Delco's All-Transistor Auto Radio
August 1957 Radio & TV News Article
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.
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
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).
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.
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.
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
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
Delco Radio Schematic <click
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.
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
1996 - 2018
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