Theory of Operation—2465B/2467B Service
Preregulator Control IC, the Inverter Drive circuit requires
an independent power supply, since it must be operational
before any of the secondary supply voltages can be
gen-
erated.
INVERTER DRIVE POWER SUPPLY. This power supply
consists of Q1062, VR1062, and their associated
components. As power is first applied, the initial charging
current through T1050 induces a current in the
transformer secondary winding (pins 8 and 9). The
alternating current is rectified by the diode bridge
composed of CR1062, CR1063, CR1064, and CR1065 and
stored in C1066, providing power for the Inverter Drive
circuitry.
When the Preregulator Control IC turns switching
transistor Q1050 on for the first time, the charge stored on
C1066 during the initial charging period is sufficient to
properly turn on one of the current-switching transistors
(either Q1060 or Q1070) for the first cycle. After that, the
alternating drive signals continue to induce current into the
secondary winding of T1050 to provide operating power as
long as the instrument is turned on.
The current rectified by the diode bridge and stored on
capacitor C1066 is regulated down to the required voltage
level by
R1061,
VR1062, and Q1062. Zener diode
VR1062 references emitter-follower Q1062 and holds the
supply output at approximately +11.4 V.
INVERTER DRIVE GENERATOR. The Inverter Drive
generator consists of U1062, U1064, U1066, switching
transistors Q1060, Q1070 and their associated
components. The circuitry alternately switches current
through each leg of the output transformer (T1060)
primary winding and produces the ac current required for
transformer action.
Out-of-phase input signals to comparator U1062C come
from two resistive voltage dividers place in either leg of
one secondary winding of T1050. The comparator detects
the phase changes (crossover points) of the secondary
current caused as Q1050 switches on and off. Every com-
plete on-off cycle of Q1050 produces a positive clock at
pin 14 of U1062C that toggles flip-flop U1064B. The
tog-
gling alternately turns switching transistors Q1060 and
Q1070 on, each with an approximate 50% duty cycle.
Comparators U1062A and U1062B, at the Q and Q
output of the flip-flop, detect the precise crossing point of
the toggling drive signals and ensure that only one switch-
ing transistor will be on at any one time. These mutually-
exclusive drive signals are buffered by inverters U1066A
and U1066B and applied to switching transistors Q1060
and Q1070 to alternately turn them on and off at one-half
the switching rate of Q1050. By alternately switching
opposite ends of the primary winding to ground, the
current flowing through switching transistor Q1050 will
flow alternately in each half of the primary winding. This
produces ac voltages at the secondary windings that are
then rectified, providing the various unregulated dc supply
voltages.
Current Limit
The Current Limit circuit, composed of transistor Q1040
and the associated components, limits the maximum
current flow in the output transformer to about 1 ampere.
Resistor R1040 (connected to the Preregulator Control IC
+ 15 V supply) forward biases germanium diode CR1040
and applies approximately +0.3 V across the base-to-
emitter junction of Q1040. Current flowing to the output
transformer develops a voltage drop across R1050 that
adds to the bias developed by CR1040. As the current to
the transformer increases, the voltage drop across R1050
also increases
until,
at around 1 A, the combined voltage
drop across R1050 and CR1040 forward biases transistor
Q1040.
The base of Q1022 is pulled negative through
R1042,
and the +15 V supply for the Preregulator IC
turns off (see Preregulator Control description). The power
supply will try to restart itself; but, as long as the
excessive-current condition persists, the current-limit cir-
cuit will keep shutting the supply down, protecting the
instrument.
Rectifiers
The rectifiers convert the alternating current from the
secondary windings of inverter output transformer T1060
to the various dc supply voltages required by the instru-
ment. Rectification is done by conventional diode rectifier
circuits, and filtering is done by conventional LC networks.
The +87 V unregulated supply is produced by a
voltage-doubler circuit. The positive plate of C1130 at the
anode of CR1132 is referenced at approximately +45 V
through diode CR1131 (to the +42 V unregulated supply).
As the positive half cycle from the 42 V secondary winding
(actually about +45 V peak) is applied to the negative
plate of C1130, the positive plate is elevated to a peak
value of approximately +90 V. Diode CR1132 becomes
forward biased and storage capacitor C1132 is charged to
about +90 V. Following cycles replenish the charge
drawn off by the loads on the +87 V supply line.
Line Signal
A sample of the ac line voltage is coupled to the Trigger
circuit by transformer T1229 and provides the LINE TRIG
signal to the Trigger hybrid. Transformer current is limited
3a-50
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