February 1932 Radio-Craft
[Table
of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
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Once again I have given selflessly and freely of my time and talent to make available resources to those
seeking certain knowledge regarding vintage radio circuits ;-) This time it is the Radio Service Data Sheet
for the Howard Model 45 A. V. C. Superheterodyne with Model A. V. H. Chassis. The info was published in a 1932
edition of Radio-Craft magazine, one of Hugo Gernsback's many endeavors. Also on the page is a
complete list of all such schematics and tuning and repair data posted thus far.
Howard Model 45 A. V. C. Superheterodyne with Model A. V. H. Chassis
Radio Service Data Sheet
The values of the components of this
receiver chassis are as
follows: Resistors R1 , R3, R5, 1/5-meg. (1/3-watt); R2, R6, 500
ohms (1/5-watt); R4, 6,000 ohms (1/2-watt); R7, 30,000 ohms; R8,
volume control, 1/2-meg.; R9, 1/2-meg.; R10, 3,000 ohms; R11, 2,000
ohms; R12, R13, 150,000 ohms (1/2-watt); R14, 2 megs.; R15- R16-R17-R18-R19,
voltage divider, 9,900 ohms; R20, R21, 10 ohms (center-tapped);
R22, 200 ohms.
Condensers C4, C5, C6, C7, I.F. trimmers; C8, C9, C10, C15, C16,
0.1·mf.; C11, 0.00025-mf.; C12, 0.001-mf.; C17, C18, 0.25-mf.; C19,
C23, 0.5-mf.; C21, 0.05-mf.; C24, 1.0 mf.; C25, C26, 8 mf. (420
volts); C27, 4 mf. (420 volts).
In the interest of obtaining best results with the Automatic
Volume Control receiver, it is important that the type '27 control
tube V9 be a selected one, with a definite plate current cut-off
when tested at 180 volts plate and 20 volts bias on the grid. This
cut-off should be less than 5 microamperes. If there is no means
available for checking the tube (in the form of a special tube tester),
an immediate check for tube performance can be obtained in the set
itself.
For instance, disconnect the antenna and short-circuit the aerial
lead, leaving the control tube out of the socket, and note the swing
of the tuning meter. Then insert the tube in the socket and if it
is a good automatic volume control tube, there should be no change
in the position of the pointer on the tuning meter. If there is
a change in the position of the tuning meter pointer, namely, a
swing toward the right, it is an indication that the A. V. C. tube
does not have a definite plate cut-off; instead, it is drawing plate
current and as a result the bias voltage on the regular R.F. and
I.F. tubes has been raised, with the consequent cutting down in
plate current.
The Model 45 speaker has a 350-ohm field, and as such it cannot
be used with the Models 35 and 40 receivers.
The receiver housed in the regular cabinet is the "Model 45";
the chassis is the "Model AVH."
The automatic volume control functions in holding the second-detector
input voltage at a definite level, a system which is different from
that in other receivers. A reduction of background noises, between
stations, will be noted.
The only service met with to date on the Model "H" receiver has
been in connection with the shorting out of the R.F. plate bypass
condenser, the red lead of which may accidentally become wedged
underneath the first I.F. coil can. The insulation does not cut
through immediately but, after being in service for a number of
days, the pressure on the insulation may be such as to gradually
cut through it, shorting out the plate bypass condenser, and thus
producing zero voltage on the plates of the R.F., first detector,
and I.F. tubes.
The A. V. C. tube is so connected by means of a 2-megohm resistor,
R14, that the grid is at absolute "B-" potential. The cathode of
the tube is connected to a point on the voltage divider which is
at 24 volts positive, with respect to "B-" or the grid. There then
exists between the cathode and the grid a potential difference of
24 volts with the grid negative by this amount. The plate of this
tube connects to ground by means of two 150,000-ohm resistors, R12-R13.
Since ground is connected to 124 volts, positive (with respect to
"B-"), there exists between the cathode and the plate a potential
difference of 100 volts. In order to bypass any R. F. energy which
may appear on the plate, a non-inductive condenser C22 is connected
from the plate of the A. V. C. tube to the cathode.
With the condition of no-signal there exists a bias of 24 volts
and a plate potential of 100 volts. Under these conditions, there
is no plate current flowing and the tube is said to be adjusted
to cut-off. Since no plate current is flowing, there exists no voltage
drop across the plate circuit resistors and, therefore, there is
no bias voltage on the grids of the controlled tubes. The only bias
on the R.F., first detector, and I.F. is caused by the respective
voltage drops across their cathode resistors. These resistors are
designed to give the most sensitive operating point.
In the case of a received signal, energy passes through the receiver
to the second-detector grid. Here the A. V. C. (automatic volume
control) tube grid, and the second-detector grid, are in parallel.
The signal voltage is fed to the grid of the A. V. C. tube through
a small fixed condenser, C11.
It will be seen that during the positive half of the incoming
cycle, the peak voltage of the signal swing subtracts from the original
bias voltage; which means that the instantaneous bias on the tube
is less than the original bias and the tube begins to draw current
in its plate circuit. Since this current flows in the resistors
in the plate circuit of the A. V. C. tube, there exists a voltage
drop across these resistors; also, the flow of the electrons is
from plate to ground so that the plate becomes negative with respect
to ground. Now, since the original potential of the cathode of the
R.F., first-detector, and I.F. tubes is positive with respect to
ground, it follows that if the grids of the respective tubes are
connected to a resistor in the plate circuit of the A. V. C. tube,
that any potential existing across this resistor is added to the
original bias and makes the grids more negative than the original
bias by the amount of the voltage drop across the resistor in the
A. V. C. tube plate.
It is at once apparent that the greater the signal voltage appearing
at the grid of the A. V. C. tube, the more plate current will flow
in the plate circuit: an increase in plate current means an increase
in bias on the R.F., first-detector, and I.F. tubes; an increased
bias on these tubes means less amplification and, therefore, less
grid swing on the second-detector and A. V. C. tube. This cycle
goes on until a constant voltage is obtained across the second-detector
input, or, in other words, until a condition of equilibrium is reached.
Since R8 is located where the tone control is normally connected,
it was necessary to relocate the tone control, C13-R9-C14. As less
resistance is included between the two condensers, they become more
effective in bypassing the higher audio frequencies; at the same
time, they resonate the primary of T2 to a lower audio frequency.
Posted February 25, 2015
[references/general/radio-service-data-sheets.htm] |