From cbfsb!att!linac!pacific.mps.ohio-state.edu!zaphod.mps.ohio-state.edu!uakari.primate.wisc.edu!ames!tulane!rouge!pc.usl.edu!jpd Tue Nov 12 23:21:57 EST 1991 
Article: 10385 of rec.radio.amateur.misc 
Path: cbfsb!att!linac!pacific.mps.ohio-state.edu!zaphod.mps.ohio-state.edu!uakari.primate.wisc.edu!ames!tulane!rouge!pc.usl.edu!jpd 
From: jpd@pc.usl.edu (Dugal James P.) 
Newsgroups: rec.radio.amateur.misc 
Subject: IC751 extension to 37MHz 
Message-ID: <1991Nov12.205246.21845@usl.edu> 
Date: 12 Nov 91 20:52:46 GMT 
Sender: anon@usl.edu (Anonymous NNTP Posting) 
Organization: Univ. of Southwestern La., Lafayette 
Lines: 97 
Originator: jpd@pc.usl.edu 
 
[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 
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.  
 
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: jpd@usl.edu 
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. 
 
 
 
 
