1957 Auto Radios: Chevrolet
April 1957 Radio & TV News Article
to Ford and Chrysler aficionados for not having similar articles
for your classic automobiles, but this article from a 1957 edition
of Radio & TV News only covers Chevrolet radios. Maybe
someday I will acquire editions with other models. Transistors were
fairly recent newcomers on the portable radio scene (on any radio
scene for that matter), so you will please excuse the absence of
them in most radios of the era. In fact, as evidenced by a companion
article in this same edition titled "Delco's
All-Transistor Auto Radio," such newfangled devices like transistors
were reserved for top-of-the-line models like Cadillac's Eldorado
Brougham. A move toward printed circuit boards, rather than the
time-honored point-to-point wiring, was well underway, and push-button
tuning was being sold to the car buying public as an indispensible
safety feature - the "hands-free" feature of yesteryear. Even though
push button tuning with memory (albeit mechanical) for storing station
locations had been around for a long time in tabletop and floor
model console home radios, miniaturization and added parts cost
made them impractical for most automobiles. Take a look at the instructions
for installation into and removal from the dashboard to get a good
sense of how complicated everything has become. In 1957, disconnecting
the car's battery did not automatically require an anti-theft unlocking
code from the manufacturer to turn it on again.
April 1957 Radio & TV News
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio &
Television News, published 1919 - 1959. All copyrights hereby acknowledged.
Here is the
equivalent report for
1957 Ford radios.
1957 Auto Radios: Chevrolet
Transistors and printed wiring spark new design trends as streamlined
tuning gains favor. With the right data, sets are accessible for
Fig. 1. The "Custom Deluxe Wonder-Bar," Model 987577 features
push-button and search tuning, push-pull output, printed
wiring, and a separate sub chassis for power supply and
While Detroit trumpets the design innovations in
the line of chromium cruisers it is turning out this year to fill
the nation's highways, often overlooked is the quiet revolution
going on in the design of the radios on the sleek car dashboards.
Trends are in evidence on at least four different fronts.
Some attempts at finding new ways to put together an automotive
receiver are in conflict with each other. Whichever way these conflicts
are resolved, their very existence underscores the fact of change.
The changes themselves, in any case, are of great interest both
to the car owner and the technician who must keep these receivers
The increased use of printed circuits, strongly
evident, is not exclusive to auto receivers. The growing popularity
of simplified station-selection devices, an occasional feature of
household radios some years ago, is becoming the exclusive domain
of the auto-radio designer - the man or woman behind the wheel can't
always spare the hand needed for conventional manual tuning. Importance
of the transistor - a "natural" in the auto radio, where the expense
of its use can be balanced out against other savings it makes possible
- has been responsible for at least two departures with conventional
design philosophy, both aiming to accomplish the same thing.
Choice of Receiver Types
Of particular interest is the
fact that no line can be drawn between one manufacturer and another
in the matter of auto-radio design, at least in one important sense:
we do not find that one manufacturer favors one type of approach,
with a second manufacturer championing another radio design. Each
maker has put the decision in the hands of the public by offering
a wide choice of radios. For example, the Delco Radio Division of
General Motors Corporation reflects in its 1957 line of radios the
full gamut of innovation mentioned earlier. In fact, all the trends
already noted plus at least one other, are evident in the choice
of models available for Chevrolet cars alone.
All of the Chevrolet receivers fall into the printed-wiring class.
On some models, printed wiring is used throughout. On others, it
is used throughout except for the power supply and audio-output
portions, which appear on a separate subchassis, conventionally
wired. One of the models with the separate subchassis is the "Custom
Deluxe Wonder-Bar" set, Model 987577, shown in Fig. 1.
Model 987575 (upper) uses low-plate-potential tubes and
transistor output. Model 987573 (lower) is a standard tube-vibrator
Of special interest because it is the top model offered for
the regular line of Chevrolet automobiles, Model 987577 sheds some
interesting light on the present unsettled situation in design.
With transistors making so much of the news, this top-line receiver
incorporating all the deluxe features being offered this year is
a conventional tube radio, as shown in Fig. 2. Until some trend
definitely manifests itself as far as transistors are concerned,
there appears to be a tendency to hedge with conventional receivers.
Schematically, Model 987577 begins with the familiar lineup
of r.f, amplifier, converter, i.f. amplifier, and detector-1st audio
stages, all powered by a 12-volt vibrator supply, with gas-filled
rectifier. Of special interest is the audio-output stage, in line
with a generally increased awareness concerning audio fidelity.
A push-pull output stage delivers up to 7.5 watts, which is quite
a bit of power within the relatively small enclosed space of an
automobile interior. The 6" by 9" oval speaker uses a high-energy
magnet. While current audio practice generally makes use of a tube
for a phase-inverter to feed a push-pull stage, note the transformer
that fulfills this function in Fig. 2.
The trigger circuit,
in the lower left-hand corner of the schematic, operates a signal-seeking
tuner mechanism, which it controls through a pair of solenoids (parts
106 and 107) and a relay (part 103). One of the solenoids returns
the tuning cores to the low end of the band; the other is used to
re-cock the power spring. Accuracy of the automatic tuning provided
with this mechanism is reported as better than ±1 kc. Although the
signal-seeking arrangement is electronically similar to that used
in earlier versions, mechanical improvement has been achieved. Also,
this tuner operates in conjunction with a mechanical push-button
station selector, to provide the user with an unusual degree of
flexibility in choosing his program material, whether he has a specific
station in mind or wants to shop around the dial at random.
Fig. 2. Delco's "Custom Deluxe Wonder-Bar," Model 987577
(schematic), is the top receiver for regular 1957 Chevrolets.
Although it provides push-pull audio output, search tuning,
and push-button selection, it uses a conventional tube-vibrator
The "Custom Deluxe" receiver, Model 987575,
provides push-button and manual tuning in a hybrid receiver that
uses a single-unit chassis, featuring printed wiring. This increasingly
familiar type of circuit, consisting of five tubes using low plate
voltages and a single transistor, operates directly from the 12-volt
auto battery without need of a vibrator or separate power supply.
A prototype of this family of hybrid sets was described in an earlier
issue of this magazine (see "No Vibrator in New Auto Sets," September,
1956, page 61, and "Low Plate-Potential Tubes," January, 1957, page
Generally speaking, Model 987575 follows the earlier
hybrid prototype, with one change in tube type and another in the
transistor type, although there are no changes in function. The
r.f. and i.f. amplifiers are 12AF6's in this version, instead of
12AC6 as used in the other receiver. The converter is a 12AD6, and
the detector-1st audio is a 12F8. To this point, the tube line-up
differs from that in a conventional receiver only in that plate
and screen voltages for these tubes are in the range of 10-12 volts.
Following the 1st audio amplifier is a specially designed 12K5 audio
driver. This serves to build up signal to provide sufficient drive
for the input to the transistor. The output transistor is a 2N278
power unit. The 2N278, which is rated as a 14-watt dissipation unit
at the usual 10 percent distortion figure, easily delivers the 6
watts of power output demanded of this stage. To maintain temperature
control, the transistor is mounted directly to a cooling fin. This
output stage is the only one which doesn't use printed wiring.
Although the most elaborate receiver for the regular Chevrolet line
is of conventional circuit design, the situation has been reversed
with respect to the set used in the Chevrolet "Corvette." Here the
same deluxe facilities - combined manual, signal-seeking, and push-button
tuning, plus push-pull audio output - as provided in the "Custom
Deluxe Wonder-Bar" radio have been included, but the design involves
five completely conventional tube types, one rectifier (but no vibrator),
and four transistors in an unusual configuration that parts company
with the hybrid set using 12-volt tubes. Two of the four transistors
in this receiver Model 3725156 comprise the push-pull audio output
stage. There is no startling departure, in audio output, from what
may be found in any number of 7-transistor portables now on the
market, except for the above-average power output obtained. The
two remaining transistors, however, are used as the heart of an
unusual power supply.
Oscillator as Power Source
Fig. 3. Two power transistors are operated as a blocking
oscillator. Their output is stepped up through the transformer
and rectified to provide "B+."
The transistor power supply,
which eliminates the need for the vibrator, is shown in Fig. 3.
Input to this pair of 2N174 or 2N290 power transistors is the 12
volts supplied by the car battery. The transistors are in push-pull
to form a blocking oscillator. In addition to the two windings,
conventional to blocking-oscillator circuits, there is a step-up
winding that feeds increased-voltage oscillator output to the plates
of a full-wave rectifier. The filtered output makes available approximately
200 volts of "B+" for plates and screens of the conventional tubes.
Readers may be reminded of the somewhat related r.f. high-voltage
power supplies in some early TV receivers.
audio-output transistors are the same type used in the Model 987575
receiver for the regular Chevrolet cars. In their class AB push-pull
arrangement they provide approximately 11 watts of power output.
The chassis consists of two sections: the r.f. portion uses printed
wiring; the audio-power chassis section contains the four transistors
and the 12X4 rectifier. The latter chassis section is made of heavy
aluminum, and has the transistors mounted to it on mica insulators,
In this way, heat generated by the transistors is conducted to the
chassis through the mica to keep the transistors from exceeding
their maximum junction temperature.
Rounding out the Chevrolet
line are two more conventionally designed units consisting of vibrator,
rectifier, and five tubes (Model 987573) or six tubes (Model 987693).
The five-tube set, known as the "Standard," is of straightforward
design with conventional manual tuning. The second set, the "Custom
Deluxe," adds an extra tube for push-pull audio output and provides
for push-button station selection. Also, like the 987577, it uses
two separately mounted subchasses a printed-wiring r.f. unit and
a conventionally wired section for the audio-output and power-supply
Car owners prone to reminiscence have been known to recall, with
some amusement, the era when the still-novel auto radio was an intruder
to the inside of the automobile, and looked it. The main portion
of the radio would mount under the dashboard somewhere, in a separate,
easily seen (and easily reached) container. The tuning head, often
at the end of a separate extension cable, might be found on the
steering wheel, roughly in the position that was occupied years
later by the hand shift. While such arrangements scarcely made for
streamlined appearance, they did provide the advantage of good access.
In a matter of minutes, the entire radio could be taken out of the
car and moved to a more convenient place for repair. In those days,
the service Charge for repairing a defective auto radio was no greater
than that for performing similar labor on a house receiver.
Fig. 4. The strictly physical dismantling procedure of the
average auto radio, prior to conventional troubleshooting.
is often more time-consuming than the electronic testing
and repair that follows. These explicit mounting details
for the line of radios used in all Chevrolet 1957 models.
except the "Corvette," should be valuable in saving servicing
Nowadays, radios must be built-in, giving the appearance of
being part of the car. Accordingly, getting them out for service
amounts to dismantling part of the car. On Chevrolet automobiles,
as on many others, the best way to get to the radio is by way of
the glove compartment. Removing this compartment provides clear
access for one hand to one portion of the receiver, considerably
simplifying the matter.
For special considerations pertaining
only to the regular line of Chevrolet cars, Fig. 4 will be found
most useful. Its exploded views, called-out details, and pinpointing
of differences for the choice of receivers available can save much
time. Once the receivers are out of the car, troubleshooting routine
The speaker may be removed by loosening four
nuts, not shown, located at each corner under the speaker grille.
This permits the grille to be lifted off which, in turn, reveals
the heads of four other screws, available from the top of the dashboard,
that hold the speaker directly in place.
report that, difficult though the removal procedure would appear
to be, trouble ends with the first effort. After one receiver has
been removed from a given automobile, subsequent removals from the
same make and year of auto present surprisingly few problems.