"WARC" Band Operation for the KWM-2/2A and S/Line
For those of you who have tried these interesting bands, the following problems will be quite familiar.
30 Meters - The digital band - overcoupling to the antenna (plate current runaway).
17 Meters - A tendency toward the above as the loading control is almost full on (capacitor fully meshed).
12 Meters - Just pop in the crystal and go. See end of article for crystal list.
A quick fix for the overall loading problem is to disconnect the rear "B" section of the loading capacitor C151-B<1>. Carefully insulate the buss, as this connection will have to be restored.
Now, connect a jumper from the rear stator terminal to the front. This fix works fine on all the bands, but before any work is done to the rig (circuit peaking, etc.), the loading network must be restored to original configuration.
All the early manuals mention this solution, however, it is strangely missing from the 9th Edition, 15 January 1978.
Here the suggestion is to add a capacitor. This has been my thinking for some time, but how to avoid drilling holes or adding a switch?
I tried Bill Orr's suggestion, as found in Ham Radio, February 1983, which is to use an external air variable across the antenna receptacle. This does not work and just loads down the transmitter. I find the needed capacity must be added in front of the final 50 ohm, 2.5 millihenry choke<2>.
My solution adds a single piece of insulated #20 stranded hookup wire from the stator of the "A" section of the loading capacitor (section nearest front of rig) C151-A<3>, to the Noise Blanker antenna jack<4>. There are some large chassis holes near L19<5> that allow a very direct route. Desolder the inductor that you will find on the antenna jack and tie it back for easy replacement if necessary. This is the only change in the rig.
The means of adding the capacitance may take several forms, so I will only describe a simple, straight forward one here. First, don't plan on using a piece of RG58 to connect your little box to the rig, as 30 inches of it will add about 130 picofarads to your radio's loading network. I made up a double male RCA plug (they are available)<6>, which then plugs directly into a small die cast box in which are the two capacitors. At the 30 Meter jack on the box, a trimmer capacitor similar to those used in the radio (C152, etc)<7> would be ideal, as the adjustment screw could be outside the box. The measured capacity at the tip of my double plug is 387 picofarads. Lacking this capacitor, 1,000 volt silver micas could be substituted.
A listing in one catalog has 330 pf and 68 pf, 1,000 volt units available. At the other jack, marked 17 meters, I have measured 112 picofarads. Same catalog - two 56 pf, 1,000 volt caps should work fine. I leave the 17 Meter capacitance plugged in all the time, except for 80 and 10 Meter operation. The loading control moves several notches clockwise, however, if you have your bands pretty well set at the 50 ohm mark, this will not matter.
In closing, the advantage is when you remove the capacitor box, the rig is back to normal, no holes, no switches and just one short length of wire. Try it, you'll like it!
John Raitt, N9BUU
S/LINE notes:
<1> C147-B in 32S-3
<2> L24 in 32S-3
<3> C147-A in 32S-3
<4> to an added rear panel jack.
<5> L15
<6> Surplus Sales of Nebraska
<7> C146 in 32S-3
