Category Archives: Radio history

1946 Two Meter Transceiver

Seventy-five years ago, this ham was just getting back on the air after a wartime hiatus, thanks to a two-tube two meter transmitter described in the June 1946 issue of Popular Mechanics. The magazine noted that for the ham licensed prior to Pearl Harbor, as well as the newly licensed, the simple set would provide an efficient way of getting on the air for local contacts while the big rig was still under construction.

The set used a 6N4 to serve as oscillator, with a 7C5 modulating. On receive, the 6N4 became a superregenerative detector, with the 7C5 serving as audio amplifier. Also shown above is a 300 volt power supply, although another supply, such as a vibrator supply in a car, could be used to power the rig. The recommended antenna was a folded diple constructed of 300 ohm twin lead.

1941 RCA BP-10 Portable

Eighty years ago today, the June 9, 1941, issue of Life magazine showed some of RCA’s offerings. The portable was the main feature, and was billed as the perfect Father’s Cay gift. The superhet weighed in at only 4-1/2 pounds, but was a full toned powerful set. It featured a built-in antenna in the lid, and the speaker was said to pay symphony music with amazing tone and volume. The set sold for $20, and if you acted fast, they would toss in the leather case.

The set appears to be model BP-10, a four-tube set powered by a 67.5 volt B battery and 1.5 volt filament battery.

Homemade Cardiac Monitor: 1961

If Junior is looking for a spectacular science project in the field of biomedical engineering, this one is a sure winner.

Sixty years ago, the June 1961 issue of Popular Electronics showed how to construct this simple cardiac monitor. The circuit was simple, amounting to a 4 transistor audio amplifier. The diagram called for 2N279 transistors, which are apparently unobtainium today, but this substitution guide lists the 2N2431 as equivalent, and it is available at a reasonable price.

The input comes from two electrodes, one of which is placed on each arm. In this position, they will pick up the currents from the heart, which are alternating currents of up to 100 kHz. The audio portions will be audible in the headphones, and also displayed on the meter. The magazine notes that you are not hearing the actual sound of the heart, merely the amplified voltage sent to the heart muscle. Other muscles can be monitored by placing the electrodes on either side of the muscle in question.

The electrodes are simply pieces of metal placed on the body. Prior to placing them, the skin needs to be scrubbed to improve the conductivity. This is done by scrubbing with Lava soap
or Ajax cleanser. (However, since Ajax is now billed as “non-scratching”, I’m not sure it would still work.) The article notes that the device is completely safe, even for children. First of all, it runs off only three volts. And the connection to the electrodes goes through a capacitor, so even that voltage has no pathway to the body.

More advanced versions are available today, but the advanced student will almost certainly bring home the blue ribbon in the science fair by building the medical device at home.

Emergency Portable Radio Batteries: 1951

This young woman shown above in Popular Mechanics 70 years ago this month, June 1951, is using her portable radio to pull in some entertainment at the beach. But she also realizes that in a civil defense emergency, the set might have a vital role to play if the bombs started falling.

The problem, of course, is that you need batteries for the radio, and many sets had oddball shapes and sizes, some of which might not be available in the runup to an emergency. Therefore, the magazine gives some pointers on substituting what’s available. Even though the batteries might not fit in the case, with a little thought, it was a relatively easy matter to power the radio for emergency information.

Two Beginner’s Sets: 1951

This young man is pulling in the proverbial strong local station thanks to one of two sets described in Popular Mechanics 70 years ago this month, June 1951. Both sets are billed as low-cost sets for young radio experimenters, and the magazine noted that such designs re not as plentiful as they should be.

The set above is a crystal set using a galena detector. The tuning coil is wound on a rectangular piece of wood under the chassis, and there is an opening on top for a slider.

The father-son team at left have upgraded the set. First of all, instead of a galena detector, this set uses a 1N34 diode. And for added volume, it employs a 3Q5-GT tube as audio amplifier. For the B+, the set uses 9 volts, but the article points out that for added volume, as much as 90 volts can be used.

If you are thinking of recreating one of these designs, you might find ideas for sourcing parts at my crystal set parts page.

1941 Headphone Crystal Set

Eighty years ago this month, as shown in the June 1941 issue of Popular Mechanics, Junior was so pleased with his new crystal set that Dad decided to make a second one for himself. The set was so small that it was built right in to an old set of “Baldwin” style headphones, which had an extra large Bakelite shell and cap to house the set.

Into the bottom of the shell was placed a spiderweb coil, and the the shaft of a small tuning condenser was mounted in a hole drilled in the bottom. A wire with a spring clip was used for the optional but recommended ground connection, with another for any convenient antenna.

Keeping Volume Controls In Stock: 1941

Eighty years ago, the hapless radio serviceman at left was making his customer wait for a relatively simply job to be completed. The one at the right, however, was able to proceed immediately with the job, thanks to having in stock a kit consisting of 18 volume controls, along with various switches and shafts. He didn’t have to wait for the exact replacement. He just located the resistor of the correct value, and then added the shaft himself, tapping it in place with a hammer. If a switch was part of the control, it was an easy matter to add that as well.

The set of 18 controls sold to the dealer for $14.97, with a total list price of $29.95. The kit came from the International Resistance Company of 401 N. Broad Street, Philadelphia, and this ad appeared in the June 1941 issue of Radio Retailing.

1946 Portables

Seventy-five years ago, the summer of 1946 was the first summer after the war, and Americans were ready to enjoy some leisure. The June 1946 issue of Radio Retailing reminded its readers that this meant there would be a big demand for portables, and manufacturers were in the process of delivering the first new sets. The season would last through fall, and dealers were advised to stock up.

There were currently 24 models on the market. Some of these included a record player, with a maximum price of $149.50. Of the 16 straight portables, the average price was $35.43, with the lowest being $17.50.

The magazine noted that only half of the new receivers were from old-name manufacturers. The remainder came from postwar startups.

Converting TVs to Larger Picture Tubes: 1951

Seventy years ago, these Philadelphia television technicians made the cover of the May 1951 issue of Radio Electronics by their ambitious service of converting small screen television to larger tubes. The magazine carried a feature penned by their boss, Larry Oebbecke of Philadelphia Television Service Corp., 19011 W. Cheltenham Ave, Philadelphia, who reported that the job could be profitable “if you know what you are doing.” He reported that his shop had been doing the work for about a year, and averaged 25-35 sets per week.

Some sets were simply not good candidates for conversion, and the author identified many. He stressed the importance of taking good notes when doing the jobs. The first conversion of a particular model might not be profitable, but armed with good notes, subsequent jobs would be much simpler. Also, the jobs required cutting a new front panel, so saving the templates was key to doing the job economically. Normally, the job would take one technician 6-10 hours to complete. In general, 7 inch sets weren’t worth the trouble to convert, since a new set would wind up being cheaper. But larger sets could be upgraded to 14, 16, 19, and 20 inch screens. Older sets with round tubes could get a more modern look with the newer rectangular tubes.

The shop provided a 90 day warranty on the conversion work, and a one year guarantee on the tube itself. The author noted that the guarantee did not extend to parts of the set on which no conversion work had been done.

1941 DeWald Radios

Eighty years ago this month, the May 1941 issue of Radio Service Dealer carried this ad showing the 1941 lineup of radios from DeWald Radio, 170-16 Jamaica Avenue, Jamaica, New York.

The ad features the model 562, an All-American Five with a suggested retail price of $19.95, as well as a portable at the same price point, the model 410.

The model 565, retailing for $24.95, was a portable that also ran off household current. Finally, the model 670 had a retail price of $33.50 and was a six-tube set tuning the broadcast band as well as two shortwave bands, 4.7-10MHz and 11.5-24MHz.

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