Theory of Operation—2465B/2467B Service
When power is first applied, the High-Voltage Regulator
circuit detects that the negative crt cathode voltage is too
positive and pulls pin 2 of transformer T1970 negative.
The negative level forward biases transistor Q1981 via the
base-drive winding of the transformer. Current begins to
flow in the primary winding through transistor
Q1981,
inducing a magnetic field around the transformer primary
winding.
The increasing magnetic field induces a current in
the base-drive winding that further increases the base
drive to the transistor. This in-phase feedback causes
current in Q1981 to increase until the primary winding
current reaches its maximum value. As the rate of change
of the primary current peaks and then reverses, the
induced magnetic field begins to decay. This decreases the
base-drive current and begins turning Q1981 off.
As Q1981 is beginning to turn off, the magnetic field
around the primary winding continues to collapse at the
resonant frequency rate of the transformer. This induces
into the base-drive winding a voltage that completely turns
off the transistor. The collapsing magnetic field goes to
zero,
then builds in the opposite direction to a maximum
before collapsing again (resonant flywheel effect). This
sequence of events occurs repetitively as the circuit
con-
tinues to oscillate.
The oscillating magnetic field in the primary winding
couples power into the secondary windings of the
transformer. The amplitude of the voltages induced in the
secondary windings is a function of the turns ratios of the
transformer windings.
High-Voltage Regulator
The High-Voltage Regulator consists of U1956A and B
and associated components. It monitors the crt Cathode
Supply voltage and varies the bias point of the switching
transistor in the High Voltage Oscillator to hold the
Cathode Supply voltage at the nominal level. Since the
output voltages at the other secondary winding taps are
related by turns ratios to the Cathode Supply voltage, all
voltages are held in regulation.
When the Cathode Supply voltage is at the proper level
(-1900 V), the current through R1945 and the 19-Mfi
resistor internal to High Voltage Module U1830 holds the
voltage developed across C1932 at zero volts. This is the
balanced condition and sets base drive in Q1981 via
integrator U1956A and voltage-follower U1956B. Varying
base drive to Q1981 holds the secondary voltages in
regu-
lation.
If the Cathode Supply voltage level tends too positive, a
slightly positive voltage will develop across C1932. This
voltage causes the outputs of integrator U1956A and
voltage-follower U1956B to move negative. The negative
shift charges capacitor C1951 to a different level, around
which the induced feedback voltage at the base-drive
winding will swing. The added negative bias causes Q1981
to turn on earlier in the oscillation cycle, and a stronger
current pulse is induced in the secondary windings. The
increased power in the secondary windings increases the
secondary voltages until the Cathode Supply voltage
returns to the balanced condition (zero volts across
C1932). Opposite action occurs should the Cathode Sup-
ply voltage tend too negative.
Cathode Supply
The Cathode Supply circuit is composed of a voltage-
doubler and an RC filter network contained within High-
Voltage Module U1830. This supply produces the —1900
V accelerating potential applied to the CRT cathode and
the -900 V slot lens voltage. The -1900 V supply is
monitored by the High Voltage Regulator to maintain the
regulation of all voltages from the High Voltage Oscillator.
The alternating voltage (950 V peak) from pin 10 of
transformer T1970 is applied to a conventional voltage-
doubler circuit at pin 7 of the High Voltage Module. On the
positive half cycle, the input capacitor of the voltage dou-
bler (0.006 /*f) is charged to -950 V through the forward-
biased diode connected to ground at pin 9 of the module
(charging path is through the diode, so stored charge is
negative). The following negative half cycle adds its ac
component
(—950
V peak) to this stored dc value and
produces a total peak voltage of -1900 V across the
capacitor. This charges the 0.006-^f storage capacitor
(connected across the two doubler diodes) through the
second diode (now the forward-biased diode) to —1900 V.
Two RC filters follow the voltage doubler to smooth out
the ac ripple. A resistive voltage divider across the output
of the filter network provides the
—
900-V slot lens poten-
tial.
Anode Multiplier
The Anode Multiplier circuit (also contained in High Volt-
age Module U1830) uses voltage multiplication to produce
the +14 kV CRT anode potential. Circuit operation is
similar to that of the voltage-doubler circuit of the Cathode
Supply.
The first negative half-cycle charges the 0.001-^f input
capacitor (connected to pin 8 of the High Voltage Module)
to a positive peak value of +2.33 kV. The following
posi-
tive half cycle adds its positive peak amplitude to the volt-
age stored on the input capacitor and boosts the charge
on the second capacitor of the multiplier (and those follow-
ing) to +4.66 kV. Following cycles continue to boost up
3a-36
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