THIS MOD IS GREAT THANKS FOR EVERYONES HELP OUT THERE!!
I have an Icom IC-706 and did the mod. remove the yellow wire
(4th from the edge of the plug in connector ) on the top side of
the IC-706 right above the FL-30 SSB filter. The yellow wire
controls either switching in the 120Mhz LP filter or the 2 meter
(pseudo HP filter).
If you just disconnect the yellow wire , the sensitivity increases
above the 148-200 Mhz region. I also noticed the FM broadcast
sensitivity went down greatly, to over come that, the yellow wire
routed from the top side back to the bottom side and attached the
VCO switch points (see note below). The +5Volts
logic high from the plated hole next to J8, allows the LP 120Mhz
to come back when the radio is tuned below 129Mhz.
I did this mod and it works great!!!!
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 fooling yourself, driving the IF
into saturation, with worst dynamic range.
I hope someone from Icom reads his, with this mod, now I can
say the rig now covers the 30Khz - 200Mhz, rather than the modified
statement 30Khz - 120 Mhz.
Now , I am very happy with the receive performance, leaving the
yellow wire, and not connecting it to the VCO switch bank logic
high point near J8, allowed me to hear 2 meter repeaters in the
82-83Mhz range, and poor FM broadcast reception.
Now connecting it to the VCO switch bank, the radio performs to
my satifaction, hearin weather at 162, forestry at 171, TV audio
near 200Mhz and remembering to turn preamp off in 118-129 aircraft
region, no 2 meter images/mixes any more in 82-83Mhz area from 2
meters. Can comfortable listen now to FM broadcast in the
88-108Mhz area. I now have no real complaints about the the IC-706.
Having fun with mine, traded in my Kenwood TS-50, and no compaints,
don't even miss the TS-50. THe IC-706 is a real great rig.
!! Have fun with your 706's .
ICOM, I HOPE YOU SEE THIS, I THINK THIS MOD IS VIABLE AND SHOULD BE
INCORPORATED FOR NEWER ICOM IC-706's!!!
The fourth yellow wire seems to control
a lowpass/highpass 120 MHz filter. In the standard configuration the
highpass filter is used only on 144-148 MHz, every other frequency uses
the lowpass. Anyway, if you want to listen to frequencies above 120 MHz
this mod is very useful. It does even remove most of the FM band
intermodulation. I just don't understand why Icom has not put the switching
limit on 120 MHz instead of 144/148.
Does this mod affect HF or 6m? After the mod, what frequencies use the LP
and what frequencies use the HP filter?
The mod affects only frequencies above 60 MHz. If you just cut or remove the
yellow wire, the HP or actually 2m bandpass filter is used all the time.
Last night I took the loose yellow wire and connected it to a VCO control
switch transistor (?) on the lower PC board. Now I have the LP filter in
use between 60 and 129 MHz and the 2m bandpass between 129 and 200 MHz.
There are five SMD transistors on the lower PC board just behind the
"MENU" button. The one in the middle controls the VCO that is in use
from 60 to 129 MHz. The single pin has 5V when within this range.
The mod affects only frequencies above 60 MHz. If you just cut or remove the
yellow wire, the HP or actually 2m bandpass filter is used all the time.
Last night I took the loose yellow wire and connected it to a VCO control
switch transistor (?) on the lower PC board. Now I have the LP filter in
use between 60 and 129 MHz and the 2m bandpass between 129 and 200 MHz.
There are five SMD transistors on the lower PC board just behind the
"MENU" button. The one in the middle controls the VCO that is in use
from 60 to 129 MHz. The single pin has 5V when within this range.
There is a trace from this pin to a plated-through hole near J8.
This is a good place to connect the wire. (I haven't even seen the
schematic so cannot say if this is the best way to do it.
But it works for me.)
129 MHz seems to be quite close to the cross-over point of the LP and
2m BP filters. Here is a listing of the (NBFM) signal strength needed
to "light" the first segment on the S-meter on each frequency:
MHz     uV/LP     uV/BP
125         .8        2
135         12       .5
145                    .15
155         30        .5
165           3
175          11
185         140
After the mod I can listen to the Helsinki airport on 134 MHz but the
Meteo on 128.4 is still drowned in intermod from the FM BC band.
Proceed at your own risk etc. And have more fun with the 706 !