HP 182C - 4-40. Gate Amplifier Function; 4-48. High Voltage Power Supply (HVPS)

Theory
Model
182C
user. Table
4-1
lists the maximum current available from
each power supply to the
plug-in compartment of the
oscilloscope. There is no
minimum current requirement
for any supply.
Table 4-1
.
LVPS Current Capabilities
Power Maximum
Safe
Supply Current Available
+
100 VDC 160 mA
+
15 VDC
750 mA
-
12.6
VDC 750 mA
-100
VDC 80
mA
4
40. GATE
AMPLIFIER.
4-41,
The inputs to the gate
amplifier
are an
unblanking
gate
from the time base plug-in.
a
chopped blanking
signal from the vertical
amplifier
plug-in and an
externally
input Z-axis
signal. These three signals may be present
singly or simultaneously,
depending
on
control settings
and signals applied.
4 42.
The unblanking gate
is
first applied as a current to
A7Q1, a
common
base
amplifier, then combined in the
low impedance emitter circuit of A7Q5 with a current
established
by
the INTENSITY, FIND BEAM, and EXT
DISPLAY
front-panel controls. Depressing FIND BEAM
shunts
the adjustable INTENSITY potentiometer to
increase
emitter current and produce an
intensified
beam.
Setting the HORIZONTAL DISPLAY to EXT supplies
additional
current
from the —100V supply. This estab-
lishes an unblanking current level to compensate for
removal of the internal unblanking signal from the time
base
plug-in. and establishes
a
nominal brightness level.
Note
The intensification function of the FIND
BEAM switch
is
removed
on
instruments
withaPlI phosphor CRT (OptionOII).
Additional information is provided in
Section VII.
4-43. The output voltage
of
A7Q5
is coupled through
emitter follower A7Q6 to
complimentary amplifier A7Q7
and
A7Q8.
Diodes
A7CR1 through A7CR4 provide a
clamping
action
to
prevent overdriving
the amplifier.
4-44.
A large
negative feedback from the
collectors of
A7Q7
and A7Q8 ensures that the amplifier gain in very
stable. Capacitors A7C6 and A7C8
provide for adjustment
of the high
frequency
feedback
and
gain. Decreasing the
capacitance of A7C6
decreases the high frequency
feed-
back and increases
gain, while decreasing
the capacitance
of A7C8 increases
high
frequency
feedback and
decreases
gain. Amplifier voltage
gain is approximately
10
for
Z-axis signals. The
gate amplifier output
is approximately;
AEq
8 collector^
(A
ICR4)
(R21 +
R22)
4-45. The
gate amplifier
output unblanking
signal is
added to the
-3200V output of
the high voltage
power
supply and
applied to
the CRT control
grid.
Voltage
level changes
of the unblanking
signal
cause corresponding
changes
to the CRT control
grid voltage.
Diodes
A7CR6
through
A7CR9 provide
isolation
protection
from high
voltage transients from the
CRT control
grid.
4-46.
An
alternate
trigger signal is used
by multi-channel
vertical amplifier
plug-ins to initiate
channel switching
action.
Transistors
A7Q2
and
A7Q3 function
as a
fast
acting switch. With
A7Q2 normally conducting and
A7Q3
non-conducting,
the unblanking gate trailing
edge causes
A703 to corxiuct arrd
A7Q2 to cease conducting. The
switching output
is
differentiated
and applied to A7Q4,
providing
a negative-going voltage pulse for vertical
ampli-
fier
channel switching.
4-47.
Z-AXIS
INPUT. The input impedance
to the Z-axis
input
is approximately 5100 ohms.
An input signal of
approximately +2
volts amplitude
is adequate
to blank a
trace of normal
viewing intensity, while an
input signal of
-
2 volts will
provide unblanking.
Since the gate amplifier
has
a
voltage
gain of about 10
.
a 2-volt input will result
in
a 20-volt change at
the CRT grid.
4-48.
HIGH VOLTAGE
POWER
SUPPLY
(HVPS1.
4-49.
The HVPS
generates three regulated
voltages. These
are applied to the
cathode (-31 SOVj,
control grid
(-3200V)
and post-accelerator (+19kV) of
the CRT to provide
the
accelerating
potential required
to
produce
excitation of
the
CRT phosphor for
a
visible
trace. All three
voltages
are regulated
by sampling the
-3150-voll supply.
The
HVPS is ^own in
simplified
form in Figure 4-3.
Refer
to this figure,
and to the schematic
in Section VIII
while
reading
the
following
explanation
of HVPS
operation.
4-50. HIGH VOLTAGE
OSCILLATOR. Chassis mounted
transistor
Q1
and transformer A6A1T1
form an oscillator
which generates approximately 26 Vac at 40
kHz. A feed-
back
winding on the transformer provides the regenerative
coupling to sustain oscillation.
Operating power is pro-
vided
by
the
unregulated +27V
supply. The supply source
is
fused
and decoupled.
4-4