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World of Ham Radio 1997
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WOHR97_AmSoft_(1997-02-01).iso
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ic251_3.doc
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1997-02-01
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IC251.3
MODIFICATIONS FOR THE ICOM IC251
SSB & CW VARIABLE POWER
Place set with the controls towards you and remove lid. If
necessary check FM and CW power. Connect set to known good dummy
load and power meter.Locate R13 and R14 on the small board behind
the front panel RF power control. With RF power control on max,
TX on FM and adjust RH pot(R13) for 10-12watts.Turn RF power
control fully down and adjust LH pot(R14) for desired low power
setting (min 200mw). Short key jack at rear of set, switch to CW,
TX and adjust R129 for 10-12watts the same as on FM. Switch off
power. Locate the brown wire to the left of the Xtal filter it
runs from next to R258 to near R327 its marked with a small b on
the main unit board layout diagram. Cut this wire at the front
end and solder to the emmiter of Q65 or the purple lead marked I
on the main unit layout. Solder the slider of a 4.7k preset to
the LH end of the resistor behind R14,then solder a 3.3k resistor
to one leg of the preset, run a wire from this resistor to the
top of R272 (scrape the paint off before attempting to solder).
Switch on. Switch to CW, TX and with front panel RF power control
fully down adjust the(4.7k preset for desired low power level
(min 200mw) SSB power is the same as CW. The front panel control
now affects both FM,CW & SSB power levels.
VOX ON FM
Place the set with the controls towards you,remove lid.
Locate R290 on the main board it is just behind the CW delay control
cut this resistor and insert a 1N4148 diode in series with it,its
anode should be away from the resistor.Connect another diode (1N4148)
to R290 then connect its ano he board on the back of the rf power
control.
The VOX switch will now work on FM
EXTENDED FREQUENCY RANGE
Place the set with the controls facing away from you,remove lid.
Locate D52 on the Driver unit (top LH corner on the diagram, the
side nearest the edge of the set) this will not be present in
solder a 1N4148 diode in this space. The set will now cover
143.800 to 148.199mhz on TX and RX
NB the PI board layout diagram is incorrect in 3 manuals I have seen
so beware!!!!
JULIAN G6LOH