Icom IC-706Mk2G IF Tap

As with my other radios (IC-756pro3, IC-910H & FT-817) I wanted to tap the IF (Intermediate Frequency) out of my Icom IC-706Mk2G to allow monitoring on an SDR radio and associated software.

The IC-706Mk2G has two IF frequencies, I tapped the 1st IF at 69.0115MHz (the 2nd IF is around 9.0115MHz) and used a PAT (Panoramic Adaptor Tap) high impedance buffer amplifier designed by David G4HUP, G4HUP has different models of his PAT board and the one used for the IC-706Mk2G is the PAT-70M, the 70M means the output LPF (Low Pass Filter) is set to 70MHz, ideal for an IF around 69MHz.

IF Tap Location and Installation

Schematic

I originally tried tapping the IF at L511/C512 which is before the 1st IF filter (FI511) but I found this had a number of problems;

  1. The RX sensitivity dropped a number of dBs, which it was not supposed to happen as this is a high impedance buffer amp
  2. There were lots of oscillator artefacts (harmonics) in the SDR display

I then tried a second IF tap point, this time it was after the 1st IF filter.  I tapped at L512/R513/D502 and at this point the issues from the original tap location went away and the final tap location all works well.

IC-706Mk2G_IF_Tap_Location

IF Tap PCB Location

The location of the IF Tap point is on the main board, which is located under the top panel (the one with the speaker in it), to the left side of the radio with the LCD facing you. Note you will need to remove the bottom panel also later on.

IC-706Mk2G_IF_Tap_Location-Radio

The Final IF tap point at L512/R513/D502 is on the front left of the main PCB as shown below;

IC-706Mk2G_IF_Tap_Location-Board

Once I found where to connect the IF Tap to, I used some RG-178 coax to connect that point on the PCB to the PAT board.  I good bit of soldering skills are needed, note the coax shield is soldered to the inductor can.

2014-11-13 23.25.22

Close up of the IF Tap point showing the centre of the RG-178 coax connected to R513 (marked 331).

2nd-IF-Tap_Close

PAT Board Mounting

There are a few ways to mount the PAT board. Dave G4HUP can supply mounting kits for various radios for a small fee.

2014-11-13 22.57.40

I simply used some back to back sticky tape which was attached to the underside of the PAT PCB and then the PAT PCB was stuck down on to the installed optional narrow filters I have fitted in my IC-706Mk2G.

If there are no optional filters then the PAT PCB could be mounted on the shielding can to the left of the current PAT mounting location shown in the images.

2014-11-13 23.25.48

Rear Panel Connection

As this is just a buffered IF tap, I use phono connections for the IF output from the radio, this is used on all my IF tap modifications.  I used to use SMAs in the past but SMAs became cumbersome for something where loss is not so important.

The IC-706Mk2G turned out to be very easy to fit a phono connector on the back panel of for the IF output. On the back panel is a small shiny blanking plate, which is easily removed with 2 screws, under which is an 8mm hole ideal for a phono connector, or SMA connector if required. This means no drilling was required.

 2014-11-12 22.25.59

The phono connector is seen fitted in the middle of the rear panel with the shiny plate on the desk.  On the inside I tightened the phono mounting bolt and put some ‘screwlock’ on it to stop it coming loose at any point.

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Another length of RG-178 was connected between the output of the PAT board and the phono connector the back panel, to do this the filter board needs to be removed to route the RG-178 under to make it all fit, this does require the bottom panel of the radio to be removed so that an RF cable connected between the filter and PA boards can be undone which is required to allow the filter board to be lifted sufficiently to be able to run the RG-178 under it.

DC Power to PAT Board

The PAT board requires 5v to 12v DC varying between 5mA and 10mA current draw respectfully.  The IC-706Mk2G has very accessible 8v and GND (0v) points over C647 which is located next to the optional filters.

 IC-706Mk2G_PAT_DC_LocationPAT_DC

It might be useful to use the R8V line to power the PAT board when mounted in the IC-706Mk2G radio, the R8V line is “8v on RX” – this would mean that the PAT board only has power when in RX, C647 is a permanent 8v.

SDR Output – HDSDR

This is how the IC-706Mk2G IF output, using the PAT interface, looks like in HDSDR. Note the shape of the signal in the waterfall, the peak in the middle is down to the 1st IF filter bandwidth but signals +-60KHz are easily seen.

Screenshot 2014-11-13 23.38.55

2 Comments

  1. Max Donoghue

    Drew,

    The reason you are only seeing the limited bandwidth is due to the filter. If you tap after the filter, it greatly restricts what you see on the panadapter. I found the same on my FT1000 Field. I decided to tap before the filter as I have done in the past. I can deal with “artifacts” if I have to. Seeing only 30 KHz of the band is a waste of time in my opinion.

    Max
    VE3TMT

    Reply
  2. Drew

    Hi Gavin, tried this on my mk2g It seems to me that the band width is very narrow on the display. I have one of these installed on my ts50 and my ts450 and am able to see all stations across around 1mhz. After this mod the 706mk2 showed some nearby(close to radios freq) stations but other more than about 100khz away showed a lot of attenuation, which increases the further you go away from the radios vfo frequency. I guess this is due to the different (final) tap position you chose (instead of the original tap point). BTW I tapped the IF off on both my kenwoods using simple capacitive coupling and a simple resistor (.01uf caps and 1k resistor on both the the centre and ground), this method didn’t quieten the receiver at all and shows a crisp clean display without any unwanted harmonics. Any chance you could show the original tap point (a photo of the actual spot on the radio) not just the final tap.Perhaps I have just been spoilt by the way my kenwoods work by tapping the IF straight after the first mixer. Long story short signals 200khz away are >-30db down when compared to my kenwwod.Many thanks Drew

    Reply

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