HP 182C - High Voltage

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