Subject: ICOM IC-706 Yellow Wire Mod Details?


Can someone point me to specific info on the details of the (in)famous
yellow wire mod for the IC-706 which opens up Receive above 129MHz?


Remove the radio top cover. With the radio faceplate facing you,
you'll see a multiconductor plug near the middle of the radio.
The fourth wire from the *left* is yellow. Remove this wire from
the connector and tape it off, or cut it, and you've done the
yellow wire mod (type 1). Be careful, there's also a yellow
wire fourth from the *right*, that's not the one you want.

What this mod does is force the radio to use the 2m bandpass
filter all the time rather than just in the range 144-148 MHz.
The "wideband" filter that would normally be selected outside
the 2m band actually cuts off at about 140 MHz, so you can't
receive signals above the 2m band (such as national wx at
162.55 MHz) with the stock radio. After the mod, the 2m BP
filter has wide enough skirts to allow you to receive national
wx, but peters out above that.

There's a downside to this (type 1) yellow wire mod. After
the mod, signals in the aircraft band will be greatly 
attenuated. That's because the lower skirt of the 2m BP
filter (now used all the time) peters out at about 130 MHz.
To avoid this problem, there is a type 2 yellow wire mod.

In this latter mod, you take the yellow wire you disconnected
in the type 1 mod and extend it to the bottom of the radio
and connect it to a switching transistor there. Remove the
radio bottom cover. You'll see 5 SMD transistors just behind
the front panel menu key. The center transistor has a trace
going over to a feedthru hole near a coax connector on the
right side of the board. Solder the extended yellow wire
to this feedthru hole. Now when you tune *below* 2m, the
original "wideband" receiving filter will be switched in
and you'll have restored aircraft band sensitivity. Above
2m, the 2m BP filter is still used, so you'll still be able
to pick up national wx.

There is a different solution, other than messing with yellow
wires, to the problem which can give you coverage all the way 
to the 200 MHz upper tuning limit of the radio. This mod requires 
you to modify the "wideband" filter so that it actually cuts off 
above 200 MHz as it should. This mod requires removing two SMD 
capacitors in the filter (and optionally piggybacking one of them 
back on top of another capacitor). The former mod is simpler since 
you don't have to worry about damaging the removed capacitors, but 
can cause image problems. To do either version of this mod, you 
really need the service manual's parts layout diagram. It is 
difficult to explain exactly where these capacitors are without 
a drawing. (Needless to say, you should also have some SMD rework
experience to perform this mod.)

Gary
-- 
Gary Coffman KE4ZV          |    You make it,     | Due to provider problems 
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