tack a 15K resistor from the AGC input (low end of R12) to
ground just for these tests. For a tuning control, you can
temporarily wire a pot and resistor to the Vt input on this
deck. Do not install the band switch at this time. Do not
connect the 24 VDC supply at this time, either. Do connect
the + 5 VDC to the deck. You will need all of your coils at
this time, plus a 100 µH, 10 µH, and 1 µH for these tests. For
each of these, tack one coil at a time onto standoff posts A
and B. With a scope and frequency meter attached to the
emitter of Q3, you should see approximately 360 mV of
clean sine wave with each coil used. Don’t worry about the
exact amplitude as this will be adjusted at a later point in
With any coil installed, vary the tuning range and you
should see a bit more than one octave of frequency span
for each coil. The exact frequency is not important here, but
each coil should give about a 3:1 frequency shift as you
increase the coil inductance. Shut off power to the deck
and temporarily connect a 51 ohm 1/4W resistor from the
RF output jack to ground. Power up and scope this point.
There should be about 2,000 mV P-P of clean sine wave
here. If all is well, power down and remove the coil.
Prep the switch by soldering a 1” piece of wire to the
S2B wiper contact. Also, one item worth adding is a thin 1”
square shim of pre-tinned copper with a 3/8” hole in its
center, shoved onto the switch mount bolt. This makes it so
much easier to install the AGC resistors by giving a
convenient ground point. Now, install the switch so that the
wiper contact of S2B is positioned right over insulated
standoff A. This should be about 1/2” above the post.
Tighten the switch mounting nut. Solder the wiper lead
and install the wiring from the RF ground plate on the
switch to post B. Add a wire with FB1 from S1a wiper
contact to the low end of R12. Now, the individual band
coils will be installed.
Starting with the lowest band and as each coil is
installed, a frequency check will be made for span and
correctness of its labeled bandwidth. The band 8 inductor
(see Parts List) is just a 1-1/2” piece of bare #22 wire. Start
here with a piece a little longer than 2”, then trim back as
needed while doing the band tests. After you cut it to the
length you need, just wind up one or two loose loops if
necessary to fit in the space allowed. This will make virtually
no change in inductance as opposed to the straight wire.
The coils that you use may not be the same value as
mine due to parts tolerances, and there are a lot of
tolerances to consider here. The SMV-1404 is a very
repeatable component, while the coils can vary by 5% or
more. Resistors, pots, and the list goes on. The laws of
probability state that half the tolerances will be positive, the
other half negative, and they will cancel each other out.
Murphy’s Law says all tolerances will add in the same
direction and render the design useless. In the real world,
it’s never this bad, but still something to be aware of.
When you purchase coils, it would be a good idea to
get a variety close to the Parts List values and some at
about 5% of the listed values. I actually ordered 50 for this
project since it was a new design. The parts are dirt cheap,
and it would be a shame to come up short and have to
order 80 cents worth of coils for $6 postage. Besides, you
can always use the extra in other projects.
Let’s get back to installing the tank coils and what you
want to achieve here. Strive to get the span needed for the
panel labeling (shown with some overshoot) on each end of
that band. I ended up with an average of 5% on the ends,
but some were as close as 1%. The AGC resistor installation
and warmup will shift that span by a very small amount. The
actual total span of frequencies should be 0.3 MHz to 150
MHz with no gaps between bands. Mine runs 0.295 MHz
to 162 MHz. Having plenty of overshoot is okay, but the
most important aspect is that the band switch labels
guarantee their stated bandwidth. Can’t quite fit everything
into my labeling? You can always tweak the Vt voltages to
32 June 2014
■ FIGURE 7.
■ FIGURE 6.