Ferrocart Vibrator Application Notes

1. GENERAL
 Although vibrators of one form or another have been in use for many
years in telephone exchanges, and other similar environments, it was
not until the development of automobile radio receiving sets that compact
and relatively inexpensive vibrators were produced capable of withstanding
the wide fluctuations of battery voltage and mechanical jarring found in
a modern automobile.  Not only do modern vibrators operate in any
physical position, function over a wide range of conditions and give long
life, but they are quiet both mechanically and electrically.
All of the synchronous and all of the non-synchronous vibrators are
identical in construction except that a different driving coil is used for
each voltage, and different numbers and arrangements of prong bases are
used.

2. NON-SYNCHRONOUS VIBRATORS

This type of vibrator is also called "Single" or "Valve Type" since it has
a reed vibrating so as to make alternate contact with a single contact
on either side, and hence requires a separate rectifier to produce direct
current for high potential supplies used in battery operated radio receivers.
They are intended for use with a full-wave or center tapped primary
winding of a step-up transformer.  The reed is energized by means of
a small electromagnetic coil which acts on a magnetic armature mounted
on the free end of the reed.  The coil is connected electrically between
the reed and the fixed contact which closes when the reed is attracted by
the coil. Thus when the starting switch is closed, the vibrator coil is in
series with one half of the transformer primary winding. The resistance
of the vibrator coil is high compared to that of the primary winding, so
that no appreciable effect is produced at this instant in the primary
winding.  However, the vibrator coil attracts the reed armature, closing
the initial or "starting" contact thereby short-circuiting the coil.  This
creates a direct path for battery current to flow through the primary
winding. The momentum of the reed keeps the initial contact closed for
a time, and then the elasticity of the reed causes it to swing back, open
the initial contact and close the second or "rebound" contact.

3. WAVE-FORM

When the primary winding of the transformer is connected directly to
the battery, a counter electromotive force is induced in all of the
transformer windings, which is in opposition to the battery potential, in
the primary winding. The induced potential remains practically constant
as long as the contacts remain closed.  When the contacts open, the
induced potential in the transformer windings starts to reverse. However,
the rate of reversal is controlled by a condenser usually connected in shunt
to the high potential secondary winding, sometimes called a "buffer"
condenser. This condenser usually is given such a value that the induced
potential in the primary winding has reversed but has not yet equalled
the battery potential by the time the alternate contacts close.  Since the
direction of current flow around the transformer core is reversed when
the alternate contacts close, the counter electromotive force during the
second half-cycle will have a polarity opposite to the first.  The result
is that the wave-form of potential in all windings consists of a series of
flat-topped half-cycles of alternate polarity.  Each flat-topped wave is
connected to the following one by a sloping line terminating in an abrupt
voltage change just as the contacts close. The slope of the wave between
flat-tops, that is, while both sets of contacts are open, is controlled by the
size of the buffer condenser.

4. RECTIFICATION

When current of this wave-form is rectified by a full-wave rectifier of
any type, a series of current impulses is obtained, each having the
characteristic flat-topped wave shape, but all of the same polarity. This
is passed through a smoothing filter consisting of an iron cored reactor
or choke, with a filter condenser, usually electrolytic, connected across
the circuit at both input and output ends of the filter reactor. The output
current and voltage from the smoothing filter is quite steady and contains
negligible ripple if the reactor and condensers are of proper values.
In the case of automobile radio receivers, the ground or common electrical
point of the receiver is connected to the low potential battery, and is
negative with respect to the high potential required for the anodes.  If
a hot cathode rectifier is to be used, the cathode must be at a potential
several hundred volts positive with respect to the battery, which is the
best available source for cathode heating current. To meet this problem,
overseas engineers introduced the first indirectly heated cathode rectifier for
automotive use, in which there was sufficient insulation between heater
and cathode to permit a potential difference of several hundred volts
between them.  Thus the heater is operated from the battery, but the
cathode operates at full positive "B" potential. From this original rectifier
developed the present 84 or 6Z4 type.

5. SYNCHRONOUS VIBRATORS

Another method of rectifying the output of the vibrator transformer is to
add a second set of contact points on the reed to engage with a second
set of fixed contacts. Such vibrators are called "synchronous" since primary
and secondary contacts operate in synchronism, also "double" since there
are two complete sets of full wave contacts, and "tubeless" since no
rectifier tube is required.  The secondary contacts are adjusted to close
after and open before the corresponding primary contacts, to prevent
destructive arcing. This results in an advantage over the non-synchronous
type of vibrator, in that the primary contacts open and close at times
when the transformer is disconnected from its load.  The transformer in
a no-load or idle condition draws the relatively small exciting or magnetizing
current from the battery, so that the primary contacts operate at moments
when they are carrying very little current.  This prevents appreciable
arcing at the primary contacts.
On the other hand, the secondary contacts are not required to open or
close with a large difference of potential across them, since the input
condenser of the smoothing filter retains nearly its full charge during
the interval that it is disconnected from the transformer secondary winding,
and the prior closing of the primary contacts produces the full no-load
potential of the secondary winding before the secondary contacts are
brought together. As soon as the secondary contacts close, the secondary
voltage drops from the no-load to the full-load value, which is not much
lower if the transformer is designed to have good voltage regulation.
When the secondary contacts reopen, the secondary voltage rises again
to its no-load value. Thus the secondary contacts operate at times when
very little difference of potential across them exists.  By the time the
primary contacts open the secondary contacts have separated far enough
to prevent a spark from occurring.

Synchronous vibrators therefore have several advantages over non-
synchronous vibrators: They eliminate separate rectifiers while costing
no more than non-synchronous vibrators which they equal in external
dimensions; they are more efficient, since they eliminate the power required
to heat rectifier cathodes, and also the space potential drop inside the
electronic rectifier; and they will handle relatively large amounts of output
power with less deterioration than non-synchronous vibrators.
 

6. SPLIT REED SYNCHRONOUS VIBRATORS
Radio receivers with output tubes having directly heated filaments present a
special problem in connection with the grid bias for the output stage.
Unless a bias battery is used there is no way to obtain a potential
more negative than the negative end of the filaments using an ordinary
synchronous vibrator, since the moving contacts of the secondary circuit
are electrically common with the primary reed contacts, which in turn
are connected to one side of the battery. To meet this difficulty, the split-
reed synchronous vibrator was developed.  It differs from the normal
synchronous vibrator in that the reed is divided longitudinally, each section
carrying a set of contacts electrically insulated from those of the other
section.  The armature is mounted on the free ends of the two reed
sections by means of small insulators, while the fixed ends of both reeds
are insulated from one another and from the frame.   The circuit is
arranged so that the secondary reed is negative, and is returned to common
or ground through a resistor which is by-passed by a condenser.  The
potential created across this resistor by the "B" current flowing through
it is then used for grid bias.  The design and operation of split-reed
vibrators is otherwise the same as ordinary synchronous vibrators.
 

Elimination of Vibrator  Interference.

1. GENERAL

The introduction of a vibrator into a radio receiving set for the purpose
of obtaining a high voltage B supply from a lower direct current supply
such as a storage battery, at once raises problems concerning the inter-
ference such a vibrator causes due to interrupting a direct current at a
constant rate. These problems are entirely apart from such questions as
mechanical vibration transmitted directly from the moving elements of
the vibrator to the radio set. The mechanical cushioning of present-day
vibrators is such that this is not an important factor.
Electrical interference from the vibrator may occur due to the following
kinds of action:-
(1) DIRECT PICK-UP from the vibrator circuit by unshielded coils,
exposed grid leads or the antenna lead itself.
(2) ANODE MODULATION of any of the high frequency amplifier
or detector tubes, due to improper filtering of the anode supply
voltage.
(8) HEATER MODULATION of any of the high frequency amplifier
or detector tubes, due to improper filtering of the direct current
connections to the heaters.
(4) CHASSIS-COUPLED VOLTAGE PICK-UP in any of the high
frequency circuits, usually grid circuits, due to the chassis base
acting as a common path for currents of signal frequencies, and
the interfering currents from the vibrator circuit.
 

2. DIRECT PICK-UP

In order to eliminate direct pick-up all high frequency coils should be
enclosed in individual shields.  Grid leads should be kept as short as
possible. The antenna lead should be shielded over its entire length from
the point where it enters the receiver to the antenna coil itself. An effort
should be made to make the mechanical design of the receiver such that
all the power supply components are grouped together and should be
kept as far away from the high frequency input of the receiver as possible.
 

3. ANODE MODULATION

Anode modulation is easy to detect and comparatively simple to cure. The
simplest method of detecting this form of interference is to connect a
resistance load of such a value that the power supply is operating under
normal load, then supply the anode voltages to the receiver from batteries;
if there is still interference, with the power supply operating under these
conditions, it is evident that interference is occurring in another portion
of the circuit. However, if the interference is reduced when the receiver
is operated from batteries, then the high frequency choke reactor in the
B output circuit, if used, is either too small, it has too high distributed
capacitance, or the associated radio-frequency by-pass condenser is too
small. Generally it need not be larger than 0.05 to 0.1 M.F.  The axis
of the high-frequency reactor should be changed to make sure it is not
coupling to either the iron-cored choke reactor or the vibrator transformer.
On tube type circuits the r.f. by-pass condenser is seldom required.
 

4. HEATER MODULATION
heater modulation is usually detected by operating the power supply
from a separate battery.  When the power supply is obtained from a
separate battery, a shielded cable should be used, grounded to the chassis,
to prevent radiation of interference from this cable which might entirely
mask the heater modulation interference.

It must be kept in mind that if any change is made which reduces the
power of an interfering noise or signal by one-half the apparent reduction
will be slightly more than detectible by the ear.  This corresponds to a
change of 3 decibels in loudness, while an actual change of approximately
10 decibels is necessary to give the impression of a 50 per cent. reduction
in loudness. Thus if the interference is coming equally from two sources,
elimination of either one will not seem to help much, but if both sources
are eliminated simultaneously, the interference ceases entirely.  The use
of an output meter on the audio output is suggested, as changes of noise
of much less than one half are easily detected, especially if the interference
is relatively steady.

It has been found that receivers having high sensitivity may require two
h.f. reactors between the battery or d.c. power supply, and the heaters.
The use of the chassis as a common connection for all of the heaters is
not recommended due to the chance of voltage pick-up in the chassis.
This may not show up on model receivers, but in production, the resistance
of the grounding may vary slightly, and cause large changes in the amount
of interference caused. The heater circuit should be grounded to the chassis
at only one point.  The usual method is to wire all heaters together,
grounding one of them to the chassis. The heater to be grounded should
be found by experimenting to find the best point, as this will vary with
different designs. Care should be taken that there are no radiating loops
formed by the heater circuit which might couple to some portion of the
high frequency amplifier.
 

5. CHASSIS-COUPLED VOLTAGE PICK-UP
Voltage pick-up due to improper grounding of the power supply and
high-frequency amplifier elements is the most common source of inter-
ference and also the most difficult to locate.  The simplest method of
locating the source of interference is to short the grids of the tubes,
starting with the output tube and determine in which stage of the
amplifier the noise is originating. A common source of trouble is found
in receivers using automatic volume control. In such receivers the tuned
circuits are completed through condensers by-passing the grid return to
ground.  When these condensers are grounded directly to the chassis, a
voltage which is developed across the common impedance between the
point where the condenser is grounded and the wiping contact of the
variable condenser is picked up and applied to the grid of the tube. In
order to eliminate this interference the by-pass condenser should return
directly to the wiper of the section of the variable condenser tuning that
particular coil.  The condenser wiper should be bonded to the chassis
through a piece of heavy flexible copper braiding. As a rule, it is desirable
to ground the variable condenser at only one point on the chassis.
6. LOCATING INTERFERENCE IN A COMPLETED RECEIVER

In order to check for interference on a completed receiver, the antenna
lead-in should be grounded through a .0002 M.F. condenser.  If the
interference appears with the lead-in short-circuited in this manner, but
does not appear with it open, it indicates improper grounding of the
primary circuit of the antenna coil.  In some cases, this type of inter-
ference can be eliminated by returning the ground end of the antenna
coil primary to the condenser wiper.  Sometimes it will be found that
there is less interference when the Automatic Volume Control condenser
or the primary of the antenna coil is grounded to some point on the
chassis rather than on the condenser wiper. This is due to an out-of-phase
voltage being picked up and balancing out the interference, or neutralizing
it.  As a rule, this method of eliminating interference leads to erratic
receivers in production, as small changes in the impedance of the current
paths will change the balancing-out effect a great deal.
In some cases, interference has been located in the grid circuit of the
first audio frequency tube, due to the ground return of the volume control
being at a point remote from the tube's cathode circuit.  Where diode
detection is used, it has been found that often a hum voltage is induced
in the last high-frequency transformer through coupling with the power
transformer. The grid lead of the first audio tube will pick up considerable
interference if it is long and unshielded, or if it runs close to the power
supply or heater wiring.

7. COMPONENTS

Although the general construction of vibrator operated receivers follows
the lines of a.c. sets, there are certain additional considerations with regard
to some of the components having to do with the vibrator circuit.
 

8. VIBRATOR

Practically all vibrators now supplied to the industry have their own
individual shields or metal housings. The shielding housing is not essential
where the entire vibrator is enclosed within a shield together with the
transformer and other components recommended to be so shielded. The
vibrator housing will nearly always require grounding, however, especially
if the housing projects into the unshielded space of the receiver. There
are several ways in which the housing may be grounded. One most common
way is to make a connection inside the vibrator, between the housing and
the prong connected to the reed, which in turn is generally connected to
the grounded side of the storage battery or d.c. source. Another method
is to omit the internal strap, and ground the housing by means of a
clamp surrounding the vibrator socket, having 6 or 8 spring fingers which
grip the lower part of the housing firmly. Such vibrator ground clamps
can also be obtained with bent or "formed" ears which fit into an annular
groove at the lower edge of the housing, thereby preventing the vibrator
from working loose from the socket, even if mounted in a position other
than vertical. Another method less often used, is to connect the housing
to an otherwise insulated prong of the vibrator base plug, grounding
the corresponding socket jack as desired for best results.
 

9. PRIMARY RESISTORS
For 6volt operation, it is generally found that improved operation is
obtained if a resistor of from 50 to 100 ohms is connected from the
reed of the vibrator to each stationary contact, the leads being as short
as possible. The rating should be from half to 1 watt. For operation on
other voltages, the resistance will vary approximately as the square of
the voltage.

10. HIGH-FREQUENCY FILTER BETWEEN  VIBRATOR CIRCUIT
AND BATTERY
In stationary radio receivers containing vibrators, it is necessary to place
a filter between the d.c. supply and the vibrator circuits to prevent
interference from coupling to the signal circuits via the d.c. supply. In
automobile receivers it is also necessary to prevent interference from
the ignition system of the car from entering the radio receiver. It has
been found that it is seldom necessary to use suppressor devices on the
spark system of an automobile, if certain filter elements are added to the
receiver, which are designed to operate at very high frequencies.
From one to three air cored choke reactors are used in the battery lead
to the vibrator circuit, having from 30 to 100 turns of sufficiently heavy
wire to carry the current.  One form of choke which has been used
satisfactorily in many sets consists of 74 turns of No. 16 A.W. Gauge
wire (0.05 inch, 1.29 mm. diameter) wound with 4 layers insulated with
paper, on a mandrel having a diameter of 5/16 inch (7.94 mm.). Single
layer chokes are also used. When multilayer chokes are used, it is usually
best to connect the inner end toward the d.c. supply; the outer end,
toward the vibrator.
To prevent interference from the vibrator, coadensers of approximately
0.5 M.F. are connected to ground from both sides of the choke nearest
the vibrator, if more than one is used. These must have very low power
factor at high radio frequencies, and must have short leads, of low
resistance material.  The ground return of these condensers should be
as short as possible and soldered directly to the chassis.  The ground
connection to the vibrator reed should be soldered to the same point as
these condensers.
To prevent spark interference from the automobile motor, low-capacity
condensers called spark plates are used, generally connected between ground
and the ends of the air cored choke nearest the battery, if more than one
is used. These condensers have a capacitance of from 10 to 100 mmf.,
usually between 20 and 50 mnif. One type of spark plate consists of a
steel plate having an area of several square inches riveted to the radio
case by insulated rivets, and insulated from the case by either mica or a
good grade of insulating or fish paper, to give the desired capacitance.
Spark plates are not required on non-automobile sets.

11. HIGH-FREQUENCY FILTER IN HIGH-VOLTAGE OUTPUT
CIRCUIT
To keep vibrator interference from reaching the anode supply circuit an
air-cored high frequency choke reactor is placed between the iron-cored
choke reactor and the cathode of the rectifier tube, or center tap of the
secondary winding of the transformer in synchronous vibrator circuits. It
has an inductance of from 0.5 to 5 millihenrys, and should be of "universal"
or self-supporting construction, having low distributed capacitance.  It
should be physically small to restrict its external field.

A by-pass condenser of from 0.0005 to 0.1 M.F. capacitance may be required
connected between ground and the side of the air-cored choke nearer the
interference. In tube type circuits, this condenser is seldom required.

In some cases, the high-frequency filter is placed on the other side of
the smoothing filter, that is, between the iron-cored choke and the tube
anodes.

12. SMOOTHING FILTER
The rectified high-voltage direct current is smoothed out by means of an
iron-cored choke reactor shunted by electrolytic condensers much as in
a.c. radio receivers. The input filter condenser may be from 4 M.F. up,
and the output filter condenser from 6 M.F. up to as high as 30 M.F. if
exceptionally good filtering is required. The choke usually has a resistance
(d.c.) of from 200 to 500 ohms, with an inductance of from 5 to 30 henrys.
 

13. SPARK FILTERS
Besides the spark plates and high-frequency chokes in the battery leads,
interference filters or condensers are required on all other leads from or
to the receiver.

In the antenna lead-in, a small high-frequency reactor as small as 20 to
40 turns, 1/4 to 1/2 inch in diameter (6 to 12 mm.) is used, with by-pass
condensers.  In many cases, the lead-in is of shielded wire, which acts
as a by-pass condenser. The other side of the antenna choke is by-passed
by a small spark plate of from 5 to 20 mmf., usually with mica insulation.
Any other leads, such as to dial lamps, external controls, etc., usually
require spark plates to prevent bringing in interference from the spark
system.
 
 

The PM237 and PM238 are the most common types as these were used in the Astor car radios installed in Holdens and Fords of the 1950's. Why they show voltage output is anyone's guess...it's the transformer that determines that, not the vibrator.

Inside a Non Synchronous Ferrocart Vibrator. Unlike the superior Oak/MSP/AWA vibrators, these ones have the can crimped on so one has to make a mess of it when servicing. Also, the Ferrocarts have a shunt driven reed which means less reliability. Vibrators would not have such a bad reputation if there were less of the shunt driven kind. Fortunately, (unless it was made by Astor) most Australian made equipment did not use this type of vibrator.

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