HP 8414A Operating And Service Manual page 96

Section VIII
Service
SCHEMATICS. CIRCUIT DESCRIPTION
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 the 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 when the Network
Analyzer is phase locked. 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, A1Q1 con¬
ducts which turns A1Q2 on. The voltage at A1TP8
goes more negative and the CRT is blanked.
A negative marker pulse turns switch A1Q4 on, its
output voltage at A1TP8 approaches ground and
appears as a positive-going input signal to the fol¬
lowing stage. This signal is more positive than the
unblanking signal; therefore, the CRT's electron
beam is intensified.
Differential Amplifier
A1Q3 and Q6 form a differential amplifier. The
amplifier gain varies with position of the front-
panel 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 at A1TP7
should be about +65V for a blanked condition,
about +70V for an unblanked condition, and about
+105V for a marker condition.
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