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
1.8-2.0 27.98-30.62 Default HAM mode selection
changed from 160M
9.95-10.5 1.78- 2.02
13.95-14.5 3.48- 4.02
17.95-18.5 6.98- 7.32
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
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