Tektronix T921 Instruction Manual page 168

Circuit Description-T921/T922/T922R
XI . 1
Diagrams 7, 8, 9, and associated circuit descriptions pertain to circuitry that is used only in the T922R and is not present in
the T921 and T922 bench versions . For circuit descriptions and diagrams of circuitry common to the T921, T922, and T992R,
refer to diagrams 1 through 6 .
SCALE ILLUMINATION AND
+32 VOLT POWER SUPPLY
The T921 and T922 bench versions use a dropping resistor
R784 and Zener diode VR784 to drop the +100 volt supply to +33
volts for use in the Vertical Output Amplifier and crt circuits (see
Fig . 7-4) . In the T922R, the Scale Illumination circuit provides a
+32 volt supply in place of the +33 volt supply .
VR818 holds the base of Q812 at +33 volts . This holds the
output voltage at about +32 volts . When the SCALE ILLUM
control (R812)
is
set fully counterclockwise, Q816 and Q818 are
biased off and transistor Q812 supplies all the current drawn by
the +32 volt supply . To increase Scale Illumination, R812 is
adjusted to forward bias 0816 and 0818 . As this forward bias
increases, the current through them increases . This causes a
corresponding decrease in the current through Q812 . The
increased current through Q816 increases the current through
DS816 and DS818, which increases scale illumination .
R784
10 WATT
REV . A, JUNE 1976
+100V
+33V I
VERTICAL
OUTPUT
AMPLIFIER
1982-63
Fig. 7-4. Bench version T922 +33 volt power supply .
T922 R
VERTICAL SIGNAL OUT
(COMP VERT SIG OUT)
Fig . 7-5 shows a simplified block diagram of the Vertical Signal
Out circuit . The Vertical Signal Out circuit supplies a sample of
the signal present in the Vertical Output Amplifierto J169 (COMP
VERT SIG OUT) on the rear panel .
Q160 and Q162 are connected as a differential amplifier . The
paraphase signal from the emitters of 0112 and Q122 (see
diagram 1) is applied totheinputsofthedifferential amplifier (the
bases of Q160 and Q162 respectively) . The output of the
differential amplifier is taken from the collector of Q162 and
applied to the base of Q166 . Transistor Q166 amplifies the output
signal and shifts its do level . The signal from the collector of Q166
is connected (through P168) to the COMP VERT SIG OUT
connector, J169 .
Fig . 7-5 . Simplified block diagram of the vertical output amplifier
circuit .