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Subject: Re: Icom 706 - modified

In article <4884r9$>, (Cecil A. Moore~) writes:
> In article <>,
>   wrote:
>>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. 
There is a trace from this pin to a plated-through hole near J6. 
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.

> thanks and 73, Cecil, KG7BK, OOTC  (not speaking for my employer)
Neither am I. Proceed at your own risk etc. And have more fun with the 706 !

73 de Arto, OH2BGN
Arto Harjula                      tel +358-0-5112 8308   fax +358-0-5112 8299
Nokia Telecommunications / PMR          

From: (Gary Coffman)
Subject: Re: IC706
Message-ID: <>
Reply-To: (Gary Coffman)
Organization: Destructive Testing Systems
Date: Sun, 19 Nov 1995 08:20:17 GMT
Lines: 93

In article <48li87$> (Manfred Lange) writes:
> (Gary Coffman) wrote:
>> On HF, though, it's a dream. I love the 1 Hz tuning, and the ability
>> to slow the tuning rate to 1/4 normal for digital. Tuning steps are
>> selectable at 1 Hz, 10 Hz, 1 kHz, and 1 MHz. I wish they hadn't skipped
>> 100 Hz, that would be useful for rapid band scans. 1 kHz is too coarse
>> for anything other than rapidly tuning to a selected band segment and
>> then shifting back to 10 Hz or 1 Hz. (Another firmware change.)
>Hi Gary,
>100-Hz-steps are possible, no problem. If you select the
>10-Hz-Function and press the "TS"-Button for about 2 Sec., the rig
>will jump into the 100-Hz-mode.

Actually, that changes you between 1 Hz and 10 Hz tuning steps. However, 
there is a way to get 100 Hz (and several other) step sizes. The fine manual
explains it on page 17 as I discovered on rereading it. Just push TS until 
the marker is over the kHz digit, hold it for 2 seconds, and you're in a 
menu that lets you select 0.1, 1, 5, 9, 10, 12.5, 15, 20, or 100 kHz tuning 
steps.  Whew! They didn't leave out a thing.

>>For those who like to scan VHF Hi-band, we've discovered a mod that
>>restores sensitivity, but at the cost of reduced sensitivity below
>>120 MHz.
>Could you post this mod here or via E-Mail?

This is the "yellow wire" mod, but one of my correspondents has
found a better way that actually allows the filter to switch at
the corner frequency (apparently around 129 MHz) as you tune. Now 
PROCEED AT YOUR OWN RISK, there are no guarantees you won't trash 
your radio, this is reverse engineering with no schematics, Icom 
doesn't warrantee this mod, etc, but here's the mod anyway. :-)

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 pass filter 
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 (more than
I wanted to feed into the radio) 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 and all that trash. This is tiny SMD
stuff. If you trash your radio, you're on your own. Thanks go
to our correspondent from Finland, whose call I do not know,
who discovered this latest mod, and to WB6JDI for relaying it 
to me.

A final PS to all you mod freaks. There may be *a better way*
than this mod to restore high band sensitivity. If you're a
worry wart, or unsure of your skills working with tiny stuff,
wait for the schematics and board layouts to come out on the 
radio. Icom says January. Then we may find a better way. If
you're brave of heart, then happy mods to you.

Gary Coffman KE4ZV          |    You make it,     | gatech!wa4mei!ke4zv!gary
Destructive Testing Systems |    we break it.     | emory!kd4nc!ke4zv!gary 
534 Shannon Way             |    Guaranteed!      |
Lawrenceville, GA 30244     |                     | 


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