P12 connections (A and B). This is for connecting to the
band switch which will eventually end up sitting right over
the top of them. They are located in the middle left of
Figure 4 and are a little hard to see. Pin 10 already has its
lead installed with a ferrite bead in place.
Lastly, the MV1404 varactor diodes I used in this
particular project came in TO-92 packages which are
becoming very hard to find. They were later replaced with
an MV 1404-9 which is a dual diode unit in a single SMD
package. It also improved performance slightly.
The 12-position band switch shown in Figure 5 is made
up of a two-pole, double-deck 12-position wafer switch.
The first deck is used for AGC level resistors. Cut a
piece of single-sided circuit board a little larger than the
size and shape of the switch deck. Drill two mounting holes
for attaching to the switch bolts and ream out the copper
side to provide an insulating barrier. Drill 12 small holes in a
slightly larger circle pattern than the switch contact points.
Remove the switch bolts one at a time and replace with 2. 5
mm x 2” screws ( 4-40 thread will work but requires a small
amount of reaming to pass through).
Nut these down to the switch, then install two spacers
of approximately 1” length on the protruding bolts. Install
the previously made up circuit board with the copper side
to the rear. Drop a couple of insulating washers over the
bolts and nut down the assembly firmly (don’t go crazy
here). This newly added circuit board is the RF ground plate
shown as such in the schematic.
The coils are mounted by shoving one lead through the
ground plate hole and then cutting the other lead to length;
merely lay it on its associated switch lug and solder. Solder
the end protruding through the ground plate and trim off
the excess. Then, continue to the next one until finished.
Installing band’s 9 and 10 coils adjacent to the wiper
contacts will alleviate some of the switch’s parasitic
reactance which becomes more dominant at these
higher frequencies. The chart in Table 2 shows the band
and associated coil value.
Given the tolerance on these coils, the labeled band
frequencies are very nominal but will be in the ball park for
a guide. All bands have sufficient overlap so that no voids
will occur. (Some of these coil’s values may be
unobtainable due to being out of stock or on back order,
but they can be made up in series with other values — just
add up values in the same way as series resistance.)
Solder a piece of #22 solid hookup wire about 3/4”
long to the wiper contact. Now when this switch is
installed, that lead will be at the bottom of the switch and
positioned directly over the area of the MC1648 P12 or
stand-off. Note that the RF deck is mounted upside down
and directly under the band switch. This switch is mounted
1-1/4” (shaft center) above the Rf circuit board surface.
You can panel mount it, or do as I did and mount the
RF deck and switch to a scrap of 1/32” aluminum bent at a
90 degree angle. This can then be powered up for basic
checkout before the whole sub assembly is installed in the
enclosure. This method facilitates checkout as a stand-alone
sub assembly and was mandatory in my case for the design
process. The sweep circuit was built on a RadioShack 276-
168B circuit board. I love these little boards as they accept
DIP devices very nicely. The board measures 2-1/4” x
2-1/2”. Layout is straightforward and non-critical; although,
I added a 16-pin header due to a multitude of lead
connections to the front panel controls. Also, do not use a
ceramic capacitor for C2 — mylar is best!
December 2013 33
Band 1 2 - 4 MHz 80 uH Band 3 5 - 10 MHz 12 uH
Band 2 3 - 6 MHz 33 uH Band 4 8 - 16 MHz 4. 7 uH
Band 5 12 - 24 MHz 2. 2 uH Band 8 40 - 80 MHz 0.18 uH
Band 6 20 - 40 MHz 0.82 uH Band 9 60 -120 MHz 65 nH
Band 7 30 - 60 MHz 0.37 uH Band 10 90 - 180 MHz 20 nH
■ FIGURE 4. RF deck —
■ FIGURE 5. RF deck and band switch —