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
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.
Here is the equivalent report for
See all available
vintage Radio News
1957 Auto Radios: Chevrolet
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.
Transistors and printed wiring spark new design trends as streamlined
tuning gains favor. With the right data, sets are accessible for service.
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.
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.
Model 987575 (upper) uses low-plate-potential tubes and transistor
output. Model 987573 (lower) is a standard tube-vibrator unit.
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.
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.
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 circuit.
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
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
Fig. 3. Two power transistors are operated as a blocking oscillator.
Their output is stepped up through the transformer and rectified
to provide "B+."
Oscillator as Power Source
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
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.
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 time.
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.
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 becomes familiar.
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 few problems.