Monthly Archives: February 2023

1948 Metal Detector

The plans for this three-tube metal detector appeared in the British magazine Practical Mechanics 75 years ago this month, February 1948. The general circuit is similar to inexpensive metal detectors sold today, although the modern versions are much smaller as they are transistorized.

According to the author, many coins, rings, and other metallic objects, some of considerable value, were found on the sites of old Roman camps in Dorset.

The general idea of this circuit is quite simple. It consists of an RF oscillator, the coil of which is held near the ground. When that coil comes into the proximity of metallic objects, the change in permeability results in a change of frequency. This type of metal detector can work quite well, but it does require a bit of practice. The detector makes a loud squeal, and the best way to use it is to tune it carefully so that the squeal just barely disappears. If it’s done carefully, the detector can be very sensitive.

Many people have bad experiences with the types of inexpensive metal detectors shown here, some of which look like toys, but that’s because they didn’t take time to carefully adjust them and practice. It’s best to start out indoors, toss some metallic objects on the floor, and then carefully adjust the metal detector and see how it reacts when the coil is brought near the item. After some practice, you can take it outside and look for treasure. The metal detector above, with which Roman coins were found, is actually the equivalent of any of these detectors, and with a bit of practice, one of these will give similar results.

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1943 Emergency Generator

Eighty years ago this month, the February 1943 issue of QST showed this emergency generator. Hams might have been off the air for the duration, but they still had an interest in emergency needs, including WERS operations.

It was powered by a Briggs & Stratton gasoline engine normally rated at 1-3/4 HP, but the accompanying article noted that it was capable of up to 2-1/2 HP maximum as shown here. It was capable of putting out 120 volts thanks to a salvaged Dodge 12-volt generator, rewound, and was capable of putting out over 1400 watts. The field coils needed power, and that was provided by a second six-volt generator also driven by the engine.

The estimated cost of the whole unit was said to be $7.50, although the author admitted that this figure might have been somewhat “under-exaggerated.” The set shown here was the second one constructed, and a third was underway.

One of the gentlemen shown on the cover, although they’re not identified, was apparently Warren Copp, W8ZQ. The article mentioned that he was the father of then-eight-year-old actress Carolyn Lee. We’re not sure exactly why that’s relevant, but like the author of the QST article, we believe that’s the kind of thing our readers would want to know.

Col. Richard G. Rogers, 1943 POW Broadcast

We’ve previously written about the prisoner of war broadcasts of World War 2, and eighty years ago today, the Washington Evening Star of February 16, 1943, carried this report of one such broadcast. Col. Richard G. Rogers was being held prisoner in Formosa, and recorded a message to his family in America. As often happened, listeners in America sent news to his family. There were many such letters sent in these cases, as documented in the book Letters of Compassion, but this is the first case I’ve heard of where one listener sent a phonograph record of the broadcast, undoubtedly recorded on their Recordio.

British Auxiliary Territorial Service, 1943

This young British woman, shown on the cover of the February 1943 issue of Radio Craft is a member of the Auxiliary Territorial Service (ATS), and was tasked with spotting enemy planes. According to the magazine, she and her colleagues shared much of the credit for winning the Battle of Britain.

The young women of the ATS took over important “desk jobs,” freeing men to serve in combat. Their training was intense, and required intelligence and keen perceptive powers. In particular, the work shown here at a radiolocation station required keen senses of sight and hearing, as well as concentration and constant alertness.

One Tube “Saskatchewan Space Charger” Receiver, 1943

Eighty years ago this month, the February 1943 issue of Radio Craft carried this circuit for a simple one-tube regenerative receiver. It had been sent in to the magazine by one Joseph Niwranski of Brooksby, Saskatchewan, and was dubbed the “Saskatchewan Space Charger. The set, which employed a 1A5GT tube, was said to perform splendidly. It reportedly had enough pep to drive a speaker on strong stations.

According to this Find a Grave entry, the author was about 17 when he designed the circuit. He died in 1992 at the age of 67.

1948 Magnetic Recording

The drama student shown above appeared on the cover of the February 1948 issue of Radio News. The young thespian was an early adopter of magnetic recording technology, one of the many civilian technological advances that came from wartime industry. She used a wire recorder from Webster-Chicago Corp. to practice her diction and delivery.

Of course, home sound recording had been possible for some time, thanks to disc records such as the Recordio. But magnetic recording media had the great advantage of being reusable.

The magazine contained a number of features, and it does contain a very complete look at the state of the art in 1948. The two competing formats were wire and tape. Wire held a slight lead in fidelity, but tape was easier to work with, especially when it came to editing and splicing. Interestingly, the magnetic recording tape of the time was actually paper tape with a thin magnetic coating. Typical speed was 8 inches per second.

Among the features of the issue was a construction article for the tape recorder shown here. The project was said to be “well within the capabilities of anyone who has a working knowledge of electronic circuits and who has ordinary mechanical ability.” The mechanical ability was important, because all of the parts had to be made. A metalworking lathe was required, although the article noted that those without one could have the parts custom made at a local shop at low cost. As one of the mechanical diagrams shown at right shows, the construction did require some mechanical skill. Apparently, 75 years ago, that level of skill was ordinary.

The recording/playback and erase heads also had to be homemade, and consisted of a coil wound on a laminated core and placed in a shielded enclosure with just a small opening for the tape. The choice of material for the core was critical, and had to be permalloy. The type of material normally used for transformer cores would not work. The best solution was to buy a particular type of audio transformer, disassemble it, and use the lamination material for winding the heads.

Back Seat Earphones: 1963

Sixty years ago this month, the February 1963 issue of Popular Mechanics carried this project designed to help parents of “youngsters who insist on raucous music and clatter from the car radio throughout a long trip.”

The solution to domestic tranquility was to let them listen to their heart’s content in the back seat, but through earphones. Then, “only plugged ears need hear the racket.”

The magazine showed how to make this small control box, which allowed switching from the car speaker to the headphones. It also included a volume control, in the form of a 20 ohm fader. Installation was easiest if the speaker were mounted on the rear shelf, and the work could easily be done through the trunk. If the only speaker were in the front, the job was only slightly more complicated, and required running two wires under the carpet.

With this modification in place, the rest of the family was guaranteed to be happy.

Amboy Lighting Company, 1923

If you needed radio supplies in the Garden State a hundred years ago, then the place to go was Amboy Lighting Company, 193-195 Smith Street, Perth Amboy, NJ, was the place to go.

According to this ad in the Perth Amboy Evening News, February 9, 1923, the store featured Atwater-Kent receivers, and had just received a shipment of Federal headphones.

1953 Hi-Fi Amplifier

Seventy years ago, this young woman was tasked with providing the high fidelity audio system for some important event, and she carried out the assignment by constructing this high-power low-distortion amplifier from the February 1953 issue of Popular Mechanics.

The amplifier, suitable for use with any high-quality speaker system, put out a respectable twenty watts with two 6L6’s running push-pull, with only 5% intermodulation distortion (IMD). At ten watts, the IMD was well under 1%.

The set boasted seven tubes, including rectifier, and as shown here, it was easily portable.

Nighttime Radio: 1923

A hundred years ago, radio was clearly a nighttime activity, as shown by this hapless radio buff on the cover of Radio News, February 1923. Hams were still mostly playing around with the medium frequencies just above the standard broadcast band. They didn’t know what the D layer of the ionosphere was, or that it was absorbing their signals as soon as the sun came up. What they did know was that their signals got out at night, but could communicate only locally by day.

So they worked the radio by night, and took care of other less essential activities (such as work and sleep) by day. It wasn’t until they started getting pushed up into the “useless” short waves that they realized that those frequencies promised world-wide communications by day or night.

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