Replacing polyvaricon tuning for varactor tuning

The supplied polvaricon tuning capacitor works well but gives a limited tuning range and only has about 180 degrees of travel. This modification will replace it with a 10 turn potentiometer and a varactor. You can obtain a 10 turn pot from Electronic Surplus Inc. for about $12.00 plus shipping. Another great source of parts is Ebay. Search 10 turn potentiometer. They currently have one for $4.40. Watch the shaft size. It's not critical but some pots have a 1/4" shaft. Measure your hole size in the chassis and check to make sure you aren't surprised. You could also use a regular pot and gain the advantage of a wider tuning range but I would highly reccomend the 10 turn. Put a large knob on it and you'll love the tuning feel.

An MVAM109 may also be used for the NTE 618. They are now available from

The NTE618 has a capacitance range of 20pfd at 8 volts and 420pfd at 1.2 volts. Also acceptable is a MVAM108 or MVAM109. Any of these will give an increased tuning range over the polyvaricon.

If you are using a bitx kit chassis, the tuning capacitor hole needs to be enlarged to 3/8". This can easily and neatly using a step drill. This is not a destructive step as the polyvaricon can be re-installed if you are careful when enlarging the hole.

I mounted all of the new components directly onto the 10 turn pot. This makes it very easy to do and undo. The regulated voltage for the 10 turn pot can be easily obtained from the "hot" side of the fine tune pot. That's also a convenient point to connect the ground of the 10 turn pot. In the left background you can see the connection to the board at at the C38 pads. I used a twisted pair wire to eliminate any capacitance variation due to wire movement. Pay attention to the polarity when hooking up the tuning capacitor as one side is ground. If you connect it backwards, you will have a short across the C38 terminals.

The completed assembly

I replaced the supplied knob with a larger one. This makes use of the fine tune pot almost optional. The completed modification makes for a very smooth tuning feel. I ended up with a tuning range of 14.130 MHz - 14.390 Mhz. This could be refined some by adjusting the turns count of L7. I reccomend winding L7 with 55 turns and then removing one at a time to get the tunning range correct. I ended up with 52 turns on mine.