Category Archives: Radio history

1961 Emergency Call Box

Sixty years ago, the February 1961 issue of Electronics Illustrated showed this prototype of a highway emergency call box from Hoffman Electronics, dubbed the “safety satellite.”

The unit is self-explanatory. In an emergency, the motorist would push the appropriate button for police, fire, ambulance, or service, which would be dispatched from a central control point.

While the FCC had not yet assigned a frequency, the prototype three-tube VHF transmitterwas operating on 72-76 MHz, using FSK transmission with about 1 watt transmitter power. While the article does not specify, from the photo of the device, it appears that the message is sent by a motor-driven keyer.

It was said to have a line-of-sight range of about 25 miles. Each emergency message would transmit for 1.5 seconds, and would represent about 5 minutes charging time with the silicon solar panels. Six NiCd cells were used. On transmit, one was used to power the filaments, and the other five went to a solid-state power supply to provide the B+ voltage.

The price tag for each transmitter was about $300, with the FM receiver running an additional $250-275. Depending on the complexity of the decoding and display consoles, the monitor station could run a total of $3000.

The system was touted as a bargain, since the cost to wire a highway would be about $3000 per mile, with one transmitter every quarter mile on each side of a freeway. This compared to about $8000 per mile to install emergency telephones. When charging, the batteries would be in parallel.

In addition to emergency highway use, the system was capable of sending telemetry for industrial applications.

1941 Minnesota Radio Stations

As we’ve previously written, on March 29, 1941, most broadcasting stations in the United States changed frequency, as the top of the dial moved from 1500 kHz to 1600. This listing shows all stations in Minnesota at that time. The first column shows their old frequency, and the last column shows the new frequency, many of which still look familiar today.

The listing appeared in the February 1941 issue of Service magazine.

St. Paul, MN, City Radio System, 1951

St. Paul Police Sgt. Hans Peterson.

Shown above, 70 years ago, is Sgt. Hans Peterson of the St. Paul, MN, Police Department, at the console of the city’s mobile radio system. The system was described in detail in a two-part series appearing in the February and March, 1951, issues of FM-TV-Radio Communications magazine, authored by the city’s Commissioner of Public Safety, Robert F. Peterson.

Peterson notes that, as in most other cities, greatly increased demands had been placed upon the police department since the war, due to added services, as well as additional crime and juvenile delinquency. And with the development of long-range aircraft, the city found itself closer to Russia than the cities on the east coast, making civil defense a concern. “Obviously, more manpower is indicated, but in St. Paul our 40-hour week and limited budget make any substantial increase in personnel out of the question.” Therefore, the city turned to technology, in the form of a modern radio system, to increase efficiency of officers. The system also handled traffic for the city of West St. Paul and the Ramsey County Sheriff’s Department.

Engineer Bob Morrison standing beside police cruiser. Selective call light is visible above center of windshield.

The system consisted of equipping 53 police vehicles, as well as fire stations and vehicles, with dual-frequency radios with selective calling capability. All outgoing calls were handled on 159.09 MHz from a console at the Public Safety Building. A leased telephone line led to the transmitter and receiver atop the First National Bank Building. The transmitter room was, understandably, kept locked at all times, and also had a CO2 fire suppression system installed. The door and fire system had monitors that were linked back to the main control point, so that the dispatcher would be aware of any intrusion or fire.

Normally, all units operated on 159.09 MHz. But the mobile units were all equipped to transmit on a secondary frequency of 158.97 MHz, which was continually monitored by the dispatcher. This allowed mobile units to make emergency calls even during longer broadcasts from the dispatcher.

A key element of the system was the selective calling ability. The console shown above was equipped with dozens of switches, one for each mobile unit. One of these could be switched on to selectively call any car. Normally, officers in the squad would be listening to the radio at all times. But previously, if they had to get out of the car to perform their duties, they were out of service for further calls. For example, if an officer were out of his squad car investigating a traffic accident and a robbery occurred just around the corner, there would be no way to alert the officer.

Selective calling provided a solution. When an officer was out of his car, he could still be alerted by a light mounted above the windshield, or perhaps a horn. These were connected the the selective calling system. If the officer was needed, the dispatcher would flip the switch, the light would come on, and the officer would know there was a priority call.

A duplicate console was connected. Shown below, radio operator Art Tweet is shown at this console. The radio operator was responsible for system operation, but in rush hours, he could assist the dispatcher.

Radio Operator Art Tweet

Police Chief Charles J. Tierney.

Fire Chief William H. Mattocks.

Commissioner of Public Safety Robert F. Peterson.

Superintendent of Radio L.A. Ginther.

1946 Personal Receiver

This gentleman has found marital bliss, thanks to a personal radio receiver he put together according to the plans in the February 1946 issue of Popular Science.

As the magazine pointed out, “instead of thinking harsh thoughts about members of your family who frown on your radio listening when they want to read or sleep, try one of these compact personal radios that make the entertainment of the airwaves your private business.” The wife is undoubtedly in that category, but he made the radio his private business by listening with headphones while she slumbers in peace. Presumably, there were harsh thoughts prior to making this set, and it probably saved the marriage.

The circuit is a three-tube regenerative set. One tube serves as a rectifier to supply the B+ voltage in the AC-DC “hot chassis” design. To run the filaments, multiple “curtain burner” line cords, adding up to 600 ohms are used. While the set is designed for headphones, it’s powerful enough to drive a speaker, and the optional hookup for a speaker is shown.

1961 Code Practice Oscillator

Sixty years ago, the February 1961 issue of Electronics Illustrated showed how to put together this code practice oscillator which is simplicity itself. The article notes “that old headache, acoustical feedback can be useful,” and it’s put to work by simply amplifying the output of a carbon button microphone, and placing the microphone next to the speaker.

According to the article, the placement of the microphone is easier done than described. You basically just move it around until the pitch and volume are right. The author reports that he wound up gluing a poker chip to his to get it just right. The circuit has no volume control, but by reducing the battery voltage, the room-filling volume can be reduced.

The jack on the side is to plug in an external microphone, and use the device as a conventional amp. The article cautions to use a long enough lead in order to avoid feedback in this case, since it’s not wanted.

The choice of speaker and PNP transistors were not critical. The author reports success with a 2N34A, 2N255, 2N256, or 2N554. In this application, a heat sink was not necessary, so the semiconductor could be mounted in any convenient configuration.

Stringing the Dial Cord

Even for the electronic genius who is repairing or restoring an old radio, stringing the dial cord is often a vexing problem. It’s a deceptively simple mechanism–you turn the tuning knob, and a string turns the tuning capacitor, and also moves a pointer along the dial. But the devil is in the details–it has to be set up so that a full 180 degree rotation of the variable capacitor results in the dial pointer moving the full length of the dial. Invariably, if you’re not careful, things don’t sync up.

An article in the February-March 1946 issue of National Radio News serves as a good primer on how to do it. The author notes that there are so many variations that even a much longer article wouldn’t cover them all. Fortunately, in most cases, the technician could write to the manufacturer and get a diagram. These days, the dial stringing diagram is often available at the same online sources that carry the schematic diagrams.

If the diagram was not available, the first step was to make a sketch. If the old cord was still in place, the first order of business would be to make an accurate diagram of exactly how it was hooked up.

If the dial cord was already broken or missing, then the first step was to try to figure it out and still draw a diagram. In this case, the diagram would serve two purposes. If the original guess was correct, then it would serve as a guide to do the job. But if it didn’t work, it would document one possibility that didn’t work, thus helping with the process of elimination.

The article cautioned that normal string could not be used for a replacement cord, since it tended to stretch. Instead, the magazine suggested silk or nylon fish cord as one possibility.

Overall, the article serves as a good introduction to the subject, and if you’re faced with this daunting task, I recommend giving it a read before trying to tackle the job.

1941 Four Tube TRF Tuner

Eighty years ago, this second period class somewhere in America was getting ready for its radio geography lesson. While the teacher points out western Africa on the map, the audio expert at the back of the room is preparing to cue in a radio lesson on the subject, which will presumably be piped in through speaker number 1 above the map.

He’s pulling in the program on the four-tube tuner described in the February 1941 issue of Popular Mechanics. It’s a TRF tuner which is selective enough to separate the local stations, but has sufficient bandwidth to provide good reception of musical selections. The set can be run through any audio amplifier. In this case, the school is progressive enough to have a public address system installed, and the students will hear the program through it.

Postwar Radio Retailing: 1946

Shown here on the cover of Radio Retailing 75 years ago this month, February 1946, this couple are taking home their first postwar radio. New civilian radios were coming back on the market after a long absence, and there was probably still a shortage, as evidenced by the crowd clamoring behind them.

The magazine reminded dealers, however, that the time would soon come when customers would stop searching for radios. Instead, they would soon be going from store to store choosing. It warned against complacency. Even though it was currently no real task to sell anything, the dealer’s goal should be to make a customer of every shopper. In particular, customers would appreciate a little extra courtesy and consideration.

Selling a scarce item with the same degree of interest that would be called for in a competitive era would have the result of making friends and getting free advertising for his goodwill.

The day would soon come when the dealer would have to fight for every dollar, and attractive selling methods now would go a long way to bring in those dollars.

1951 Crystal Set

Seventy years ago this month, the February 1951 issue of Boys’ Life magazine showed how to put together this simple “non-crystal crystal set”. Regular readers will recognize it was the venerable “foxhole radio,” which used a razor blade and pencil lead for the detector.

The coil is wound on a flat piece of wood, measuring 4 by 2 inches and a quarter inch thick. Paper clips are used for many of the components, such as the slider and connectors.

For ideas on sourcing some of the parts, see our crystal set parts page.

Scouts Listening to the Radio, 1941

Eighty years ago this month, the February 1941 issue of Radio Retailing showed these Boy Scouts tuning in a program. The magazine reminded dealers that every boy should have a radio in his room, and that this was particularly true for scouts, who needed versatile portable sets for use at home or in the woods.

The Scouts are not identified, other than being from Troop 132 somewhere. The uniforms don’t look quite right for BSA uniforms, and the pennant in the background has the letters “ORIA BC” visible. I’m guessing that it reads “Victoria, BC,” and these are actually Canadian scouts.

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