Crt Circuit - Tektronix 221 Instruction Manual

Circuit Description—221 Service

CRT Circuit

The CRT circuit provides the high voltage and control
circuits necessary for operation of thecrt. A block diagram
of the CRT circuit is shown in Fig. 4-5. A schematic is
shown on diagram 5.
A network consisting of CR733, C733, CR734, C734,
CR735, and CR736 is connected as a voltage doubler that
provides approximately 1100 volts to the crt circuit. The
1100 volts is connected to a 110 V Zener diode VR738,
which provides a 1000 volts supply to the Intensity circuit.
It is further divided by R734 and R735to provide the Focus
control voltage. The 1000 volt supply is also returned to
4-8
Fig. 4-5. Simplified block diagram of the CRT circuit.
the Secondary Inverter stage and serves as a reference for
the regulating circuit.
Deflection plate blanking is used for controlling the crt
beam. When an unblanking signal is received, Q754 turns
on and forward biases CR756, which lowers the deflection
blanking voltage to approximately 40 volts, which allows
the electron beam to pass to the crt screen. The CRT GRID
BIAS adjustment R739 controls the maximum usable
intensity. The TRACE ROTATION adjustment R751 con­
trols the current through L751, which affects both the
vertical and horizontal rotation of the crt beam. FOCUS
control R733, used in conjunction with ASTIGMATISM
adjustmenet R752, is adjusted to obtain a well-defined
display. UNBLANKING adjustment R753 is adjusted for
optimum trace brightness.
REV. B, AUG 1974