KB2LJJ Radio Mods Database
After the mod you can listen to the airport on 134 MHz but 128.4 is still drowned in intermod from the FM BC band.
Proceed at your own risk
The receiver sensitivity above 120MHz (except the band between 144-148MHz) is very poor and also the transmitted FM
deviation for NARROW FM is too small; therefore I did some tests and came to the following two modifications.
To carry out these modifications it is necessary to use the IC706 service manual, which can be bought at any ICOM dealer.
1. When choosing NARROW at FM, then during transmitting the max.
deviation is reduced from 4.8kHz to 2.4kHz. It is still desirable
to choose NARROW at FM because the receive performance is better
then.
After changing resistor R272 from 1K to 8K2. (at the bottom side of the MAIN UNIT), then the max. deviation during
transmitting goes from 4.8kHz to 4.3kHz when choosing NARROW at FM.
The modulation at AM is also changed now, but this can be corrected with potmeter R271.
2. To improve the receiver sensitivity between 120 - 144MHz and
between 148 - 200MHz the following modifications can be carried
out.
At the PA UNIT, change the 60 - 200MHz bandpass:
1) Remove C53(20p), C152(20p), C153(12p) and C154(20p).
2) Short-circuit L49(82nH) by soldering an interconnection at the
place of the removed C153.
3) The inductance of L16, L17, L18 and L19 must be reduced somewhat.
This can be done by separating the windings somewhat with a small
screwdriver.
Now the sensitivity is good up to about 175MHz. Also the sensitivity in the airband is much better now.
For receiving above 175MHz the low-pass filters at the ANT2 input have to be changed (components around L16, L17, L18,
L19.
I would not recommend that, because the spurious suppression during 2M transmitting becomes worse then!
To expand the band on the VHF portion.
(I have not tried this mod.
I received this infomation via e-mail)
Caution: This is quite involved. If you are not too good you might be advised to get someone who has the experience to handle
this mod!
1. Remove the main board from the unit.
Keep the cutout hole away from you. This I consider the top of
the board.
2. Remove the shield from the top of the board. There are 30-40 solder
points from the shield to the board.
3. Under the board on the top of the board, remove R-353 and Q-38.
They are located to the left of the IC-36 chip on the top of the
board under the shield.
4. On the underside of the board, Locate IC-32. There are two IC
chips.
IC-32 will be just down and to the right of the shield.
5. Using a pointer, point at the left upper pin, and go toward the
top of the board. You will find a trace that stops. It comes out
http://www.kb2ljj.com/data/icom/ic-706-MKII.htm (19 di 30)23/08/2009 23.05.07
