Tektronix CURVE-TRACER 576 Instruction Manual page 78

Circuit Description-Type 576
0 to +4.5-volt Variable Supply. The 0 to +4.5-volt vari-
able supply consists of diode bridge D758A, B, C and D,
filter capacitor C759, comparator 0767 A and B, emitter
follower 0774, short protection 0772 and series regulator
0778. This circuit operates in essentially the same manner
as the -75-volt supply circuit. In this circuit, however, the
reference voltage at the base of 0767 A is variable from 0
volts to +4.5 volts by the READOUT ILLUM control,
R760, and divider R762 and R763. The output current of
the supply is limited by 0772.
+5·volt Supply. The +5-volt supply consists of error
amplifier 0780, short protection 0784 and series regulator
0787. The supply shares diode bridge D758A, B, C and D
and filter capacitors C758 and C759 with the +4.5-volt sup-
ply. Any variation in the +5-volt supply voltage is amplified
by 0780, causing the base voltage of 0787 to vary in
opposition to the variation of the supply. The current con-
ducted through R788 by the supply is thus regulated,
which in turn regulates the +5-volt supply. 0784 provides
short protection by turning on whenever the current
through R788 becomes excessive. When 0784 turns on, the
base voltage of 0787 is pulled down, limiting the current
through R788.
+12.5-volt Supply. The +12.5-volt supply consists of
diode bridge D790A, B, C and D, filter capacitor C791,
comparator 0795A and B, emitter follower 0803, short
protection 0800, and series regulator 0808. This circuit
operates in essentially the same manner as the -75-volt
supply. Short protection of the + 12.5-volt supply when it is
shorted to a more positive voltage is provided by 0725 of
the -75-volt supply. If the +12.5-volt supply voltage is
pulled up, the base of 0725 is also pulled up, turning on
0725. With 0725 turned on, the base of 0729 is pulled
down turning off the -75-volt supply, which will turn off
the+ 12.5-volt supply.
+15-volt Supply, Camera Power. The + 15-volt supply
consists of error amplifier 0810, emitter follower 0817,
short protection 0814 and series regulator 0819. The sup-
ply shares diode bridge D790 and filter capacitors C790 and
C791 with the +12.5-volt supply. Any variation in the
+15-volt supply voltage is amplified by 0810, causing an
opposing variation in the voltage at the base of 0817. This
opposing voltage variation is transmitted through the emit-
ter of 0817 to the base of series regulator 0819 where it
controls the current conducted by R819 and thus regulates
the supply. When enough current is conducted by 0819 to
turn on 0814, the voltage at the base of 0817 is pulled
down, thus limiting the current through 0819.
+5o-volt Supply. The +50-volt supply consists of diode
bridge D821A, B. C and D. and filter capacitors C822 and
C823. It is a floating unregulated supply used to power the
step amplifier output.
+10Q-volt Supply. The + 1 00-volt supply consists of
diode bridge D828A, B, C and D, filter capacitor C829,
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error amplifier 0834, emitter follower 0840, short protec-
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tion 0837 and series regulat_or
08~6_.
Any variation in volt- .
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age by the +100-volt supply
IS
ampl1f1ed by 0834 and trans-
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mitted through 0840 to the base of 0846. Since any varia-
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tion in the supply is inverted by 0834, the base voltage of
0846 will always move in opposition to a variation of the
supply. The current conducted by R846, therefore, also is
conducted so as to oppose any change in supply voltage.
When enough current is conducted by 0846 to turn on
0837, the voltage at the base of 0840 is pulled down, thus
limiting the current conducted by 0819.
CRT Voltage Power Supply
The CRT power supply produces two high voltages, -4
kV and +225 volts, for operation of the CRT and its related
controls. In addition, the +225-volt supply is used by the
display amplifiers. The source of power for the two supplies
is a high frequency (about 28kHz) Hartley oscillator which
consists of 0851 and the two primaries of transformer
T850. The collector of 0851 is connected through the col-
lector primary, R850 and L850 to the +100-volt supply.
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When current flows through the collector primary. a mag-
netic field is built up in the transformer core. Due to this
field, a reverse base current is caused to be conducted
through 0851 by the base primary and 0851 is eventually
turned off. With 0851 off, no current flows through the
collector primary. The residual field in the transformer core
now causes forward base current to be conducted through
0851, turning it on. As 0851 turns on, current again flows
through the collector primary, thus beginning a new cycle.
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The frequency of the oscillator and thus the output current ....,
of the secondaries is controlled by the voltage on pin 2 of
the base primary.
-4 kilovolt Supply. The -4 kV supply consists of half-
wave rectifier D870, filter capacitors C870 and C871, and
divider resistors R875 through R883. This supply is a half-
wave rectified supply with D870 forward biasing on nega-
tive transistions of the voltage on the -4 kV secondary.
The -4 kV supply voltage after being filtered by C870 and
C871 is reduced by Zener diode D882 to provide the
-3890 volt cathode voltage. The grid voltage is controlled
by the divider made up of R882 and INTENSITY control
R883. The voltage on the focus screen of the CRT is con-
trolled by FOCUS control R880.
The -4 kV supply is regulated from a reference supply
which is generated by the winding between terminals 6 and
5 of T850. This reference supply consists of half-wave recti-
fier D866 and D869, and filter capacitor C866. The regula-
tor circuit consists of error amplifier 0859 and emitter fol-
lower 0855. Any variation in the reference supply voltage
is transmitted to the base of 0859 through divider
R860-R864. The variation is then amplified and inverted by
0859 and transmitted through 0855 to the base of 0851,
where it regulates the drive of the oscillator. Any variation
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in current conducted by the -4 kV supply is conducted by
R899, which causes the decoupled supply voltage at the,;
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emitter of 0859 to vary, thus compensating for current . . . ,
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variation in the -4 kV supply.
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