Kenwood TS-790 IF output for SDR panadapter

I recently got a second hand TS-790 with the 1.2 GHz unit installed and after a good cleaning and service I've done, I decided to search the options I have to get the IF from the radio to feed my SDRplay wideband receiver.

There is no doubt that software defined radios are the future... I thought it would be very nice to have a panoramic view of the bands while using the same antenna.

After search I found the TS-790 service manual and I started to study it. This rig has 2 independent receivers (main and sub) and as a result has also 2 IF's. The IF for the main receiver is 10,695 MHz and the IF for the sub receiver is 10,595 MHz.

I decided to go for the sub band IF because I didn't wanted to mess with the transmit circuits used in the main band. Looking at the block diagram it seems that the signals from all the bands (VHF/UHF/SHF) are going into the XF1 filter and then to Q5 fet amplifier. So the best place to tap for the IF would be the drain of Q5 fet. Respectively, if someone wants to get the IF from the main band should look for the drain of Q56 fet.

Kenwood TS-790 IF Block diagram

At the schematic diagram below you can see the exact tap point. Sorry for the bad quality of the pictures, but the scanned service manual is very bad...

KENWOOD TS-790 Schematic with SUB IF TAP POINT

Of course, you can't just solder a simple wire and get the IF. You need to add a suitable buffer amplifier to isolate you from the circuit and that way you don't have losses at your sub receiver.You have to use a good quality RF cable and place the buffer amplifier as close as you can to the tap point. 

You will also need to get somewhere the 13,8 volts to give power to the buffer amp. You can build your own buffer or you can buy one, as I did. I chose to buy from G4HUP.  

There is a lot of work needed to actually reach the IF tap point. If you feel that you cannot do it, then DON'T TRY IT. You may damage your expensive radio. Ask for help from someone that is more qualified to do this.

I take no responsibility for any damage that this modification may do to your radio. Proceed at your own risk !!!  

Remove the bottom cover of your radio. You will see the IF board. Then you have to remove the IF board. To do that you must unscrew first all the screws keeping the board in place and also unplug all the cables. There are many cables !!! It's highly recommended to take a high resolution photo of the board with all the cables attached in case you forget their position.

Then locate the IF tap point. In practice I soldered the cable directly to the L3. I find it's a good place and not very risky to burn the FET or other components. Don't forget the shield of the coax, use as ground the case of L2. Use a good quality RF cable. Secure the cable to the board with some tape. Check my photos below. (G4HUP does not provide the input RF cable, he provides only the output IF cable with a male or female SMA connector). I got the male SMA option to connect direct to the SDRPlay receiver input.

IF TAP POINT DIRECTLY TO L3, SHIELD TO L2

You will also need power for your buffer board. A good and safe place is the pin 7 of the ACC 4 connector. It's at the back side of the board, a little far, but I didn't have any RF problems. This pin provides 13,8 V from the main power switch of the radio. If you search carefully you may find a closer place to get the power. I was in a hurry to test this mod, so I didn't searched more :-)

13.8 V from power supply

The buffer board is secured with double side tape on the chassis at the front. For easy installation, the IF output cable is routed on the top of the board, however you can route it from the bottom. Use tireups to secure the cable from moving front and back.

Buffer board in place

General view of the IF board with buffer installed

By using the tap point at L3 you will only have a maximum usable bandwidth of about 50 KHz because of the filter on all bands (V/U/S).
If you want to see more bandwidth at your waterfall, you can try a different tap point at L1. This will give you at least 1 MHz (and much more), which in some cases might be more usefull.

If you go up to the top of the page you will see at the IF schematic diagram a GREEN point. This is L1. Click on the picture below to see in detail my second test with the tap point at L1.

Don't try to get the IF without a buffer amplifier - isolator, or you will loose RX sensitivity and maybe damage your radio. This is NOT recommended and NOT guaranteed.

IF tap point before the filter (at L1) for maximum bandwidth

I think I'll stay with the second solution for maximum bandwidth, it makes more sense to me for my needs and works good. I have tested the VHF band and the results are excellent. Same performance on receive and almost 2 MHz (and much more) of bandwidth at my waterfall. But please have in mind that tapping at L1 will NOT give you more bandwidth at UHF and SHF bands, since the signal coming from the 430 and 1200 RF boards is filtered before. If you want more bandwidth at these bands you have to tap to the RF boards. I didn't do it because I'm mostly interested at VHF band.

My favorite SDR software is HDSDR. It's free and easy to use. Configuration is very easy. Just set the IF frequency to 10.595 MHz and start listening !!!

HDSDR IF CONFIGURATION

Of course there some other options needed to be set, but this is out of the scope of this guide. For example you can set the software to control your TS-790, sync frequencies in both directions, etc... There are many tutorials online, just search them. And there are other software packages available, such as SDR Console, SDR# and others. 

So many options and a totally new way to enjoy my radio and hobby...

73, Fanis
SV1BGM