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One Transistor Pocket Radio
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One Transistor Pocket Radio Reflex and regenerative circuits are combined in this sensitive and stable radio.
By Alvin Mason The little receiver described here gets around both of these weaknesses. It uses a combination of reflex and regenerative action to cut size and components to a minimum and increase sensitivity to striking proportions. The complete unit measures only 4" x 2 1/2" X 3/4". And it's powerful enough to pull in every local station on the dial with no external antenna at all! Reflex Circuit Because of the "reflex" action of the circuit, a single transistor is made to amplify the signal twice - once at radio frequencies and again, after detection, at audio frequencies (see "How It Works"). To simplify the circuit, a diode is used as a detector, leaving the transistor free to do nothing but amplify. Also acting to increase the circuit's simplicity and stability is the regeneration hookup. The circuit is designed so that the amount of positive feedback or regeneration doesn't control the overall sensitivity as is usually the case with regenerative detectors. What's more, there is no regeneration control or annoying oscillation to contend with.
Parts List
B1-15 volt battery (two Eveready 404E's or . equivalent in parallel)
One transistor does the work of two in this highly efficient circuit. The signal is amplified twice-once at radio frequencies ,and, after detection, at audio frequencies. How It Works One transistor and one diode are employed in a circuit that combines the advantages of both reflex and regenerative action. Because the signal passes through transistor Q1 twice - once as r. f. and once as a.f. - the transistor is properly described as operating in a "reflex" circuit. Adding to the already high efficiency of this circuit is the regeneration furnished by gimmick capacitor C6. In operation, the r.f. signal picked up by antenna coil L1 is tuned by coil-capacitor combination L1-C1 and induced into secondary coil L2. Fed directly into the base of transistor Q1, the r.f. signal is amplified and passed to transformer T1. A portion of the signal from Q1's collector is returned to Q1's base by capacitor C6 to provide additional gain through regeneration , The signal induced in T1's secondary is detected by diode D1, smoothed by capacitor C5, and returned to the base of Q1 through volume-control R4 and coupling capacitor C2. Transistor Q1 again amplifies the signal, this time at audio frequencies. The audio signal from Q1's collector is fed through the primary of T1 to the earphone. Since the remarkable efficiency of this little set doesn't depend on regeneration alone, only a limited amount of regeneration is used. Its stability is evidenced by the fact that, once adjusted, the set is as stable as most non-regenerative detectors. Although a Philco r.f. transformer was used as T1 in the model, this particular transformer is available only from authorized Philco distributors and may prove hard to get. However, T1 is in no way critical - a number of transformers were substituted for the Philco unit, and most of them worked satisfactorily. The Argonne AR-162 (available from Lafayette Radio, 165-08 Liberty Ave., Jamaica 33, N. Y., for $2.95) seems to be a good substitution. A miniature output transformer measuring only 1" x 3/4" x 3/4", the AR-162 has identical center-tapped primary and secondary windings of 500 ohms with a d.c. resistance of 18 ohms. You'll have to remove the transformer's strap and laminations to fit the unit in the small plastic box specified in the parts list. But you'll find that this bit of disassembling proves no problem (see illustration on next page). The windings are light enough to be held in place with a strip of transparent tape. The center-taps are not used. Construction The chassis is a piece of Formica or phenolic board about 4" x 2 1/2" x 1/16". Depending on the size of the components, the chassis should fit into a small plastic box measuring about 4" x 2 1/2" x 3/4". Homemade printed circuitry was used on the model, but standard wiring will do just as well. Most of the component leads are long enough to permit point-to-point wiring, but a transistor socket was used to prevent possible damage to the transistor when soldering. Coil L2 consists of six to nine turns of No. 26 insulated hookup wire wound on the "ground" end of L1 and spaced 1/16" from it. "Gimmick" capacitor C6 is made up of two 1/2" lengths of insulated hookup wire twisted together several times to form a small capacitor. It's a good idea to layout all parts and drill most of the holes in the chassis before starting assembly. Since the wiring is relatively simple, you should be able to take your time and do a good job. As with any construction project, time spent in careful wiring will payoff in the long run.
Parts are mounted on a phenolic board; R2 and C6 are on the reverse side. Operation After ail parts have been mounted and soldered in place, double-check all connections. Now, with the switch off and battery B1 in place, plug in the transistor. Turn on the set and rotate the volume control to full on. Select a station, preferably the strongest one on the dial. Listen for distortion. If necessary, either loosen the coupling in capacitor C6 by untwisting the leads slightly or by snipping off the leads bit by bit until the distortion disappears. Once adjusted, the set should be nearly as stable as the superhet in your living room. And it's a safe bet that in sensitivity and portability this little unit will have few equals.
Coupling transformer T1 must be a miniature unit. If Arqonne Type AR-162 is used, it can be reduced in size by removing the strap and laminations.
Posted December 1, 2022 |
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Designing and constructing a one-transistor
pocket receiver is a challenge to any experimenter. A good many "pocket" receivers
are either too large or too bulky for true "pocket" operation. Or they simply don't
possess enough sensitivity and gain to pull in stations without an external antenna.
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