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Suitcase Phonograph for portable receivers

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Georg Richter
Georg Richter
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25.Apr.16 22:20

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This article was issued in "Radio For The Millions", in 1943 prepared by the editorial staff of Popular Science Monthly


Suitcase Phonograph For Portable Receivers

You can "broadcast" a record directly from the suitcase phonograph to any portable radio

THIS SUMMER there will be thousands of battery-operated portable radios in use on beaches, in parks, and on picnics and excursions everywhere. They will range from the camera-style midget or "personal" radios to the "twenty-pounders," capable of bringing in Europe on the short-wave band.. This battery-operated "wireless" phonograph was designed for use with these portable sets. As the name implies, the unit will transmit recorded sounds to the portable, through the medium of radio waves. The radio in turn reproduces them through the loud-speaker. No connections between the phonograph and the portable are necessary..

This combination of radio and phonograph will enable you to fill the interludes between radio programs that appeal to you with recorded dance music, symphonies, or whatever you wish. The volume control will make it possible to lower the sound of your music so that it will not disturb your neighbors, or turn it up to fill a room with music for dancing.

Underside of the phonograph's Masonite baseboard, showing transmitter, batteries, and turntable motor

Being light and compact (12 by 8 3/4 by 4 3/4 inches) the phonograph can be carried along as easily as a small suitcase. It is entirely self-contained and self-powered. Two small batteries (similar to those used in the personal-type radios — a 1,5-volt flash-light cell and a 67,5-volt "B" battery) supply all the current necessary to operate the transmitter. The power of this transmitter is so small that it will not radiate signals beyond a few feet. It is the only type of transmitter that can be operated without a license under Federal Communications Commission regulations.

Sketch of the set-up of the suitcase "wireless" phonograph

The transmitter uses a 1A7GT converter tube which acts as a combined modulation and radio-frequency amplifier. The electric impulses from the phonograph pick-up are modulated by the first and second grids of the tube. In a 1A7GT, or similar tube, the second grid acts as a plate. The modulated signal is then superimposed on the carrier wave, generated by the second portion of the tube, and transmitted to the receiver, where it is amplified and transformed back into sound.

Wiring diagram

The transmitter is tuned by a 0.00014-mfd. (or 140-mmfd.) tuning condenser and a standard four-prong broadcast coil. To cut down costs, the four-prong socket for the coil and the eight-prong (or octal) socket for the tube are both wafer types, inverted so that they can be mounted on the Masonite baseboard and used instead of the more expensive molded socket.
The suitcase into which the phonograph is fitted can be purchased almost anywhere for less than $1. A spring motor can be picked up secondhand for about $2.50, or it may be ordered direct from a manufacturer. An eight-inch or smaller turntable must be used, otherwise a larger suitcase than is shown in the accompanying illustrations will have to be purchased. A Masonite panel 11¼" by 8¼" is used for mounting the motor, pickup, and transmitter parts. In the set shown here, the panel was left in its natural brown finish, since this blended with the brown-striped covering of the suitcase. The two batteries are strapped to the undersides of the panel with a flexible brass band 3/8-inch wide. This will keep them from knocking about in the bottom of the case, and they may easily be replaced whenever necessary.

To operate the phonograph, place the seven-foot antenna near the back of the portable receiver (where manufacturers usually place the loop antenna). Then turn the transmitter on by means of the switch on the 500,000-ohm volume control, and tune the unit to some free spot on the receiver's dial by rotating the 140 mmfd tuning condenser.. Records can then be played on the phonograph, and will be heard over the set's loudspeaker.

Detail of the suitcase

 

A stage of mounting, before installation of the turntable

Completed phonograph seen from above, with eight-inch turntable. The seven-foot antenna is coiled.

LIST OF PARTS
Converter tube, 1A7GT.
Volume control and S.P.S.T. switch, 1/2 megohm.
Crystal pick-up.
Four-prong broadcast coil, 350 to 565 meters.
Tuning condenser, 140 mmfd.
Octal and four-prong wafer sockets.
Carbon resistor, 10,000 ohms, 1/2 watt.
Mica condenser, 50 mmfd.
Seven-foot antenna.
Small, brown-striped suitcase.
Masonite panel, 11 1/4 by 8 1/4"
Midget "B" battery, 67,5 volts.
Standard flashlight cell, 1,5 volts.
Spring-wound motor with 8" turntable.

  
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