June/July 1940 National Radio News
of Contents] These articles are scanned and OCRed from old editions of the
National Radio News magazine. Here is a list of the
National Radio News articles I have already posted. All copyrights are hereby acknowledged.
The old pushbutton radio tuners
were an ingenuous bit of electromechanical wizardry. For those too young to have experienced
them, operation was simple - turn the radio tuning knob to your broadcast station, pull
out the lever/button, and then push it all the way back in. Done. The next time you pushed
that button, the mechanism would slew the tuning dial to that position, taking the tuning
elements (usually just a variable capacitor) with it. For most modern electronic radios,
you program the station button by pushing and holding it for a few seconds until a beep
is heard. My father never quite got the hang of tuning the pushbutton radio in his old
Rambler (vacuum tubes) or even his 1972 Chevy pickup (transistorized, but with mechanical
tuner). He was never an early - or late for that matter - adopter of new technology,
so it was not surprising. I am surprised, though, at the number of times I have had to
show a Millennial type how to program his/her late model car radio.
My 1941 Crosley 03CB console radio has two banks of
pushbuttons, in case you want to see some real-world examples.
Make Extra Profits Changing Push-Button Radios to New Frequencies
By J. A. Dowie, N. R. I. Chief Instructor
Some time within the next 12 months, approximately 90% of the broadcast stations in
this country are scheduled to change their frequencies to the new values set forth in
the Havana Treaty. This means that everyone of the approximately eight million push-button-tuned
radio receivers must be readjusted. It will be harvest time for radio servicemen, so
be prepared! Study carefully this timely article which tells how you, as an N. R. I.
student or graduate, can get your share of this profitable business.
"How do you do, Madam. I am not the census taker, but there is a question I would
like to ask you. Does your radio receiver have push-button tuning? It does - well that
means some of the push-buttons will have to be reset when our local radio stations change
to new frequencies. Over eight million sets like yours will need this readjustment, for
stations all over the country will change frequencies at the same time. Servicemen can't
possibly take care of all these sets at one time, but by lining up the work in advance
and scheduling jobs carefully, I can promise to have your receiver readjusted within
twenty-four hours of the change-over time. By concentrating my work in this one vicinity,
I can go from one set to another with a minimum of time wasted in travel and keep the
cost of the work down to a very low figure.
"How much will it cost - well, it won't be more than $3 regardless of what make of
receiver you have, and may be a lot less. Let me take a look at your set and I'll give
you a definite figure. Here it is - and there's the push-button mechanism on top of the
chassis. It is one of the simpler types, so the charge for readjusting it will be only
$1.50. Is that all right? Fine - I'll make out a job ticket now, and phone you when the
change-over is being made, so we can set a mutually convenient time for the work."
Fig. 1 - Table giving changes in channel assignments as set forth
in the Havana Treaty. A broadcast station now assigned to a channel in Col. 1 will be
changed to the channel on the same horizontal line in Col. 2. All figures represent kilocycles.
Conversations like this will be heard all over the country this summer, as radio servicemen
prepare in advance for the biggest rush of work in the history of radio.
The forthcoming frequency reallocation comes as a result of the recent ratification
by Mexico of the Havana Treaty. This treaty was drafted in Havana, Cuba, two years ago
by representatives of Canada, Cuba, Mexico and the United States and is formally known
a the North American Broadcasting Agreement. The chief purpose of the treaty is to reduce
interference between stations, particularly the interference created in this country
by the high-powered Mexican border stations. Under the treaty, the border stations such
as 180,000-watt XERA and 50,000-watt XENT lose their present high-power assignments on
The exact date at which the frequency shift will take place has not been set by the
Federal Communications Commission at the time of going to press with this issue, but
ample notice will undoubtedly be given the public in advance by radio stations and newspapers.
About 90% of the radio stations in the United States will change frequency, so it is
extremely unlikely that any present setting of four or more push-buttons on a receiver
will remain correct after the change-over.
The accompanying table indicates the general trend of the frequency changes. In individual
cases, changes not in accordance with the tabulated values may be made to avoid interference
on adjacent channels or for other reasons. Note that present frequency assignments (Column
1 in. Figure 1) will remain the same after the change-over for all channels between 550
kc. and 720 kc. inclusive. All other stations will, in most cases,. have their frequencies
increased. No stations will; be dropped.
Naturally, if you are to get your share of this profitable business, you must know
how the various types of push-button systems work and must know how to set them up.
Types of Automatic Tuning Systems
Although manufacturers have used many different schemes for providing automatic tuning,
we can divide these into three groups according to the operating principle employed,
1. Mechanical Automatic Tuning Systems. By pressing a button or rotating
a telephone-type-dial, the listener himself provides the force required to rotate the
gang tuning condenser to the setting for a desired station. This is a purely mechanical
action, with no electrical switching whatsoever; tuning is essentially instantaneous.
2. Electrical Automatic Tuning Systems. Pressing a button switches an entirely
new set of condensers, preadjusted to a particular station, into the tuning circuit of
the receiver in place of the gang tuning condenser. The action here is entirely electrical,
hence tuning is instantaneous.
3. Electro-Mechanical Automatic Tuning Systems. Pressing a button closes
the circuit to a small electric motor, which then rotates the gang tuning condenser to
a desired station. Electrical switching here causes a mechanical force to be applied
to the gang tuning condenser. A certain amount of time is required, once a button is
pressed, for the motor to complete the tuning process.
In all three systems, the initial adjustments which insure accurate automatic tuning
to desired stations have been made by the radio dealer at the time of the installation.
Printed tabs having the call letters of the desired stations are attached to the push-buttons
themselves or to the escutcheon surrounding the buttons, to identify the station selected
by each button.
Mechanical Automatic Tuning Systems
Courtesy Philco Radio & Television Corp.
Fig. 2 - Three views of an automatic tuning system used on a number
of Philco receivers which also have A.F.C. The numbered parts on these diagrams are all
identified in the Philco service bulletin on automatic tuning.
Fig. 3 - Operating principles of direct push types of mechanical automatic
tuning systems employing a cam with either a lever and roller or a plunger. Pressure
on the button in the direction indicated by the heavy arrow serves to rotate the gang
tuning condenser (geared to the cam shaft) to the correct setting for the station assigned
to that button.
Mechanical automatic tuning systems may be divided into two general groups according
to the manner in which they are operated by the listener:
1. Rotary or telephone-dial types, in which the listener himself provides the rotary
motion which turns the tuning mechanism to the correct setting for a desired station.
Automatic stops prevent him from moving beyond the correct setting.
2. Direct push types, in which the listener applies a direct push or force to a button
or lever. Either a gear, cam or lever arrangement is used to convert this force into
the rotary motion required to turn the tuning condenser to the correct setting for a
Philco Automatic Tuning Dial. The general appearance of the Philco telephone
type automatic tuning dial with cover plate removed is shown in Fig. 2A. Constructional
details of this mechanism can be seen by studying the front-view diagram in Fig. 2B and
the cross-section view in Fig. 2C.
In setting up this Philco automatic dial tuning system, a station is first tuned in
the conventional manner, with the a.f.c. or magnetic tuning control in its "out" position.
The key (item 15) at the bottom of the dial is then adjusted by inserting a screwdriver
in its slot, pressing the key in slightly so it is free to rotate, then turning the key
until a click is heard. The receiver is now tuned for maximum output by turning the key
back and forth slightly. This procedure is repeated for each other station selected.
Direct Push Types of Mechanical Automatic Tuning Systems.
A number of different mechanisms are being used to convert an ordinary direct push
on a button into rotation of a tuning condenser to the correct setting for a desired
station; let us look over a few of them.
In receivers employing mechanical automatic tuning units, there will be one complete
set of parts like those in Fig. 3, 4 or 5 for each station which is to be tuned automatically.
Pressure on the station-selecting button will cause the tuning condenser shaft to rotate.
The cams for the different stations are mounted side by side on a cam shaft which is
geared to the tuning condenser shaft, the cams being separated by spacing washers and
held in position by friction. A mechanical locking device is provided for locking each
earn rigidly in position once it is adjusted for a station.
In some systems a straight plunger, with or without a roller, is used in place of
a lever arm.
In another system of the direct-push type, illustrated in Fig. 4A, the earn is somewhat
egg-shaped and the roller is replaced by a U-shaped or forked metal piece. Pressing the
button makes the forked plunger take the position shown in Fig. 4B, holding the cam in
a definite position.
A finger and rocker mechanism which provides mechanical tuning in still another manner
is illustrated in Figs. 5A and 5B, and a sketch of this unit is shown in Fig. 5C. For
each station there is a plunger (flat metal strip) sliding freely through two slots in
opposite sides of a metal frame. At one end of this plunger is the push-button; clamped
to one face of the plunger is a metal "finger" which can be set at any desired angle
to the plunger and held in position by a locking screw and clamp arrangement (omitted
from Figs. 5A and 5B to simplify the diagrams, but shown in Fig. 5C). Pressing in a button
makes the rocker rotate to the same angle as the finger; on the rocker is a gear segment
which meshes with a gear on the tuning condenser shaft and thus provides the correct
tuning condenser setting for the station assigned to that button.
Electrical Automatic Tuning Systems
Fig. 4 - Egg-shaped cam and plunger mechanism used in the mechanical
automatic tuning systems of some Philco receivers.
Fig. 5 - The diagrams at A and B show one of the finger and rocker
units used in the mechanical automatic tuning systems of some Crosley receivers (including
the Crosley Safety-Tune auto radio), while the sketch at C shows the complete tuning
unit with a gear drive to a gang tuning condenser. Tightening the screw on the plunger
locks the finger rigidly in position. To set up a button for a station, this screw is
loosened so the finger can rotate, the button is pushed all the way in, the station is
tuned in manually, and the screw is tightened to lock the finger at the correct angle.
A spring returns the button to its normal position when pressure is released, leaving
the tuning condenser at the correct setting for the desired station.
Fig. 6A - Sprague electrical automatic tuning unit with built-in trimmer
condensers. The switch blades in the foreground are normally held apart by the strip
of insulating material mounted on each plunger; when a button is pressed, the metal inset
moves down between the blades, shorting them and closing the circuit to one set of trimmer
Courtesy Sprague Products Co.
Fig. 6B - Bottom view of the Sprague unit, showing the trimmer condensers
and their adjusting screws, Each push-button controls one upper and one lower trimmer
on this gang assembly. One terminal of each trimmer is grounded to the frame of the unit,
and this frame is in turn grounded to the chassis.
ig. 7 - RCA electro-mechanical automatic tuning system. Shaft coupling
units are designated as BB.
Instead of rotating the tuning condenser when a new station is desired, electric automatic
tuning actually removes the variable condensers in the tuned circuits and replaces them
with new condensers which were previously adjusted to the correct values for that particular
Push-button switching mechanisms like that shown in Fig. 6A are used in electrical
automatic tuning systems. When one of the buttons on this unit is pressed down, the button
which formerly was down is released, removing that set of condensers, and an entirely
new set of condensers is switched in. The entire process of switching is practically
instantaneous. It is common practice to mount the set of preadjusted condensers right
on the switching mechanism. In Fig. 6B is a bottom view of the unit in Fig. 6A; as you
can see, there are two trimmer condensers, each adjusted by a screw, for each of the
buttons on the unit.
To secure better frequency stability, some manufacturers are using a push-button switching
system to substitute adjustable coils instead of trimmer condensers in the oscillator
tuned circuit. Special coils employing pulverized iron cores which can be moved by means
of an adjusting screw to change the inductance of the coil are used for this purpose.
A fixed condenser, usually of the temperature-compensating type, provides the necessary
capacity for the oscillator circuit. Because of the higher cost of variable-permeability
iron-core coils, they are usually used only in the oscillator circuit. A slight change
in trimmer condenser capacity will have far more detuning effect in the oscillator tuned
circuit than in a preselector tuned circuit.
Initial Adjustments. It is neither advisable nor necessary to make each adjustable
part in an electrical automatic tuning system cover the entire 540 to 1,500-kc. broadcast
band. A more economical and stable construction is secured by limiting the tuning range
of each adjustable coil or trimmer condenser to a definite section of the broadcast band;
for example, one set of adjustable parts may be designed to tune from 540 to 900 kc.,
another set may coyer the range from 700 to 1,300 kc., and the third and final set might
coyer the range from 1.000 to 1,500 kc. There is enough overlapping between these three
groups so that a station near the limit of one group may also be tuned in by another
Instruct the customer beforehand to turn on the receiver at least half an hour before
you arrive, so the chassis will reach its stable operating temperature.
To reset a button for a station, tune in the station manually and note the nature
of its program at that time. Now push in the button assigned to that station. Locate
the oscillator trimmer condenser or variable inductance controlled by that button, and
adjust until the station is heard with maximum audio output. For best results do not
depend upon your ears, but use an output indicator or the tuning indicator in the receiver
(if available). With this done, locate the preselector trimmer condenser which is controlled
by this button and adjust for maximum output in the same manner; you will note that this
adjustment is quite broad, whereas the setting of the oscillator trimmer was quite critical.
Repeat this procedure for each other push-button.
Electro-Mechanical Automatic Tuning Systems
Electro-mechanical automatic tuning systems will generally include the following sections:
1. A small electric motor which drives the gang tuning condenser through speed-reducing
gears and which can be reversed by means of a switch.
2. A switching mechanism which can be adjusted too stop the driving motor at predetermined
positions which correspond to the gang tuning condenser settings for desired stations.
3. A group of push-button-controlled switches, each of which starts the motor and
connects into the motor circuit the proper switch mechanism for stopping the motor at
the correct point (these may be located at any reasonable distance away from the receiver,
making remote control tuning possible).
44. A means for silencing the audio system of the receiver during the interval when
the motor is driving the tuning condenser, in order to prevent annoying blasts of sound
as the receiver is tuned past strong undesired stations; a means for releasing temporarily
the a.f.c. system while the tuning motor is in motion or just after it stops, in order
to allow the desired station to "take hold" of the a.f.c. system.
RCA Electro-Mechanical Automatic Tuning System. A top view of this tuning
mechanism is shown in Fig. 7A, and the simplified diagram in Fig. 7B shows how the system
operates. The metal disc with an insulated segment and a slot on opposite sides is one
of eight discs which are mounted on a common shaft and held in position by friction;
beneath each disc is a spring contact. During manual tuning the metal discs turn with
the gang tuning condenser but the electric motor remains motionless since the pin on
its shaft is not engaged with the crank arm on the speed-reducing gear.
To make the preliminary station-setting adjustment for this RCA mechanism, one of
the buttons is pressed, and after the motor has stopped, an adjusting key (provided with
the unit and kept in a special adjusting key receptacle when not in use) is inserted
in the adjusting hole corresponding to this button; this places the key in the slot on
the metal disc, exactly as shown in Fig. 7B. The a.f.c. system is turned off by means
of a switch on the receiver panel, and the receiver is now tuned manually to the station
desired for that button. Removal of the adjusting key completes the adjustment for this
station; the process is then repeated for each other button. It is not necessary to lock
the metal discs in position, since there is sufficient friction to prevent them from
slipping during normal receiver operation.
Conclusions. In general, the telephone dial types of automatic tuning units will be
the most difficult to readjust. If your preliminary examination indicates that trouble
may be encountered, it will be wise to write in advance to the manufacturer for instructions
on setting up the tuner.
Posted October 28, 2015