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.,
) 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.
See all available
vintage Radio News
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 car.
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.
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.
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
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
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
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.
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