the audio frequency signal coming into T1 modulates the
plate voltage, causing the amplitude of the RF output to
go up and down with the audio.
Searching for Parts
Before getting too excited about this project, I
needed to find sources for the critical parts. The Knight-Kit was over 50 years old, and I wondered if the 12K5
and oscillator coil (L1) were still available. A good friend
turned me on to a company called Antique Radio
Electronics. Zowie! It was tube heaven! They had all kinds
of cool things. A 12K5 was only $3.95 and the coil was
$8.95. I’m not sure how many of these items they keep in
stock, so you might make sure they are available before
you get started. I noticed that the 12K5 also showed up
on eBay and MDB Ventures.
The only hitch with the Knight-Kit broadcaster was
range. It didn’t transmit very far. Of course, the little
telescoping antenna was woefully short. I built a proper
loading coil but it was quite large and not practical, so I
gave up on that approach. Boosting the current through
grid 1 to 56 mA raised the plate current to 4.1 mA and
improved the RF output. I found that a good earth ground
also helped reduce the 60 Hz pickup and added a ground
plane. The ground (green) wire is shown in Figures 7 and
9. Finally, I changed the output capacitor from the
original 100 pF to 0.01 µF, which raised the output
considerably. The improved signal was strong enough to
reach all the radios in the house, but only if they had
decent receiving antennas. I was unable to get an output
power measurement that I believed, but I compared its
output to a 100 m W Ramsey AM Broadcaster Kit and
both seemed about the same.
FCC (Federal Communications Commission) Part 15
specifies a limit of 200 feet for an unlicensed AM
transmitter. I also saw a limit of 100 m W in the FCC OET
Bulletin No. 63. Either way, the range of the Retro-Shield
should not interfere with your neighbor’s radios. If you
think there is a problem, simply reduce the value of C10.
Now, a word about AM radios. Most home stereos
definitely need an external AM antenna to receive signals
from the Retro-Shield. Some stereos come with a goofy
little AM loop antenna, but it’s better than nothing. Failing
that, a 10’ hank of wire would probably be enough. I used
a small transistor radio to check the reception throughout
the house, and the signal was pretty good everywhere.
Of course, you don’t have to put together a stack of
■ FIGURE 7. The
speaker is mounted
on top of T1 with
three boards as I did; you can build the broadcaster as a
stand-alone unit. I sometimes use it to send i Tunes from
my PC to the radio in the garage. Next Halloween, I plan
to pipe some scary music to a portable radio outside.
This project is not a slam dunk. You have to use your
imagination and expertise to build and test the system.
The Retro-Shield has both high (1 MHz) and low
frequency (audio) signals running around. Try to keep the
RF signals as isolated as possible. The LM386 IC had some
pickup problems at first, but a friend suggested adding C2
and it solved the problem.
I selected the Arduino proto shield for the top
broadcaster board, just to keep it in the Arduino family. A
piece of perf board would be fine too, since only two pins
(Vin and GND) are needed to carry the 12 volt power
down to the shields below. A ground plane perf board
would probably be even better. J1 routes the audio up
from the VoiceShield.
The proto shield board has a small number of tiny
traces that carry signals and power to various places. All
these traces should be cut so they do not interfere with
the broadcaster circuitry. The layout is shown in Figure 8.
I didn’t show individual connections because it would be
much too busy. Just use point-to-point wiring; the
components are grouped. The parts on the bottom of the
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