Category Archives: Radio history

12 Volt Power in 6 Volt Car, 1952

1947MarQSTFinding the right power supply for a particular piece of equipment has always been an issue, as shown by this Hints and Kinks item from the March 1952 issue of QST.  There was a lot of surplus gear on the market, much of which ran on 12 volts. Most vehicles, on the other hand, had 6 volt systems. Kermit Karns, W0MYH, of Kansas City, had a 1948 Chevrolet with a 6 volt power system and submitted his solution for running 12 volt equipment in his car.

He simply added another generator and another battery to the car, essentially giving him two independent 6 volt systems, and one 12 Volt system for his radio gear. An additional 6 volt generator, identical to the one that came with the car, was mounted under the hood, and the second battery was stowed away under the driver’s seat. One of the two regulators in the diagram was for a positive ground vehicle, and the other was for a negative ground system.

In addition to being able to run his radio equipment in the vehicle, the added advantage was that the car now had two electrical systems. Karns noted that one was used primarily for the starter with the other loads attached to the second system.



1922 AM Transmitter

1922MarPM1A hundred years ago this month, the March 1922 issue of Popular Mechanics carried the full construction details for this AM transmitter for the radio amateur. Heretofore, constructing such a transmitter was an expensive proposition, since it invariably required a motor-generator to provide the high voltage. This transmitter, however, used a “new type of transformer” to supply the 500 volts of B+ from household current, “as simply as screwing in a lighting bulb or plugging in an electric iron.” Two rectifier tubes could be of “any of the various types on the market,” and the transmitter itself used three UV-202 tubes.

The price of all of the parts was said to be about $125. According to this inflation calculator, that works out to over $2000 in 2022 dollars, so this was not the project for the impecunious.

The article noted that the exact range would, of course, vary with local conditions. But the author reported that music and voice had been transmitted from 100 to 200 miles, and under especially good conditions, an extreme range of 500 miles had been recorded.

The author, whose later call sign was W9DCX, went on to become the magazine’s radio editor. He died in 1955, as noted in his obituary in the magazine.

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1947 RCA-Victor 65U Radio-Phono

1947Mar10LifeThis ad appeared 75 years ago today in the March 10, 1947, issue of Life Magazine. It’s for RCA Victor’s model 65U radio-phonograph. It’s a typical “All American Five” broadcast radio with automatic record changer. It has a large elliptical speaker, and it’s a nice sounding radio.

I actually own one of these that I got back into working order a few years ago. The automatic record changer (said to be capable of 12 ten-inch records or 10 twelve-inch records) stopped working a long time ago, but the phonograph still plays manually. The original cartridge was no good, but I substituted a more modern ceramic cartridge, held in place with a piece of foam rubber and rubber bands. The lid can be closed completely while records are playing, a relatively uncommon feature of tabletop sets of that age.



1962 Four Transistor “Half Pocket” Portable

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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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