Icom IC-706MKII Manual page 18

KB2LJJ Radio Mods Database
did this and it opened up the rx between 154-200 mhz.
(It was printed in CQ VHF a couple of months ago.)
For those who like to scan VHF Hi-band, there is a mod that restores sensitivity, but at the cost of reduced sensitivity below 120
MHz.
This allows the filter to switch at the corner frequency (apparently around 129 MHz) as you tune.
PROCEED AT YOUR OWN RISK
There are no guarantees you won't trash your radio.
Remove the radio top and bottom covers according to the manual.Remove the speaker. Now with the radio facing you and right
side up, you'll see a single connector with discrete wires at the rear of the control board. The fourth wire from your left should be
a yellow wire
(caution, the 5th and 6th wires on the other end of the connector are yellow too, don't get confused).
Disconnect this wire from the connector. This will enable the 2 meter band-pass filter to work outside the 2 meter band, and will
restore sensitivity between about 130 MHz and about 165 MHz (above and below that range, sensitivity still stinks, especially
above 165 MHz).
Before this mod, a low pass filter with a corner frequency around 129 MHz was in the circuit all the time except when you were
actually tuning inside the 2 meter band, and that killed high band sensitivity completely. Before removing the yellow wire,
sensitivity at 155 MHz was 30 uV for an S1 indication,after the yellow wire is removed, sensitivity increases so that only 0.5 uV
is required for a S1 reading. At 165 MHz after the mod,sensitivity is 3 uV for an S1 reading, before the mod, a very large signal
was required for an indication.
Disconnecting the yellow wire has the unfortunate side effect of reducing sensitivity between 60 and 129 MHz since the radio
never switches from the 2m bandpass filter to the low pass filter. Signals below 60 MHz don't go through either filter and are
unaffected by the mod.
Now here's how to get the low pass filter back when tuning below 129 MHz. On the bottom board, right behind the MENU
button,
there are 5 SMD transistors. These transistors apparently switch the VCOs for the various band segments. The one in the middle
of the 5 apparently switches the 60-129 MHz VCO. The single pin (one side of the SMD transistor has 2 pins the other only 1)
switches to 5 volts when this VCO is active, and is low otherwise.
This is exactly what we need to feed the yellow wire. There's a board trace coming from this pin over to a feedthru hole near J8.
Connect the yellow wire you disconnected earlier here.
The filter should now toggle between low pass and 2m bandpass as you tune below and above 129 MHz, and you should have
good sensitivity both above and below this frequency (at least up through 165 MHz).
Remember, CAUTION WARNING This is tiny SMD stuff. If you trash your radio, you're on your own.
As far as FM Broadcast intermod in the Aircraft band, I noticed the IF is a little overdriven into compression, so I turn the
preamp off(greeen to no light---preamp switch), and noticed no difference in sensitivity, intermod in aircraft band disappeared.
Running the preamp in the Aircraft 118-129 does not really help sensitivity, even though the S-meter shows higher signal levels
(jumps around alot due to saturating IF when signals are not there!!! )
the noise floor actually, degrades, thus I leave the switch (no preamp-black instead of green). Leaving it on green is just driving
the IF into saturation, with worst dynamic range.
Now connecting it to the VCO switch bank, the radio performs to my satifaction, hearing the weather at 162, forestry at 171, TV
audio
near 200Mhz and remembering to turn the preamp off in 118-129 aircraft region, no 2 meter images any more in 82-83Mhz area
from 2 meters.
I can listen to FM broadcast in the 88-108Mhz area.
Q: Does this mod affect HF or 6m?
A: The mod affects only frequencies above 60 MHz. If you just cut or
remove the yellow wire, the 2m bandpass filter is used all the
time.
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