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. As shown
in this 1957 issue of Radio & TV News magazine, 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
Dashboard of Cadillac's "Eldorado Brougham" (top) showing the
new receiver. General Motors' special. limited-production five-passenger automobile
is shown below.
Thirteen transistors make this design Landmark a superior instrument for a Limited-production
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.
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.
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.
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
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
Delco Radio Schematic <click to
Fig. 2 - The schematic of Delco's
fully transistorized car radio for the Cadillac "Eldorado Brougham" shows many special
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
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 August 12, 2021(original