From: 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 band pass
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 band pass 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.
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