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
of Contents]These articles are scanned and OCRed from old editions of the Radio & Television
News magazine. Here is a list of the
Radio & Television News articles
I have already posted. As time permits, I will be glad to scan articles for you. All copyrights
(if any) are hereby acknowledged.
Here is the equivalent report for
1957 Ford radios.
See all available
vintage Radio News articles.
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 service.
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 audio output.
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 in repair.
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.
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
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
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.
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 46).
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.
The push-pull 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 stages.
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
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.
Service technicians 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
PPosted November 19, 2013