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