completes installation. I use #24 or #26 wire here.
The rest of the active components are inserted and their
leads are used as solder tie points. All other circuit nodes
above ground get soldered to insulated standoffs. The two
standoffs (A and B) near the MC1648 pins 10 and 12 will
be used for primary testing and final S1 band switch
connection. When doing RF circuit construction, always
keep two things in mind: an RF ground plane is essential
and lead lengths must be short. How short? Well, RF design
engineers have an old saying, “if you can see the leads, they
are too long.” Impossible in the real world, but you get the
idea. The PCB is mounted to a subchassis with three 1/2”
metal standoffs. This chassis will include a 3/8” hole in its
vertical section to mount the band switch. Set this subassembly aside for now; further testing will be done later.
The RF band switch (S1) shown in Figure 5 is a two-pole eight-position wafer switch. I used an old ceramic
Centra-Lab 4 deck wafer switch that I had on hand. All
decks are held in place with long #4-40 screws and spacers.
These switches are easy to disassemble and rework. All
wafers have a full 12 positions on them, and one metal
bracket with bending tabs is used to lock the stop for the
actual number of positions desired.
The number of decks on the switch does not matter,
but you need at least two decks. Unnecessary decks are
discarded, and the wafer shaft gets cut back so it only drives
two wafers. Since the tank circuit coils on the RF deck have
+1.6 VDC of bias voltage on them, we need an RF ground
on their low side which has to be isolated from DC. This is
accomplished by cutting a piece of single-sided circuit board
to the approximate shape of the switch wafer. This is then
aligned with the wafer, and the position tabs and mounting
bolt locations are then transferred to the ground plate.
Since there are no moving parts involved here, the plate
requires no drive shaft connection. The mounting holes will
be drilled for #4-40 clearance and the coil lead holes will be
about 0.050”. The ground plate will be mounted with the
foil side to the rear. When ready to install the coils, one end
is slid through the switch’s position tab as far as it will go,
then the other end trimmed to allow it to pass through the
ground plate. Center the coil and solder, and trim the
When purchasing your switch, there are several things
to keep an eye on. Look for mounting bolt construction and
a full 12 position tabs on each deck. Also look for the least
amount of metal brackets that seem unnecessary. The
number of decks is unimportant just as long as it has at least
two as mentioned, because the switch will ultimately be
30 June 2014
■ FIGURE 3.