HP 8414A Operating And Service Manual page 75

SI
Switch SI, in the test position, opens the signal
path from the filter output so that an external volt¬
age can be applied at the rear-panel horizontal and
vertical output connectors for troubleshooting the
deflection circuits.
Vertical and Horizontal Deflection Drivers
The deflection drivers are differential amplifiers
with push-pull outputs. Both output voltages of
each driver are at about +55 Vdc with no input
signal applied. A IV change at either driver's input
should provide about a 10V change in each of its
outputs, one output going 10V more positive the
other 10V less positive. The front-panel horizontal
and vertical centering controls vary a dc bias to the
associated driver producing the same effect as an
input signal.
Intensity Modulator Inputs
The Intensity Modulator controls the CRT grid to
cathode bias. Intensity Modulator input signals
cause the CRT's electron beam to be turned off for
blanking or intensified for frequency markers.
There are three signal inputs to the intensity mod¬
ulator: one from the Network Analyzer mainframe
which unblanks the CRT (8410A Network Ana¬
lyzers unblank the CRT only when the Network
Analyzer is phase locked); the second, a rear-panel
connector which may be connected to the sweep
oscillator blanking output to blank the display dur¬
ing sweep retrace; the third, another rear-panel
connector which may be connected to a sweep
oscillator frequency marker output to display fre¬
quency marks on the display by brightening the
display at the point which represents the frequency
of interest.
Switches
Switch A1Q2 is turned off by an unblanking signal
from the Network Analyzer. Although the output
of A1Q2 at A1TP8 is always a negative voltage,
when A1Q2 is turned off its output appears
positive-going to the following stage and the CRT
is unblanked. Switch A1Q1 is normally off. When a
positive blanking pulse is applied to its input, Q1
conducts. A1Q2 turns on, the voltage at A1TP8
goes more negative, and the CRT is blanked. A
negative marker pulse to switch A1Q4 turns Q4 on.
Its output voltage at A1TP8 approaches ground
and appears as a positive-going input signal to the
following stage. This signal is more positive than
the unblanking signal, therefore, the CRT's elec¬
tron beam is intensified.
Differential Amplifier
A1Q3 and Q6 form a differential amplifier. The
amplifier gain varies with position of the intensity
control; however, with the intensity control set for
normal intensity (about -3.6V at A1TP5) and the
intensity limit set for about +110V at A1TP2 the
amplifier gain is about seven or eight. For example,
if the voltage at A1TP8 changes from -5V (Q2 and
Q4 off) to -0.1V (Q2 off Q4 on), a change of about
5V, the output at A1TP3 should change from
about +70 to about +105V, a change of about
35V.
Emitter Follower
The output of emitter follower (A1Q7) should be
about +65V for a blanked condition, about +70V
for an unblanked condition, and about +105V for
a marker condition.
CRT, CRT Power Supplies
+150V Power Supply.
The +150V power supply is
a regulated supply. Its output provides collector
voltage for the deflection drivers.
High Voltage Oscillator.
The high voltage oscil¬
lator is a free-running multivibrator whose frequen¬
cy, 20 kHz ±2 kHz, is determined by the L and C
of Tl's primary winding.
CRT Cathode Supply.
One of Tl's secondary
windings supplies power to both the CRT's cath¬
ode supply and anode supply. The cathode supply
consists of a half wave rectifier and pi section
filter. Its output voltage is about -2450 Vdc. In
addition to providing dc bias to the CRT cathode,
the cathode supply's output voltage is used in a
voltage divider to +150V to provide about -2000
Vdc to the focus control.
CRT Anode Supply.
The anode supply consists of
a voltage doubler and pi section filter. Its output
voltage is about +5100 Vdc.