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grid of tube V303B, and the corners of the square
tors R313 and R314 produces a positive voltage at
the cathode of tube V303A greater than the posi-
wave would be rounded instead of spiked. The
positive spike is a desirable feature when this
tive voltage applied to its grid through resistors
wave form is later applied to the univibrator.
R311 and R312. This effectively biases tube
V303A at cutoff so normally, with no signal ap-
2-8. Positive Pulse-Selector Stretcher
plied, tube V303A is at cutoff and tube V303B
conducts. When a positive input signal is applied
to the grid of tube V303A, the grid becomes posi-
a. The positive pulse-selector stretcher is one-
tive with respect to the cathode. When the input
half of a miniature twin-triode tube, V304A. Its
signal reaches a value of approximately 150 volts,
purpose is to eliminate the negative portion of the
and tube V303A conducts decreasing the plate
applied input signal a:nd to develop a positive out-
voltage and creating a negative swing which is
put pulse, at each of two output points, for each
applied, through capacitor C304, to the grid of
input pulse. One output is used as a trigger for
tube V303B, This negative voltage, in turn, de-
univibrator V305, and the other to drive audio
creases the plate and cathode current of tube
output tube V304B. The stretching is done on the
V303B. Because both cathodes are interconnected
time base to increase the audio component in the
the decreased cathode voltage reduces the effective
pulse applied to the audio tube.
bias on tube V303A and aids the positive grid
b. The rectangular output wave from the plate
voltage of tube V303A. A regenerative action
of tube V303B is applied to an RC circuit consist-
takes place, and leaves tube V303A conducting at
ing of resistor R319 and capacitor C305. The RC
maximum current and tube V303B at cutoff, Any
time constant of this circuit is very small com-
increase of the grid-signal voltage of tube V303A
pared to the width of the input wave and, as a
past the triggering point has no effect on the out-
result, this wave is differentiated. The differen-
put voltage of tube V303B. The two tubes remain
tiated wave is applied to the grid of tube V304A,
in this steady state condition until the signal volt-
whit h is set at the cutoff point by a fixed negative
age at the grid of tube V303A drops below 120
bias voltage applied between the grid and cathode.
volts. At this time, regenerative action again
Negative pulse on the grid have no effect, be-
takes place, because the negative signal voltage
cause the tube is already biased to cutoff. How-
produces a positive voltage across plate resistor
ever, a positive grid pulse produces a plate cur-
R315, which is applied to the grid of tube V303B
rent pulse. This plate current pulse flows through
through capacitor C304. This positive voltage in-
resistor R322 (the cathode resistor of univibrator
creases the drop across cathode resistors R313
tube V305) to charge capacitor C306. Conse-
and R314, and effectively returns the bias on tube
quently a sharp positive pulse, produced by the
V303A cathode to the cutoff condition that origi-
capacitor-charging current, appears across resis-
nally existed with no signal applied; tube V303A
tor R322 and is used 10 trigger univibrator V305.
returns to a nonconducting condition and tube
V303B conducts. The two tubes remain in a
c. The waveform across cathode resistor R321
steady state until the signal voltage applied to the
of tube V304A is shown at jack J313 (fig. 26).
grid of tube V303A is raised to approximately 150
(The second spike is caused by the recovery of the
volts, at which time the circuit is triggered and
univibrator. ) The stretched pulse results from the
the action is repeated. Thus when one tube is held
exponential decay of voltage caused by the dis-
at plate current cutoff and the other tube is con-
charge of capacitor C306 through resistors R321
ducting, a rectangular waveform of fixed ampli-
and R322. The stretched pulse is applied to the
tude is created at the recurrence frequency of the
grid of audio output tube V304B. A momentary
input signal, regardless of input waveform and
contact, INTER S.C. position of switch S203 on
amplitude. `The output of this stage is the signal
the (control panel, is connected across resistor
voltage developed at the plate of tube V303B.
R322 to short the univibrator trigger pulse for
testing purposes. This switch can be used to help
c. Capacitor C304 immediately applies the volt-
isolate trouble in the signal converter circuits and
age developed across resistor R315 in the plate
to determine whether the recorder circuits are
circuit of tube V303A directly to the grid of tube
functioning properly. (Recording at zero is ob-
V303B rather than through resistor R316. With-
t a i n e d when the switch is depressed.) Tube
out this capacitor the steep voltage, rise and fall,
V304A actually functions as a cathode follower to
developed across resistor R315 by operation of
provide a signal for triggering the univibrator at
a low impedance point.
tube V303A would be delayed in application to the

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