Modifications for the ICOM - RogerK

end of the 10 seconds whether the signal is present or not. In essence the mod. consists of changing the 'MS' scan regime fromthe 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'spossible that ICOM revised the design after the handbook was printed, or perhaps the 10-second characteristic of my particularradio is a result of long-term component drift. Whatever, the point here is that the same pause will be used for the 'MS' scanmode 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-operativein '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.1. Remove the four bottom cover screws and CAREFULLY remove the bottom cover, disconnecting the speaker plug as you liftit away.2. If your IC-551D does not have the optional FM board installed, go to Step 3. Remove the FM board by disconnecting thewhite plug from the left corner and removing four screws (two at each end). Lift the FM board away carefully as it's stillattached to the radio by a flying cable, and place it on the front corner of the case.3. 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).4. Cut a length of light, flexible wire 50-60mm long and strip 3mm of insulation from each end. Tin both ends of this jumperwire with solder.5. 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 whiteplugs/sockets (headers) along the upper edge of this PCB. On the left-most header (closest to the centre of the radio) countthe wires from right to left as they exit from the plug. This is a six-wire plug/socket (J2/P11). The second wire from theright 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 exposedwire 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 rearside. If J2/P11 have been fitted this way in your radio, you must still use the second wire from the right, since the PCBlayout does not change.However, the wire may not be dark-green as the colour sequence may be reversed.6. Look down the left-hand edge of the same PCB. Observe the white plug/socket header, the first one you come to as youlook down the edge of the PCB. The header is J4/P7. Count the wires as they exit plug P7. The third wire (countingdownwards) is an orange wire. This is the 'MS' wire. To make this wire accessible, prise P7 off J4 with a screwdriver andpull P7 up towards you. At some convenient point, cut the wire, creating two ends, one going into P7, the other comingfrom the loom. Ignore the wire going into P7, it will not be used. Instead, take the wire which comes from the loom, strip3mm 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.7. 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.8. 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 itsheader plug. Ensure that both of your soldered connections and the cut-off end of the 'MS' wire are clear of metalframework, 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 scanningby 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 prolongedtransmission, reception is briefly interrupted every 10-16 seconds as the scanning re-starts. When two or three of thefrequencies 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 isnow adjustable by the 'SCAN SPEED' control, accessible by removing the small cover in the top of the case. It's probably best toset the control at the slowest setting (fully anti-clockwise). If set too fast, scanning may not stop on active frequencies. No otherfunction 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 MHzconverter connected to the IC-551D.