TM 11-6625-847-45
off dc amplifier A1Q2, which, in turn, shuts off driver
1A1Q1 to the reference established by 4.7-volt Zener
A1Q1 and test transistor 1A1Q1. When the short-circuit is
diode A1VR2. The output at the emitter of test set
removed, the regular will recover and resume regulating
transistor 1A1Q1 is developed across the voltage divider,
action.
consisting of resistors A1R7, 1A1R4, and A1R8. Variable
resistor 1A1R4 on the front panel of the test set provides
adjustments of the base voltage of A1Q4 to maintain a
c. The dc-to-dc converter is a saturable core
constant output voltage from the regulator with variations
oscillator used to produce the dc and ac operating
in component values because of age and temperature.
voltages required by the SM-442A/ GRC. The +27-volt dc
Assume that the + 20-volt dc output instantaneously
primary power is applied to pin 7 of connector J2, from
increases to +22 volts dc. The voltage across the voltage
which it is applied through a pi-section filter network to pin
divider will increase the base bias of transistor A1Q4.
9 of transformer T1. The pi-section filter consists of
Transistor A1Q4 will have an increased rate of
inductor L1 and capacitors A2C1, A2C2, A2C3, and
conduction, increasing the voltage developed across
A2C4. The +27-volt dc is applied through the transformer
resistor A1R6. This decreases the forward-biasing of
winding to the collector of both transistors Q1 and Q2 and
transistor A1Q3, increasing the voltage at the collector of
through resistor A2R1, the transformer winding, and
transistor A1Q3. This increased voltage will decrease the
resistors R1 and R2 to the bases of transistors Q1 and
forward bias on dc amplifier A1Q2, increasing the voltage
Q2. The differences in the two transistors will cause one
on the collector of dc amplifier A1Q2. The base voltage
of them to turn on first. Assume that transistor Q1 turns
of dc amplifier A1Q2 is stabilized by Zener diode A1VR1;
on first. Application of +27 volts dc then will induce a
therefore, the emitter-to-base voltage on driver A1Q1 will
voltage in the windings of transformers T1 with the
decrease, decreasing the voltage on the collector of driver
following polarities: pin 9, plus; pin 2, minus; pin 1, plus;
A1Q1. The collector voltage of driver A1Q1 is the base
and pin 3, minus; X therefore, transistor Q1 is more
bias for test set transistor A1Q1; therefore, the decrease
forward-biased by the positive voltage on its base and is
at the collector of driver A1Q1 causes test set -transistor
driven toward saturation. Transistor Q1 will conduct into
A1Q1 to conduct less. This action increases the collector-
saturation, at which time the magnetic field created in the
to-emitter resistance to drop the voltage back to +20 volts
windings will collapse since the current becomes constant.
dc. A similar sequence of events will occur if the +20
The polarities of the windings will be reversed, turning
volts dc decreases; however, the reverse will occur in all
transistor Q2 on. This action will continue, producing a
the circuits to increase the conduction rate of test set
square wave ac signal across the primary of transformer
transistor A1Q1 and thereby decrease the collector-to-
T1. Resistors R1 and R2 are base current limiting
emitter resistance to increase the voltage at the emitter of
resistors. Diode A2CR1 will clamp pin 10 of transformer
test set transistor 1A1Q1 to +20 volts dc. Capacitor A1C5
T1 at ground so that maximum drive can be applied to the
provides filtering for the + 20 volt dc output line.
conduction transistor to drive it into saturation.
Capacitor A1C4 provides filtering feedback for transistor
d. The 54-volt ac output from dc-tide converter
A1Q4; therefore, any ripple on the + 20-volt dc output line
switch Q1, Q2 is stepped-down in transformer winding 4-
will be fed 'back into the regulator circuit, ,and. in turn, to
5, filtered, and applied to pins 1 and 9 of connector J2 as
test transistor 1A1Q1 which is 180 out-of-phase with
u.3 volts ac. The 54-volt ac is stepped-up by transformer
itself and allow the ripple to be canceled. Capacitors
winding 6-12, full-wave rectified by diodes A3CR1 through
A1C1, A1C2, and A1C3 provide high frequency filtering.
A3CR4, filtered, and applied to pin 6 of connector J2 as
If the + 20-volt dc line becomes short-circuited, the
the + 125-volt dc
resulting ground with forward-bias diode A1CR1, shutting
1-10
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