Gakken likes radios. They’ve published at least 4 such kits so far. And I know that I said I was only going to buy the numbered mook kits, but I was feeling withdrawal and had to buy something to build, since the 8-bit microprocessor kit wasn’t coming out for at least another 4 weeks. The 1 vacuum tube radio costs 3150 yen (approx. $36 USD).
AM radio transmissions consist of using a transmitter broadcasting at a specific frequency and varying the strength of the signal based on what you want to send (voice or music). The easiest method is to just hook up a microphone to the volume control of the transmitter and send your voice that way. Receiving the transmission then requires using some kind of circuit tuned to the desired frequency, and stripping that base frequency out to just leave the amplitude variations in place. This means that the receiver will have an antenna, a tuner circuit and an amplifier. While there can be many radio broadcasts simultaneously, only the broadcast on the frequency the tuner is set at will be strong enough to hear at that moment. Even so, the audio portion may still be too weak to hear as-is, which is why we need the amplifier.
This kit uses a special twist on the tuner circuit – the antenna is also part of the tuner. It’s made up of two coils, and turning the inner coil acts as a fine tuning control. The rough control comes from a variable capacitor, and the high/low frequency switch throws in a parallel capacitor to select between the 500-900KHz and 900-1600 KHz bands.
If you’re not familiar with vacuum tubes, you can imagine them as a really big, early version of a transistor. In order for a vacuum tube to work, it needs two power supplies – one for the primary signal power and the other for the heater filament. This kit uses three 9-volt batteries for the main power, and a 1.5V C cell for the heater. Rotating the C cell battery housing acts as the radio’s on-off switch. Kind of clunky, but elegantly simple. Audio out is just a mono earplug. (Fortunately, because we’re just using 1.5 V on the heater, the tube never gets hot enough to be a burn hazard, unlike with older-style tubes.)
The kit has less than 15 pieces, and the circuit board is preassembled in the base housing so you don’t even need to mess with it. Suggested assembly time is 60 minutes. I took my time, rereading the instructions to make sure I got them right, and being careful while wrapping the antenna coils to avoid kinks – which is what took up all the time. You need to pull the wire a little tight – otherwise the wrappings will get too close to the edge of the form at the end and will unwrap on you – but not so tight as to snap the wire. The form is designed for an interleaved, alternating wrapping. If two consecutive wraps are on the same side of the tabs, you missed alternating on one of the tabs somewhere. The book says to start out with a 15 cm loose end, but that leaves a lot of slack on one end and not the other. Ideally, it’d be better to trim both ends of the coil to be about 4-6 cm when you’re done, but the insulation on this wire isn’t easy to remove, so I just balled up the slack to hide it at the back of the radio. Final assembly time was about 1 hour and 40 minutes.
You want to be a little careful in the handling of the vacuum tube, since strain at the base of the pins can crack the glass and let the vacuum out. There’s a set of holes on the base of the kit for straightening out the pins before putting the tube in the socket.
The kit works fairly well, but there’s no volume control on it and the sound level was pretty weak. You may want to add an audio jack to plug in a powered speaker. To operate the radio, rotate the C cell housing to the “on” position (the LED will light), then adjust the VC knob until you get some kind of voice, music or squealing. Slowly rotate the inner antenna coil (clockwise or counter-clockwise, doesn’t matter) to fine tune the signal. The radio is directional, so you’ll need to rotate the entire kit to obtain the optimal signal strength. In fact, the mook offers a suggestion to use a map for triangulating on the transmitter. Get a city map and a compass. Lay the map down pointing north and mark where your house is on it. Tune the radio and rotate it to get the strongest signal, then use a ruler to draw a line through your house in the direction indicated by the outer antenna. Next, go somewhere perpendicular to that line, about 1.5 or 2 km, and repeat the process. The transmitter then will be in the approximate area of where the lines cross.
The unnumbered mooks only discuss subjects directly related to the kits, so they’re about half the pages of the numbered kits. This mook discusses the history of radio, talks about de Forest and Fleming, gives the theory of vacuum tube operations, and shows photos of various types of antique radios. The suggested mods are to add a bigger antenna, wrapping the antenna coil around an umbrella, and to use a telegraph key to send morse code to a second radio (requires cutting up the circuit board and adding a few extra parts).
Overall, the cheaper Gakken kits tend to be hit or miss. When they work well, they’re really fun and offer lots of replay value. Otherwise, they just become something different to be placed on a bookshelf and forgotten. If you want a good, working AM radio, go to Radio Shack and plop down $10. If you like wrapping coils and learning how radios work, this is a good kit to pick up.