John McLean's Loading Coil Modification for the MFJ-969 Antenna Tuner

Hallelujah! Those on the list may recall some problems I was having with my MFJ-969 tuner, particularly the roller inductor, well I fixed it!!

I had repeated failures with the roller inductor, caused by poor contact between the roller and coil, and also the roller bar and roller, causing unpredictable VSWR. I almost destroyed my roller inductor in the process, melting the solder where the coil is soldered onto the frame bracket. Another problem was where the insulated frame, some sort of plastic / fibre almost failed due to heat.

My first attempt was a switchable inductor. It was 2" in diameter, with taps every 30 degrees. It was from there that I realized that you only needed less than 1 turn, so the modification you see in the picture here was tried.

I removed the roller inductor, and made up a 2 turn coil, 2 inches in diameter, out of 16 gauge tinned copper wire. A friend supplied me with a 12 position switch, which I connected to the coil. I tapped the coil (inductor) at 30 degree increments, and connected these tap points to the switch, and voila!! Low VSWR, and no more problems!

I found that the best operating inductor switch position is in the third position from the start of the inductor, so you probably only need to put in a maximum of 6 taps on the coil.

Success! If you wish, you can install a 6 position rotary switch and tap it every 30 deg or so, so the operator can fine tune it. Since I made the modification, I find that more power is available at the tube, with a lower VSWR. A bonus is that the RF is not just heating the inductor, but it is now ending up where you need it - at the tube!

John McLean, NSW, Australia

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Loading Coil Modification to MFJ-969 Tuner

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