Transistor Topics - Heathkit TCR-1, MOBIDIC
April 1960 Popular Electronics
For many years, Popular Electronics had a monthly column titled "Transistor Topics" that reported on news in the world of those newfangled semiconductors. To wit, this article from the April 1960 edition begins, "Each month, more and more transistorized consumer products are developed as replacements for vacuum-tube designs." The Heathkit TCR-1 clock radio is featured for its six-transistor superheterodyne AM receiver circuit. A mechanical clock is still used since other than using Nixie tubes, digital displays were not commercially available. The MOBIDIC "super" computer is also covered for its total transistorization. At about 4 feet wide and 6 feet tall, it is hard to believe that the "MOB" portion of the acronym stands for "mobile."
April 1960 Popular Electronics
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By Lou Garner
Each month, more and more transistorized consumer products are developed as replacements for vacuum-tube designs. In view of this trend, a radio kit recently introduced by the Heath Company (Benton Harbor, Mich.) takes on special significance. Dubbed "Your Cue," Heath's TCR-1 is frankly designed as a transistorized table-model clock radio. If desired, the set can be converted into an attractive "portable" by fitting it in an optional leather carrying case.
Battery-operated, and hence completely shock-proof, the TCR-1 is as at home in the kitchen or bathroom as it is in the living room, den, "rec" room , bedroom, or office. With no dangling cord to plug in, the set can be placed anywhere rather than near an electrical outlet.
Basically a six-transistor superhet using p-n-p types in the common-emitter arrangement, the TCR-1 boasts a number of circuit innovations as well as several interesting operating features. It tunes the AM broadcast band from 535 to 1620 kc.·and has a standard 455-kc. i.f. Its audio amplifier can deliver a maximum of 300 mw. to its 4" x 6" loudspeaker, more than ample for good room volume. Radio battery life is from 100 to 500 hours, depending on whether standard penlight or mercury cells are used. A separate battery powers the clock; operating life is approximately four months, using a mercury cell.
Referring to the block diagram of Fig. 1, the set features a high-gain built-in loop antenna and a tuned r.f. amplifier ahead of its converter stage. This is in contrast to the more common practice of having the antenna coupled directly to the converter, and it insures better image rejection and overall sensitivity.
The trend to transistors has now reached the clock radio with this all-transistor circuit by Heath. One battery powers the radio, another powers the clock.
Although a single i.f. stage is employed, adequate selectivity is assured by the use of a double-tuned i.f. transformer. The d.c. component of the detected i.f. signal is coupled back to the r.f. amplifier for automatic gain control (a.g.c.); the r.f. stage amplifies the a.g.c. voltage which is then applied to the i.f. stage.
The audio signal from the detector is applied through the receiver's volume control to an audio amplifier which, in turn, drives a push-pull output stage. The power amplifier is operated Class B to insure minimum battery drain at low volume levels. A special feedback network permits the entire audio amplifier to function as a tone generator, supplying a pleasant "alarm" signal.
The TCR-1's clock-actuated switch allows the user to choose any of several modes of operation, depending on individual preferences. The set can be used as a conventional radio whenever desired. At night, a special "Lull-to-Sleep" control will turn it off automatically after a playing time of up to 60 minutes. The clock can be set to turn the receiver on automatically, supplying either an alarm tone or a radio program. Used in the bedroom, the TCR-1 permits the listener to go to sleep with music and waken to either music or an alarm. In addition, an earphone jack is provided for personal listening.
From the builder's viewpoint, the TCR-1 is fairly easy to assemble. A conventional chassis and point-to-point wiring techniques are used. The instruction manual furnished is clear and well illustrated with pictorial diagrams.
"MOBIDIC." In the field of computer design, too, the transistor has virtually supplanted the vacuum tube. Few - if any - computers in current manufacture or in the design stage use tubes. Most use transistors, diodes, and other solid-state amplifying and control devices.
Transistorized computer console, the "MOBIDIC," is already under production at Sylvania for the U. S. Army Signal Corps.
One of the most versatile of military computers is Sylvania's transistorized "MOBIDIC" (Mobile Digital. Computer - pronounced, naturally, "Moby Dick." This computer is used extensively by the army for routine business calculations as well as for such battlefield work as logistics, combat surveillance, tactical operations, scientific and analytic computation, map compilation, and determining artillery target assignment.
Readers' Circuits. The circuits in Fig. 2 were submitted by readers from opposite ends of the continent. John Gottcent of 173 Warwick St., Brooklyn 7, N. Y., sent in the circuit given in Fig. 2(A), while the one shown in Fig. 2(B) is the work .of Larry Gorney, K6EBX, of 1536 E. Ave., Q-11, Palindale, Calif.
At first glance, the two circuits appear similar. Both employ a diode and two transistors; both are designed for operation on a 3-volt battery, both tune the AM broadcast band; both use modified direct-coupling between stages; both require an external antenna for optimum performance; both employ standard magnetic earphones and, finally, both can be assembled using standard commercial components. But with all these similarities, the two circuits are nonetheless very different in operation.
Referring, first, to Fig. 2 (A), we see that John's receiver consists of a conventional LC tuned circuit followed by a two-stage RC-coupled audio amplifier which uses p-n-p transistors in the common-emitter arrangement. Coil L1 is a standard ferrite loop-stick antenna coil (such as Lafayette No. MS-11), C1 a common 365-μμf. tuning capacitor, and R1 a familiar 1-megohm volume control with s.p.s.t. switch (S1). Capacitors C2 and C3 are 0.5-μf. and 0.01-μf. units, respectively; 200-volt paper tubulars may be used. Any of several diodes can be employed - the popular 1N34 or, if you prefer, a 1N48, 1N68, or CK705. Transistors Q1 and Q2 are both G.E. Type 2N107's. The 3-volt battery, B1, is made up of a pair of penlight cells connected in series.
In operation, signals picked up by the antenna are selected by tuned circuit L1-C1 and applied to the diode detector. From here, the detected audio signal is amplified by Q1 and coupled through R1 and C2 to Q2. Additional amplification is supplied by the second stage, with Q2's output driving the pair of magnetic earphones. Capacitor C3 serves as a high-frequency bypass across the 'phones.
Note that no effort has been made to supply a separate source of bias current for Q1's collector. This current is obtained through Q2's base-collector resistance; hence, Q2's leakage resistance will play an important part in overall circuit operation. In some cases, it may be necessary to interchange Q1 and Q2 (identical types are used) or to try different p-n-p types for Q2 until best performance is attained.
Fig. 2. Diodes serve as detectors in the transistorized receiver circuits submitted by readers John Gottcent and Larry Gorney. Note use of regeneration in Larry's circuit (B.)
Larry's circuit, in contrast, takes advantage of the complementary characteristics of p-n-p and n-p-n types of transistors to achieve direct-coupling between stages. Referring to Fig. 2(B), Q1 is a popular p-n-p type (Raytheon's CK722) , while Q2 is Sylvania's familiar n-p-n type, the 2N35.
As in the first circuit, L1 is a ferrite loop-stick antenna coil and C1 a standard 365- μμf. tuning capacitor. L2 is an "extra" winding added to L1 and consists of 10 to 15 turns of 22- to 28-gauge enamel wire wound on L1's form about 1/4" from the coil itself. Larry used a type 1N34A diode, but other general-purpose units will work as well. Capacitor C2 is a 0.05-μf. paper or ceramic capacitor; working voltage is not critical. Resistor R1 is a 220,000-ohm, 1/2-watt carbon resistor, and R2 a common 500,000-ohm potentiometer. Any s.p.s.t. switch can be used for S1-toggle, slide, rotary, or push-button. As before, the 3-volt battery, B1, may be made up of a pair of penlight cells connected in series.
In operation, r.f. signals picked up by the antenna system are selected by tuned circuit L1-C1, detected by the diode, and coupled through C2 to common-emitter amplifier Q1. Enough r.f. "spills" through the detector and Q1 to permit feedback winding L2 to provide regeneration, increasing circuit gain and improving selectivity. Transistor Q1's base bias is furnished through R1. The amplified audio signal is applied through regeneration-volume control R2 to Q2 which, like Q1, is connected as a common-emitter amplifier. Finally, Q2's output drives the magnetic earphones.
Either of these circuits can be assembled on a conventional chassis or on a Bakelite or fiber mounting board. Follow good wiring practice, keeping the signal leads short and direct and observing all d.c. polarities. Don't install batteries or turn the circuit on until you have double-checked all connections for wiring errors and accidental shorts.
Both circuits give optimum performance when used with moderate-length (25' to 100') external antennas; a ground connection is optional. In strong signal areas, a shorter antenna may give satisfactory results. Moderate-impedance (500- to 2000- ohm) magnetic earphones should be used. In the second circuit-Fig. 2(B) - try interchanging L2's connections, as in any regenerative circuit, using the connection which gives maximum gain.
Product News. With the Citizens Band "booming," Radio Manufacturing Engineers, Inc. (RME), Washington, Ill., has introduced a fully transistorized, hand-held transceiver for use in the Class D Citizens Band. This compact instrument, about the size of the familiar "Walkie-Talkie," incorporates seven transistors and one diode.
Webster Electric Company (1900 Clark St., Racine, Wis.) , manufacturer of "Tele-talk" intercoms and "Ekotape" recorders, has introduced a line of transistorized d.c. to d.c. power converters. A typical unit, Type 2D12, has an input rating of 12.6 volts at 3 amp. and can supply 250 volts d.c. at up to 100 ma.
Lafayette Radio (165-08 Liberty Ave., Jamaica 33, N. Y.) has issued the second edition of its popular Semi-Conductor Directory. This useful publication, supplied free on request, has been expanded to 36 pages and lists the latest in diodes, rectifiers, and transistors.
That's it for now, fellows - we'll be back next month with more news.