Subject: Re:Icom 706 - modified

From: harjula




In article <4884r9$14to
> In article <1995Nov8.090310.1
>  

>>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    arto.harjula






Newsgroups: rec.radio.amateur.antenna,rec.radio.amateur.equipment,rec.radio.amateur.misc,uk.radio.amateur

From: gary
Subject: Re:IC706

Message-ID: <1995Nov19.082017.5687
Reply-To: gary
Organization: Destructive Testing Systems

Date: Sun, 19 Nov 1995 08:20:17 GMT

Lines:93



In article <48li87$21o
>gary
>> 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

--

Gary Coffman KE4ZV          |    You make it,     | gatech!wa4mei!ke4zv!gary

Destructive Testing Systems |    we break it.     | emory!kd4nc!ke4zv!gary

534 Shannon Way             |    Guaranteed!      | gary
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