In 1957, only a top-of-the-line automobile deserved a radio containing 13 discrete transistors and four crystal diodes. Only buyers of such a top-of-the-line vehicle could afford the luxury offered by an electronic marvel that promised instant-on music with superior sensitivity and selectivity over the vacuum tube models that lesser humans endured. Delco's Model 7268085 was up to the task as it populated the dashboards of Cadillac's Eldorado Brougham. Modern day radios use a single IC (e.g., NXP SAA7706H) for performing all reception, filtering, amplification, and tuning functions, with superior performance compared to the Delco without all the interstage tuning transformers, Rs, Ls, and Cs. Most of the rest of the circuitry in your car radio is for microprocessor control of the user interface and that single RFIC. See the companion article titled, "1957 Auto Radios: Chevrolet," in this edition.|
Delco's All-Transistor Auto Radio
Thirteen transistors make this design Landmark a superior instrument for a Limited-production car.
Dashboard of Cadillac's "Eldorado Brougham" (top) showing the new receiver. General Motors' special. limited-production five-passenger automobile is shown below.
What kind of radio belongs inside an automobile. that incorporates many engineering firsts, in addition to such luxuries and conveniences as interior carpeting, gold-finished drinking cups, and a built-in vanity including a perfume atomizer filled with "Arpege"? The car is the super-deluxe, limited-production "Eldorado Brougham," being produced by the Cadillac division of General Motors. Unable to find an auto radio of current design sufficiently distinctive for use in this "car of the future," GM's Delco Radio division has come up with an auto receiver of the future as impressive, in its own field, as the vehicle in which it makes its debut.
Thirteen transistors and four crystal diodes are used in Model 7268085, shown in Fig. 1. The completely tubeless set, as depicted, uses a separate audio-output subchassis. A superficial check of the unit reveals nothing out of the ordinary: the housing, although handsomely styled, is conventional; features and provisions - including tone control, variable rear-and front-seat speaker selection, push-button station selection combined with automatic search tuning, and a sensitivity control - have been incorporated in top-quality auto radios before this.
Awareness of the radio's distinctiveness comes with an examination of its specifications and of the generous design evident from the schematic of Fig. 2. Beginning with the receiver's input characteristics, Delco engineers rate the unit as having a sensitivity of one to three microvolts. There is always some question as to just what a sensitivity figure of this kind means in terms of AM reception, but the available data further specifies that one microvolt of signal at the antenna will produce one watt of audio power output, which is usually quite adequate for the interior of any car.
In terms of maximum power output, the push-pull transistor stage can deliver up to 10 watts. As far as the transistors themselves are concerned, response up to about 18,000 cps is available. Actual response is limited to less than this by the speaker used and by AM bandwidth considerations, Although it looks like the conventional 6"x9" oval speaker, the transducer used is rated as having response from 60 to 9000 cps - more than sufficient for the response inherent in conventional AM broadcasts. A quality detector circuit preserves whatever fidelity is inherent within AM frequency-response limits. The intermediate frequency chosen for the design is 262 kc.
Fig. 1. Top view of Model 7268085. The separate push-pull audio-output subchassis is to the right of the main chassis. which includes r.f., i.f. detector, and automatic station-seeking circuitry.
Some of the features cited begin to suggest answers to one of the first questions likely to come to mind: "What in the world are they doing with thirteen transistors? How can considerations of good design justify that substantial number when the finest available transistorized portable radios use no more than seven or eight?" Fig. 2 shows that one transistor is used as the r.f. amplifier, another as the mixer, and still another as the local oscillator. To this point already we find a configuration involving three transistors, for superior performance, where conventional circuits use one (converter only) or at most two (r.f. amplifier and converter).
Not content with these measures, the designers, moving over to the i.f. strip, have gone all out for the most selectivity and sensitivity they can get. No less than three i.f. amplifiers are used, making a total of six transistors to this point in the signal chain, where previous all-transistor receivers generally use three.
A diode detector, when shunted across a final i.f. stage in conventional fashion, is a low-impedance device that amps the i.f. stage considerably, thus reducing the gain that is realized from the i.f. strip. In its place, the "Brougham" radio uses another transistor as an infinite-impedance detector. In addition to permitting the third i.f. to operate with no significant loading, this type of circuit provides other advantages. Distortion in diode detectors, for example, is often at the mercy of variations in depth of modulation or of r.f. signal amplitude. Sensitivity to such variables is less in the infinite-impedance detector, making for better audio quality.
Where economy is the overriding factor, the type of circuit just discussed is not considered, not only because it involves additional circuit elements in itself, but because it does not provide a simple method of deriving an a.v.c. or a.g.c. voltage. This type of control, taken for granted nowadays, must either be abandoned or provided for with still more circuit additions. With quality performance as the determining factor, Delco engineers have not hesitated to call for still another transistor, and an added crystal diode, to boot. Part No. 175, the crystal diode, is a separate a.g.c. detector. Associated with it is the eighth transistor used in the set, an a.g.c. amplifier, to afford really tight control over changes in signal level. A diode in the a.g.c. line going to the r.f. amplifier provides some delay to this stage with respect to the a.g.c. signal applied to the i.f. strip, thus permitting optimum operation in the front end of the receiver.
Although it has some interesting particulars, the three-transistor audio portion of the set is not an unusual configuration in general: a single transistor acts as the audio driver, through a transformer, for the push-pull two-transistor output stage. This accounts for eleven of the transistors. Only two remain unaccounted for. These are simply replacements for the tubes one usually finds fulfilling the functions of trigger amplifier and relay control in other receivers using automatic signal-seeking systems.
Delco Radio Schematic <click to enlarge>
Fig. 2. The schematic of Delco's fully transistorized car radio for the
Cadillac "Eldorado Brougham" shows many special features.
A dose look at the audio portion of the set shows some interesting details, included in the interests of maintaining quality performance. Part No. 131, a service adjustment for establishing optimum bias for the output stage, is not an innovation: a similar control may be found in the audio-output stages of other hybrid transistor radios designed for use in vehicles. In this design, it is implemented by Part No. 127, a thermistor. The thermistor is a resistor whose value varies depending on its ambient temperature. As used here, it is a regulator or auxiliary bias adjustment. As the stage heats, its operating characteristics change. The thermistor compensates for these changes, thus maintaining a constant operating point for the 2N278 transistors in the output stage, and assuring continued good performance from this circuit despite temperature changes.
A glance at the interstage coupling transformer (Part No. 200), between the audio driver and the push-pull output stage, reveals a third winding where two would seem to do the job. This added emitter winding to the 2N109 audio preamplifier stage actually provides negative feedback to cancel distortion.
In production right now, enough units of the thirteen-transistor radio have been made for the engineers, as is their wont, to come up with their first design change in the unending search for something just a bit better. As shown, the audio-output circuit operates class AB. Minor component changes in the output circuit during later production have resulted in class A operation.
Of practical interest is the fact that the receiver (class AB version) drains only 6/10ths of an ampere from the auto battery. This means the radio could be run uninterruptedly for several days while the car is not in use without running down the battery. Even at that, more than 50 percent of the current drawn is consumed by the two pilot lamps! If these two are disabled, the radio can run on the storage battery for weeks.
Included as regular equipment in the "Eldorado Brougham," the price of the radio would doubtless be prohibitive if it were available separately. However, that is neither a new nor an unexpected situation with respect to pioneering developments in general. The important thing is that we have our first all-transistor, quality auto receiver. With the barrier broken, this forerunner is not likely to be the last of its kind.
Posted November 13, 2013