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World of Ham Radio 1997
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WOHR97_AmSoft_(1997-02-01).iso
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ic251_1.doc
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1997-02-01
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ic251.1
ICOM 251E MODIFICATION
The following modification was carried out to enable the front
panel power control to be used on CW/SSB as well as FM.
The modification is in two parts (1) the front panel and (2) the main
circuit board.
(1) FRONT PANEL
***************
(i) remove four retaining screws and top cover.
(ii) remove four retaining screws and bottom cover, disconnecting the
speaker.
(iii) if you have a Mutek front end board fitted remove it now. MAKE
NOTE OF THE CONNECTIONS TO IT! ***WRITE IT DOWN***
(iv) remove the four front panel frame screws
(v) disconnect the wiriing harness and remove the front panel. NOTE
WHERE ALL THE PLUGS GO! ***WRITE IT DOWN***
(vi) locate the orange wire linking the mode switch and the board
behind the power control potentiometer.
(vii) desolder it from it's present position on the mode switch and
move it to the common side of the switch. That completes the front
panel mod.
(2) MAIN CIRCUIT BOARD
**********************
(i) remove the internal PSU if fitted.
(ii) remove the VCO and screening plate on the underside of the main
circuit board.
(iii) now turn the rig over and find the brown wire that places
TX9V(FM) on to Q64/Q65. If you look at the board layout in the manual
the wire runs from sectors 3F to 4F and is marked "b".
(iv) Cut this wire where it joins the board, near R327/R46.
(vi) Now find the purple wire marked "k" on the board layout on the
edge of sectors 5F/6F, locate this point on the underside of the PCB.
I did this by standing the rig on one side panel and shining a light
upon the component side of the board. Pointing to the wire with a
pencil or radio driver and looking at the track side to see the
shadows.
(vii) once you have identified the point on the trackside of the PCB
desolder the wire and clear the hole that is left.
(viii) Join the purple and brown wires, then solder them back into the
hole from where the purple wire came from. If the brown wire is to
short replace it, as I did. I used a purple wire striped from a ribbon
cable.
This now means that when the radio is switched to transmit TX9V is
placed on to Q64/Q65 and not TX9V(FM) as before. That completes the
mod.
(ix) Reassemble the radio in the reverse order to which you
disassembled it, leaving the top cover off.
(x) The only thing to do now is to check that the rig still works ok
and to reset the high and low power levels. This is done by adjusting
VR13(high) and VR14(low) with a power meter and dummy load connected
to the aerial connector and if possible a two tone audio generator
feeding into the microphone socket at around 600mV should de used to
set the rig up on SSB. The power reading on the RMS power meter should
be approximately half the PEP. When the desired power levels have been
set fit the top cover.
continued in ic251.2