High Voltage - HP 182C Operating And Service Manual

Model
182C
Theory
Figure 4-3.
High Voltage
Power Supply
Block Diagram
4-51.
HV
RECTIFIERS. The 40-kHz
oscillator
output
is
stepped up by
the
secondary windings
of
A6T1.
Two
half-wave
rectifiers
and
a
voltage multiplier
are
used to
develop the high voltages necessary for
CRT
operation.
4-52.
The
CRT
grid voltage,
approximately
-3200V,
is
developed by half-wave
rectifier
A6CR1
and
filter
A6C1, A6C2,
and
A6R1
through
A6R5. The
display
intensity
lower
limit,
determined
by
the
CRT
grid
voltage
level,
is
adjusted by
A6R2. The
CRT
cathode
voltage,
approximately -3150V, and the focusing
voltage,
approximately -2270V,
are
developed
by
half-wave
rec-
tifier
A6CR4
and
filter
A6C3,
A6C4
and
A6R7.
Resister
A6R8
is
part of
a
voltage divider
which drops
the
-3150V
to
-2270V for focus
control.
Diodes
A6CR5
and
CR6
prevent
the
CRT
grid
from becoming
positive
with
respect to the
cathode.
The
CRT
post accelerator
voltage,
approximately
19 kV,
is
developed by high voltage
multiplier
assembly
All.
4-53.
HV
REGULATION.
Variations
in
high
voltage
output are fed
back
to
the high voltage regulator circuitry
consisting of
A10Q1. A10Q2. A10Q3, and
associated
components. The
regulator
controls the high voltage
oscillator bias
to
maintain high voltage
at a
constant
level.
If,
for
example, the
CRT
cathode
voltage
tends to
decrease (go
more
positive), a
positive-going
signal
is
applied
through the regulator to the base of
oscillator
01.
The
oscillator
then
conducts
for a greater
period of
time, causing
a larger
voltage
change
at
the primary of
A6T1.
This increases the
secondary voltage to restore
cathode voltage
to
the desired
level.
4-54.
The
high voltage regulator
monitors
CRT
cathode
voltage
through coupling
network
A6R9
and A6C5.
Resistors
A10R3
and
A10R4
form
a voltage
divider
between
-f
100V and
the coupling
network
output.
A10R3
adjusts
the operating
level
of
FET
A10Q1.
High
voltage
fluctuations are
sensed
by
A10Q1
and amplified by
A10Q2
and
A10Q3.
Diodes
A10CR4
and
A10CR5
provide
clamping
action to prevent overdriving
the high
voltage
oscillator.
The
regulator
output
is
applied through
the regenerative
winding
of
A6T1
as bias to the base of
Q1, thereby
controlling high voltage
oscillator drive.
4-5B.
CALIBRATOR.
4-56.
The
calibrator
uses a
1-kHz
free-running
multivi-
brator.
A
voltage divider provides the output
at
two
amplitudes: 10 volts
and 250
millivolts.
The
calibrator
output
is
a
negative-going
waveform.
4-57. Transistors
A2Q1
and
A2Q2
oscillate
at
a
rate
determined by
the time constant of associated
RC
components.
A2CR1
disconnects the collector of
A2Q2
from
the negative discharge of
A2C3
and
A2Q2
cuts
off,
and
provides
a faster risetime.
Diodes
A2CR2
and
A2CR3
protect the
transistors
from
voltage
breakdown,
A
filter
network,
A2L1
and A2C4,
isolates the multi-
vibrator
from
the
—100V
supply.
4-58.
With
A2Q2
conducting, the voltage
divider
consisting of
A2R17,
A2R18
and
A2R19
effectively
divides the
-100V
supply
voltage,
The
values selected
for
these
resistors
permit the
output of
10V
and
250
mV,
These two
outputs
are available
at
the instrument front
panel
and
may
be used for
probe compensation
adjust-
ment and
horizontal or
vertical
sensitivity calibration
checks.
4-5