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

   
 
ham | what is new | links
 Welcome to Www.Hampedia.net  submit to reddit
26 February 2021, 5:45 UTC 


SDR


ELAD
SDRplay

Transceivers


Kenwood
Midland
Yaesu
Alinco
Icom
Other

Antennas


M2
Par
Gap
Sirio
Araki
Maco
Nil-Jon
Maldol
Comet
Hustler
Nagoya
Watson
Hy-Gain
Dressler
Cue Dee
Solarcon
Diamond
Cushcraft
Quicksilver
Moonraker

Power supplies


Powerbox
Kenwood
Diamond
Samlex
Mascot
Daiwa
Yaesu
Alinco
Icom
H&H

Rotators


M2
CDE
Yaesu
Create
Kenpro

Antenna tuners


MFJ
LDG
SGC
Icom
Yaesu
Mizuho
Dentron
Kenwood
Sommerkamp
Barker & Williamson

Repeaters


GME
Icom
Zodiac
Kenwood

Transverters


Microwave Modules
Tokyo Hy-Power
SSB Electronics
Tecnostudi
Kenwood
Yaesu
Minix

Meters


Diamond
Comet
Bird
MFJ

Ham resources


Propagation:
  Sporadic E skip (Es)
  Backscatter
  Tropo
  Meteor Scatter (MS)
  F2 Skip
  Aurora
  Trans-equatorial scatter (TE)
  Lightning scatter (LS)

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) :
    Icom IC-751 (IC 751 IC751) improvement for better CW full break-in
    Icom IC-751 (IC 751 IC751) Cooling, remote jacks
    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) to 37 MHz

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.

I 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 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 shown:

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.

CB Equipment


President
Midland
Uniden
Zodiac

 Power amplifiers


Beko
Icom
Alinco
Drake
Yaesu
Daiwa
Collins
Kenwood
DanAmps
Ameritron
Autophon
Eto/Alpha
Command Technologies
E.F.Johnson
Dentron Radio Co.

FRS/GMRS


Bell
GPX
ENC
Tekk
Icom
Sony
Alinco
Drake
Cobra
Maxon
Vertex
Conair
Uniden
Jenson
Ranger
Midland
Whistler
Magnum
Unwired
Coleman
Motorola
Audiovox
Memorex
Columbia
Samsung
Standard
Kenwood
Cherokee
Panasonic
Freetalker
Vtech Vtalk
RadioShack
Fanon Courier
Sharper Image
General Electric
Oregon Scientific

PMR446


Sommerkamp
FreeQuency
Goodmans
Kenwood
Lafayette
President
Binatone
Motorola
Multicom
Albrecht
Maycom
Topcom
Xtreme
Ondico
Telcom
Maxon
Vertex
Zodiac
Yaesu
Cobra
Alinco
Icom
Alan
ITT