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Icom IC-706 series

introduction permalink: http://www.amateurtele.com/index.php?artikel=280&id=#1244
I started using Yaesu equipmend as a hamradio operator since a lot of fellow radio operators used also Yaesu equipment. I had for example the FT-817, FT-857, FT-897, FT-7800, FT-1500M, FT-450, FT-2000 and so on. For a little while I had the IC-7200, but never really experienced Icom equipment. Until I saw the IC-7300 showd up. The same weekend I sold my FT-2000 and bougt a used IC-7300. And it’s a great rig! That’s the moment I discovered the pro’s of Icom. As I was looking for a “FT-897” like radio I found the IC-706. I found the IC-706 for a good price and bought one. I discovered later that there are three versions. A couple of days later I was offered an IC-706mkIIG and upgraded to the newer version. The IC-706 is therefore sold due to the upgrade. The price was right, but there’s a technical problem with the radio. I was told that the power dropped after a while. I like a challenged one in a while and this seems a problem that could be repaired. To get familiarized with the rig I started “digging” in the service documentation and other related information on the internet. And I started this “chapter” of my website gathering and sharing relevant information of the IC-706 series rigs.

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model overview permalink: http://www.amateurtele.com/index.php?artikel=280&id=#1245

IC-706 model differences
IC-706
IC-706mkII
IC-706mkIIG
DSP
no
optional
yes
RX
30 kHz… 200 MHz

30 kHz… 200 MHz

30 kHz… 200 MHz
430…470 MHz
max RF power
1,8…50 MHz: 100 W
144 Mhz: 10 W

1,8…50 MHz: 100 W
144 Mhz: 20 W

1,8…50 MHz: 100 W
144 Mhz: 50 W
430 Mhz: 10 W
TX band
160 m
80 m
40 m
30 m
20 m
17 m
15 m
10 m
6 m
2 m

160 m
80 m
40 m
30 m
20 m
17 m
15 m
10 m
6 m
2 m

160 m
80 m
40 m
30 m
20 m
17 m
15 m
10 m
6 m
2 m
70 cm


more RF output power permalink: http://www.amateurtele.com/index.php?artikel=280&id=#1246
It’s possible to get more RF power out of the rig bet readjusting some settings. But I strongly advice not to do so! The final power amp will wear much faster, the PA will likely fail and most of all it’s likely that the amount of spurious radiation will be significant. In the “best” situation at the HF bands the rig can produce 150 Watts instead of 100 Watts. But to gain one s-point at the receiving end you need a factor four more power. To improve the signal noticeable you need at least a factor ten more output power thus at least 1.000 Watts. The difference between 100 and 150 Watts is negligible and that’s definitely not worth tortureing the rig.

final amp SRFJ7044 FET’s obsolete permalink: http://www.amateurtele.com/index.php?artikel=280&id=#1247
The “older” IC-706 series rigs are equipped with (round shaped) SRFJ7044 FETS. Nowadays (2018) these FET’s are hard to obtain since they’re not in production anymore. The later IC-706mkIIG’s are therefore equipped with newer (square shaped) MOSFET’s. Since output FET’s tend to fail, I’ts good to know there are replacement parts. Icom can (or could?!) supply a complete PA board as a drop in replacement, but that’s rather expensive. A cheaper solution is to replace the two HF FET’s by RD70HHF1 MOSFET’s and the V/UHF FET by a RD70HVF1 MOSFET. Be aware that it’s not a drop in replacement! The PA board will be needed some mechanic work. The shape of the newer MOSFET’s is square instead of round. The board should be filed down to the desired shape. Some small components will also be needed tot replace. This operation is not suitable for the faint hearted and it’s advices to leave this to the pro’s or experienced repairmen. And of course the final amp will be needed adjustment…

IF filters permalink: http://www.amateurtele.com/index.php?artikel=280&id=#1248

Icom IF filters
filter model
IF frequency
mode
bandwidth (-6dB)
shape factor
-60 dB / -6 dB relation
filter type
note
FL-44A
455 kHz
SSB
2,4 kHz
1,75
narrow
6 mm taller
won't fit in the IC-706
FL-52A
455 kHz
CW
500 Hz
2,0
narrow
6 mm taller
won't fit in the IC-706
FL-53A
455 kHz
CW
250 Hz
2,0
narrow
6 mm taller
won't fit in the IC-706
FL-96
455 kHz
SSB
2,8 kHz
1,8
wide
won't fit in the IC-706
FL-222
455 kHz
SSB
1,9 kHz
1,66
narrow
won't fit in the IC-706
FL-257
455 kHz
SSB
3,3 kHz
1,9
wide
won't fit in the IC-706
FL-70
9 MHz
SSB wide
2,8 kHz
1,8
wide
FL-80
9 MHz
SSB narrow
2,4 kHz
1,5
narrow
FL-100
9 MHz
CW narrow
RTTY narrow
500 Hz
2,8
narrow
IC-706 optional filter; narrower than default
FL-101
9 MHz
CW narrow
RTTY narrow
250 Hz
3,2
narrow
IC-706 optional filter; narrower than default
FL-102
9 MHz
AM
6 kHz
3,33
wide
FL-103
9 MHz
SSB wide
CW wide
RTTY wide
2,8 kHz
1,8
wide
IC-706 optional filter; wider than default
FL-223
9 MHz
SSB narrow
1,9 kHz
1,9
narrow
IC-706 optional filter; narrower than default
FL-232
9 MHz
RTTY / CW
350 Hz
2,9
narrow
IC-706 optional filter; narrower than default
FL-272
9 MHz
SSB
2,4 kHz
2,0
narrow
default IC-706 filter

TX limits modification permalink: http://www.amateurtele.com/index.php?artikel=280&id=#1249
introduction
Originally the IC-706 has transmit frequency limits. These limits(and likely repeater shift frequencies) are set by the manufacturer using region settings. The allowed transmit frequencies van differ from country to country. Normally these regions settings are programmed into the microprocessor. But since it isn’t convenient to write different software for each region, the region settings can be set by placing or removing jumpers. Usually these jumper are low impedance surface mount resistors or diodes. In general (also for Yeasu for example) the region settings can be changed by removing or adding resistors. In the case of the Yaesu IC-706 series radios, diodes are used.

reasons for modification / why do
The main reason to expand the TX possibilities is that it’s possible to use the rig as a wideband signal generator. This could be convenient to determine the resonance frequency of a (homemade) antenna.

important notes / why don’t
Remind that it’s still illegal to transmit on other frequencies than allowed by law. Since these IC-706’s are rather old, you don’t have to worry about voiding the guarantee. ;-) Also remind that the internal filtering is designed for the original intended ham bands. Transmitting outside of the ham bands (is illegal and) could also generate unsuppressed spurious radiation.

the modification
By only removing diode D2030, all the transmit boundaries are gone. The diode is located as shown on the image below. The diodes are located on top of the mainboard. A CPU reset isn’t necessary for the IC-706 series rig. For example for Yaesu rig’s the CPU reset is needed to load the “new” region settings. If you don’t have SMT soldering experience, it’s wise to leave this modification to the pro’s or experienced service personnel.

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fan modification permalink: http://www.amateurtele.com/index.php?artikel=280&id=#1251
heat problem
The IC-706 rigs are equipped with a fan cool the rig. The fan is controlled by a fan controller circuit wich detects the rig temperature and TX/RX status. Normally a rig produces a lot of heat during transmit. Unfortunately the (rather small) rig generates also quite some heat during receive and tends to get realy hot. The hotter components get (especially electrolytic capacitors) the faster they age. And excessive heat can/will lead to frequency drift. This could be a problem using some digimodes.

solution / modification
Luckily the heat problem can be solved by adding just one resistor to the rig. With some soldering experience this could be performed yourself. The schematic for the modification is shown below.

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The simplefied schematic shows the original fan circuit (except for the 200 Ohms resistor). The rig is fed with +14 VDC and lead trough an 4 Amps glass fuse and this signal is switched by a relay. This power relay switches the rig on and off. This voltage is switched by transistor Q571 and connected to the fan. The (not shown) fan control circuit switches the transistor to activate the fan. By adding one 200 Ohms resistor between 14 VDC and the positive (red) lead of the fan, there's a current flowing of 46,5 mA trough the resistor and fan. The voltage "over" the fan is now 4,7 Volts. The fan is now spinning rather slow when the rig is turned on. The fan could barely be heard, but the air flow is sufficient to keep the rig rather cool. When transmitting or if the temperature of the rig is too high, the fan will switch to full speed at 14 VDC. The resistor also reduces the current trough the switching transistor Q571 when activated. The heat generated in the 200 Ohms resistor is 433 mW. 46,5 mA of current trough a 200 Ohms resistor results in 9,3 Volts over the resistor. 9,3 Volts × 46,5 mA = 433 mW. Therefore it's adviced to use an 1 Watt 200 Ohms resistor.
The most logic point to get 14 VDC to hook up to the resistor is at a winding end of coil wound trough a ferrite bead with component number L613. This is the +14 VDC feedpoint for the tuner connector. Solder the resistor lead to the center of coil L613. Beware that the three coil windings are not soldered to eachother. To identify the corect coil, locate the tuner input and follow the leads to the PA board. Next to these leads are two ferrite bead coils. The one near to the corner of the board is the correct one. The other coil is connected to ground. It's wise to verify this by measuring the voltage with a volt meter.

tuning "hack" permalink: http://www.amateurtele.com/index.php?artikel=280&id=#1252
The IC-706 series radio's are equipped with a tuner connector. Primerely this is for communications with a connected optional Icom tuner. When the [tune] button is pressed, the tuner will tune. But there's a (rather old and known) trick circulating on the internet for "fooling" the rig. By connecting an electrolytic capacitor and a resistor in parallel to the "start" and "key" connection and "+14V", it's possible to switch the rig into CW mode and zend out a 10 W carrier for tuning fo approximately ten seconds when the [tune[ button is pressed. The TX time is related to the value of the capacitor (and resistor). As read on the internet the values of the capacitor should be 10...4.700 uF and the resitor should be 2k... 3M3. 1.000 uF and 92 k should be the "standard" values to go for. If a shorter of longer tune time is desired, change the capacitor and resistor values. The tuning can also be stopped by pressing the same [tune] button which is used for starting of the tuning. These two components can be soldered to a plug or soldered inside the radio. Remind that these components should be removed when used an original Icom tuner.

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