proper positions to do so. Getting false results from the
test was very confusing and annoying until I realized it was
operator error. They add virtually no extra cost and add
very little load to the power supply requirements (about 3
Referring to Figures 2 and 3, the completed front and
rear views are shown. The front panel measures 4. 5” wide
x 4.0” high. The circuit board measures approximately 3”
square and is a modified RadioShack board (#276-168). I
just love these boards as they readily accept DIP sockets
and have power busses running right underneath them. As
seen in Figure 2, the board just folds over the back side of
the front panel and is mounted to the front panel center
post by one 8-32 screw. The depth of this whole assembly
is approximately 2”. I built it this way so as to have easy
access to the board component side for calibration and/or
troubleshooting, and yet still have easy access to the
underside with only the removal of one screw.
One thing to note in the photo is that the Manual
sweep pot and the Vernier pots are shown as single-turn
pots. These were ultimately changed to 10-turn pots of the
same value ( 10 KΩ) for reasons I’ll describe later. I wanted
an enclosure as small as possible, but with the control
panel angled for comfortable operation, so the slope front
style immediately came to mind. At that point, I started
cruising the Internet for a metal box that would
be suitable, but at a lowest available price of $75,
it was out of the question. Since this circuit did
not require metal for shielding protection, I next
Prices were still too high, and nothing had a
good fit for my front panel size. Since I decided I
didn’t need a shielded box and I happen to be a
woodworker, wood was my next logical choice.
The material I used was some leftover scrap
aircraft plywood about 0.15” thick.
I cut out six panel pieces and glued them
together. Once cured, I finger-smeared a fillet of
Bondo auto filler along all the inside corners for
added strength. All in all, though, a rectangular
box would be sufficient — especially with fold-down rear feet that would hold it at the proper
angle when in use.
As can be from Figures 1, 2, and 3, the
layout is fairly simple and straightforward. The
cable leads to the generator were brought out the
rear of the adapter box through a grommetted
hole. The 14” cable was just laced every couple
of inches and terminated in a seven-pin plug that
looks similar to an XLR style. A mating jack was
installed in the rear panel of the RF generator to
accept this. From there, the jack was wired to the
proper locations on the control board as shown
in Figure 4. Alternatively, you could just run leads
out of a hole and use an outboard inline connector. Two
resistors will have to be installed in the generator for the
Vt and sense leads; you should have room left on the
control board for these.
Too tight? Just solder one end of them snug to the
board in a vertical position and tack the incoming leads to
a short stub on the other end. Tie in the power leads
wherever convenient. I did use shielded leads for the
Vt/sense lines, and tied their shields to ground ONLY at
the adapter unit end. This was probably overkill as all lines
exiting the circuit board were designed for low
impedance; generally less than 100Ω.
The power required to operate the adapter unit is ± 12
VDC at 15 milliamps. If you built your generator power
supply as originally published, it can handle the added
load. If not, then pre-check it with a suitable resistive load
for those requirements to make sure it’s up to the task.
After the first prototype was completed and in the
testing stage, it was apparent that the Manual sweep pot
and Vernier pot needed more resolution than the single-turn pots originally installed. I wanted to replace them
with three-turn pots in these locations. They were hard to
find; only 10-turn pots were commonplace on eBay and
quite cheap. I did locate some three-turn pots from a small
west coast distributor and promptly put my order in for
them. I paid a bit more than I wanted to, but they were
the only ones I could locate.
In the meantime, I wanted to continue my testing, and
34 February 2017
■ FIGURE 3.
■ FIGURE 2.