Category Archives: Radio history

1962 Four Transistor “Half Pocket” Portable

1962MarRadioElec

Sixty years ago, this listener was probably the first on her block to own a transistor radio.  And she was almost certainly the owner of the smallest radio.  Smaller than a pocket set, this one was billed as a “half pocket” portable in the March 1962 issue of Radio Electronics.

The four transistor set measured only 1-11/16 x 1-1/2 x 11/16 inches, with a 2N345 serving as regenerative detector, with three 2N207’s amplifying the audio.  It was “about the size of a petite ladies’ cigarette lighter,” and smaller than some hearing aids.  It was said to pull in local stations with good volume and clarity, and required no external antenna or ground.

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Prewar Radio Allocation Table

1942MarRadioRetailingFor a snapshot of how the radio spectrum was allocated 80 years ago, this chart appeared in the March 1942 issue of Radio Retailing. You can click on the image above for a full-size image, and on most browsers, click again to enlarge.

While America was now in the war, the allocations above are really the last prewar allocations, as they would have appeared on December 6, 1941. Since then, for example, Amateurs had left the air for the duration. At the time, the amateur bands were on 160, 80, 40, 20, 10, 5, and 2-1/2 meters. Postwar, the allocations would be similar on 160 through 10, with the addition of the 15 meter band a few years after the war. The VHF allocations shifted slightly to 50 and 144 MHz.

With some changes, the TV channels were in their postwar configurations. FM broadcasting would move from the 42-50 MHz band up to its present allocation at 88-108 MHz.



Laco Kitcraft Model 200 One-Tube Radio, 1947

1947FebPS5LacoKitcraftSeventy-five years ago this month, the February 1947 issue of Popular Science showed this simple one-tube radio kit, the Laco Kitcraft Model 200.  The kit sold for $6, and featured a single 1L4 (or 1T4 or 1U4) tube, and required 1-1/2 volts for the filament, and anywhere from 22-1/2 to 90 volts for the B+.

According to the magazine, it was perfect for the youngster who wanted his or her own radio.



Anemometer With No Moving Parts

1957FebPE10If Junior wants to take home the blue ribbon at the next science fair, this project will almost certainly provide it. When Junior announces to the science teacher that he or she is going to build an anemometer (an instrument for measuring wind speed) with no moving parts, the teacher will be mystified, and will wonder whether it is even possible. But when they see the completed device in action, they will be astonished at its simplicity.

1957FebPE11The anemometer consists of a Wheatstone bridge circuit, which consists of four resistors. Two of the resistors are actually thermistors of equal value. As long as their resistance remains equal, the meter shows a reading of zero. But if they are unequal, then the meter displays a current. The two thermistors are placed outside at the spot where the wind is to be measured. When they are energized, they heat up slightly, which causes their resistance to change. As shown at left, both are mounted in a small plastic container, but one of those containers has small holes drilled in it. When it is exposed to the wind, it is cooled, but the other thermistor is not. The stronger the wind, the greater the cooling, and the current increases. In other words, as the wind increases, it is shown on the meter.

Once the meter is built, it needs to be calibrated, and that requires Junior to “enlist the services of a competent automobile driver” on a “highway which permits maximum state speed limits.” The driver accelerates to 60 MPH, and Junior holds the thermistor assembly out the window, as far as possible. (We note that Junior should take care not to have the arm amputated by a passing truck.) Junior then adjusts the instrument so that it indicates a full scale reading on the meter. The measurement is taken again at different speeds, and the meter reading is noted.

When Junior is awarded the blue ribbon for this elegantly simple design, the teacher will undoubtedly be thinking, “why didn’t I think of that?”

The original construction article, from the February 1957 issue of Popular Electronics, called for a “matched pair” of thermistors, since they need to have equal values. While it might not be possible to buy a matched pair, there is an inexpensive alternative. Junior can buy this set of 100 thermistors on Amazon at a very reasonable price. It includes 10 each of different values, including the needed 2kΩ. Junior just needs to measure all ten, and then use the two that are the closest in value. The remaining 98 thermistors can be used for other experiments. In fact, by adjusting the values of the other resistors, another value of thermistor could be used.

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1937 Two Tube Regen

1937FebPS02Eighty five years ago, the February 1937 issue of Popular Science carried the plans for this two-tube regenerative receiver for the broadcast band up to 65 meters (about 4.6 MHz) with two plug-in coils. The set used two type 49 tubes, with the first one reflexed to serve as both RF and AF amplifier. The second tube served as regenerative detector.

The remarkable part of this receiver was that the B+ was only 11 volts. It used two dry cells for the filaments, which were also hooked in series with a 7.5 volt battery. The magazine noted that this made the receiver ideal for portable use, since the batteries were small enough to be carried in a pocket.

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1942 Three Valve Emergency Receiver

1942FebPracWirelessEighty years ago, the January and February 1942 issues of the British magazine Practical Wireless showed the construction details for this three tube emergency receiver. The magazine had received many demands for a receiver capable of good performance on the medium waves, but with components that could be obtained with a minimum of difficulty, given the wartime conditions. The editors settled on this three tube design, with one tuned RF stage, which was found to perform adequately, but “shorn of refinements which would normally have been incorporated in times when components were easily and quickly obtainable.”

The main design was published in the January issue, with the February issue showing some refinements that would make the set more sensitive and selective. The February issue also showed how a two-tube version could be made, which might be necessary due to wartime parts shortages.

The regeneration control on this set is interesting, and something I haven’t seen before. It is a variable differential capacitor, which has two separate stators, and one rotor. The idea is evident from the diagram symbol. They are also sometimes called a split stator variable capacitor, and they apparently are a thing.

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1957 Two-Tube VHF Superregen

1957FebPE31This young man is exploring the action bands of the VHF spectrum with a two-tube superregenerative receiver from the February 1957 issue of Popular Electronics.

The set tuned 28-175 MHz, which included the FM and TV bands, as well as amateurs, police, fire, and aircraft. The magazine noted that the layout of the circuit was critical, and cautioned builders to construct it according to the photographs, in addition to the schematic. In particular, short lead lengths in the RF stage were critical.

A 12AT7 served as RF amplifier and detector, with a 6AF4 serving as audio amplifier. A signal generator was suggested for final calibration, but in the absence of one, TV and FM broadcast stations could be used to figure out the dial positions. Three plug-in coils were used for band switching.

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1922 Boys’ Life Receiver

1922FebBLA hundred years ago this month, wireless column of the February 1922 issue of Boys’ Life showed scouts how to put together the simple radio receiver shown here.  The magazine noted that just a few years prior, there was still little to hear on the airwaves in most of the country.  Near the coast, it would be possible to pick up Morse transmissions to and from ships, and in larger cities, there might be a few signals here and there.

But in most of the country, there had been little to listen to.  But that ways changing, and by 1922, just about anywhere in the country, there were plenty of interesting signals just waiting to be pulled in.  In fact, even in areas without newspapers, the radio could be used to pull in stories straight off the news services, and it was possible to get sports scores long before your neighbors could.  There were even concerts being listened to by hundreds of thousands of people in many states.



TV Relay Tower, 1952

1952FebRadioElecI remember as a kid seeing a structure similar to this one. I don’t remember exactly where, but it was somewhere along the route between Minneapolis and Duluth, with one horn facing south, and the other facing north. I asked my parents what it was, and they said it was for relaying TV signals. It made perfect sense to me, since the two horns looked a lot like TV screens.

I didn’t know what was inside, but now I do. The electronics were on the top floor, the B+ power supply was on the third floor, the filament power supply was on the second floor, and a backup generator was on the ground floor.

This diagram appeared 70 years ago this month in the February 1952 issue of Radio-Electronics.  Click on the image to see the full size version.



1962 Phone Answering Machine

1962FebPSTelephone answering machines didn’t really become a thing until the 1970s, and even then, they were expensive and rare. At first, even their legality was dubious, since The Phone Company was jealous of anyone making any kind of direct electrical connection to the Public Switched Telephone Network.  I didn’t have an answering machine myself until well into the 1980s. Surprisingly, there was a time when the phone just kept ringing when nobody was home, and there was no way to leave a message. Similarly, if your phone rang, the only way to find out who was calling was to pick up the phone and talk to them.

For those who were unwilling to wait for the future, the February 1962 issue of Popular Science showed you how to put together your own answering machine. And since there was no direct connection to the phone line, you didn’t have to worry about provoking the ire of The Phone Company. When the phone rang, an inductive coil placed under the phone would sense it, and it would trigger a solenoid which would physically lift the button on the phone. It would also turn on the tape recorder, which had been left in the “play” position. The tape recorder would then play your outgoing message through the speaker, which was close to the telephone handset. At the end of the outgoing message on the tape, you had painted silver paint on the back of the tape. When this passed through an outboard sensor you had added to the recorder, it would trip a second solenoid, which would flip the switch on the tape recorder from “play” to “record”. The microphone was placed next to the receiver, and it would record for thirty seconds. At the end of this thirty-second piece of tape, there was another section of silver paint, which would reset the whole contraption for the next call.

You would need to prepare the tape in advance for as many calls as you expected to get, including multiple outgoing messages and silver paint sections.

For the outgoing message, you were instructed to tell the caller to leave their 30 second message when they heard the “click.” Presumably, the sound of the tape recorder switching over from playback to record would be sufficiently loud to serve as the cue.

To hear your messages, you would first glance at the tape to see if it had been used. If so, you would need to first remove the tape from the external switch, since running the tape through it unnecessarily would quickly remove the silver paint. You would then plug the tape recorder directly into the wall, rewind, and listen to the tape, which would include both your outgoing messages and the incoming messages.

As you can see from the schematic below, the control circuit used tubes, to switch the relays and solenoids. Since one of the relays was used to turn the tape recorder on and off, the tape recorder itself would need to be a solid state model, since there would not be time for the tubes to warm up.

After it was built, the device had to be adjusted. In particular, there was a sensitivity control for the circuit triggered by the ringer. In order to adjust it, the phone had to be ringing. And, of course, the only way to do that was to call someone else and ask them to call you.

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