Introduction:
Here is a modification for the IC-551D 6-metre multimode
radio. The mod. may be applicable to other ICOM radios of that era, eg,
the 2-metre version (IC-251?, IC-211?). It is easily carried out and
consists of soldering a jumper wire between two other wires. No major
disassembly is required.
Background:
When the 'VFO' switch is set to the 'MS' position, the
IC-551D can be made to scan three user-programmable frequencies. Unfor-
tunately, after scanning has stopped on an active frequency, it never
re-starts, even if the signal disappears from the frequency. The radio
remains on that frequency indefinitely. The user is forced to manually
re-start the scanning, and the cycle repeats when an active frequency
again opens the squelch.
Modification:
This mod. will cause the scanning to pause on an active
frequency for approx 10 seconds; scanning automatically resumes at the
end of the 10 seconds whether the signal is present or not. In essence
the mod. consists of changing the 'MS' scan regime from the band-scan
'MODE A' regime (permanent stop) to the band-scan 'MODE B' regime
(pause).
Note:
The IC-551D handbook says that the pause in 'MODE B' is sixteen
seconds, whereas in my radio it is close to ten seconds. It's possible
that ICOM revised the design after the handbook was printed, or perhaps
the 10-second characteristic of my particular radio is a result of
long-term component drift. Whatever, the point here is that the same
pause will be used for the 'MS' scan mode as is already used in the 'B'
scan mode. So in your particular radio the pause may be 10-16 seconds.
Scan Rate:
After modification the scan rate becomes adjustable by the
'SCAN SPEED' control, which was previously rendered non-operative in
'MS' scan mode, although it was operative in the two band-scan modes,
A and B.
Procedure:
Remove the power plug from the rear of the IC-551D. Turn
the set upside-down with the rear closest to you. From this point on,
all expressions such as 'left', 'right', 'up', 'down', refer to the
radio as viewed by you in this orientation.
-
Remove the four bottom cover screws and CAREFULLY remove the
bottom cover, disconnecting the speaker plug as you lift it away.
-
If your IC-551D does not have the optional FM board installed,
go to Step 3. Remove the FM board by disconnecting the white plug from
the left corner and removing four screws (two at each end). Lift the FM
board away carefully as it's still attached to the radio by a flying
cable, and place it on the front corner of the case.
-
Remove the sub-bracket on which the FM board was mounted by
removing the screws at each side (in my radio: 2 on LHS, 1 on RHS).
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Cut a length of light, flexible wire 50-60mm long and strip
3mm of insulation from each end. Tin both ends of this jumper wire with
solder.
-
Observe the printed circuit board mounted vertically about
40mm behind the front-panel controls. This PCB is the 'DRIVER' board.
This is a CPU/digital processing board and has nothing to do with the
transmit driver stages. Observe the three white plugs/sockets (headers)
along the upper edge of this PCB. On the left-most header (closest to
the centre of the radio) count the wires from right to left as they
exit from the plug. This is a six-wire plug/socket (J2/P11). The second
wire from the right is a dark-green wire. This is the 'VFO B' wire.
DO NOT cut this wire. At some convenient point, strip about 4mm of
insulation from it with a razor blade. Tin the exposed wire and solder
one end of your 50-60mm jumper wire to it.
Caution:
In some photos in the ICOM documentation J2/P11 are shown
fitted to the front-panel side of the PCB, rather than the rear side.
If J2/P11 have been fitted this way in your radio, you must still use
the second wire from the right, since the PCB layout does not change.
However, the wire may not be dark-green as the colour sequence may be
reversed.
-
Look down the left-hand edge of the same PCB. Observe the
white plug/socket header, the first one you come to as you look down
the edge of the PCB. The header is J4/P7. Count the wires as they exit
plug P7. The third wire (counting downwards) is an orange wire. This is
the 'MS' wire. To make this wire accessible, prise P7 off J4 with a
screwdriver and pull P7 up towards you. At some convenient point, cut
the wire, creating two ends, one going into P7, the other coming from
the loom. Ignore the wire going into P7, it will not be used. Instead,
take the wire which comes from the loom, strip 3mm or so of insulation
from it and tin it with solder. Solder the free end of your 50-60mm
jumper wire to this tinned end.
-
Push P7 back down into the wiring and press it back onto J4.
This is a little tricky, but is best done with a short, skinny, screw-
driver.
-
Re-fit the FM board sub-bracket and re-fit its screws. Re-fit
the FM board to the bracket and re-fit its screws. Plug in its header
plug. Ensure that both of your soldered connections and the cut-off end
of the 'MS' wire are clear of metal framework, shield compartments. The
rest of the re-assembly is in reverse order to that described earlier.
Testing:
Turn on the IC-551D. Program three frequencies into memory
positions 1,2,3. Set the 'VFO' switch to 'MS' and initiate scanning by
pressing the 'MS/MW' button. When one of those frequencies becomes
active, scanning will pause for 10-16 seconds; scanning resumes auto-
matically at the end of the pause, whether the signal is present or
not. When receiving a prolonged transmission, reception is briefly
interrupted every 10-16 seconds as the scanning re-starts. When two or
three of the frequencies are active, each transmission is heard in
rotation for 10-16 seconds, eg. 1,2,3,1,2,3.
Scanning can still be stopped and re-started by pressing the 'MS/MW'
button. The rate at which the frequencies are scanned is now adjustable
by the 'SCAN SPEED' control, accessible by removing the small cover in
the top of the case. It's probably best to set the control at the
slowest setting (fully anti-clockwise). If set too fast, scanning may
not stop on active frequencies. No other function of the IC-551D is
disturbed by this modification.
I use my IC-551D in the 'MS' scan mode to monitor my three favourite
2-metre repeaters, using a home-brew 144-50 MHz converter connected to
the IC-551D.
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