its much easier to stay in the ham
bands (especially WARC bands) with the new program, and the
default band of 10M much more suits my tastes than the old 160M
default band. 

Thanks to VE6MGS [mark@ve6mgs.uucp] for his great help, without
which I could not have accomplished this. 

73 es  bonne chance de tom N5OFF@W5DDL.AARA.ORG
-- James Dugal,	N5KNX		Internet:
Associate Director		Ham packet: n5knx@k5arh
Computing Center		US Mail: PO Box 42770  Lafayette, LA  70504
University of Southwestern LA.	Tel. 318-231-6417	U.S.A.

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