Power Supply; High B + Indicating Circuit; Video And Highlight Driver Circuits; Horizontal Sweep - Zenith ZVM-1240 Service Manual

Refer to the following circuit descriptions for more in-
formation.
Power Supply
The following description is written for the ZVM-1240.
The ZVM-1240-E oper,ates similarly.
The full-wave bridge rectifier circuit comprised of
diodes CRX501 through CRX504 rectify the AC volt-
age when power switch SX501 is closed. Capacitor
CX507 develops approximately + 155 VDC with 1.7
Vpp typical ripple (+335 VDC for the -1240-E and
-1240-EM versions). CX508 and CX509 provide addi-
tional filtering. The DC potential of + 155 volts is ap-
plied to pin 12 of transformer TX502. Resistors RX505
and RX510, and capacitor CX513 provide a self-start-
ing bias circuit for transistor OX501 .
When a voltage develops at the output of the bridge,
the base of OX501 is driven positive, forcing the tran-
sistor into conduction. As a current path is established
through OX501, pin 10 of TX502 approaches chassis
ground potential. As current passes through the pri-
mary of TX502, pins 10 to 12, a magnetic field induces
voltage into both secondary windings.
The voltage induced into the winding tied to the emitter
of OX501 -gradually goes positive until conduction
through the transistor stops. When conduction stops,
the field collapses and the collector-tuned circuit rings
at a frequency determined by CX512 and the primary
of TX502. This keeps OX501 reverse biased. When
the sine-wave voltage at the collector of OX501 tries
to go negative, it induces a negative voltage at pin
7 of TX502 and the transistor begins to conduct to
saturation again.
The voltage induced into the winding, pin 1 to pin 2
of TX502 is rectified by CRX506 and filtered by CX511
to provide a + 24 VDC input to the emitter of OX502.
This transistor, along with OX503, OX504, and as-
sociated circuitry, form the power supply regulator.
RX503 is the B + adjustment and should be set for
+ 16.2 VDC output. CRX505 is the power indicator.
Circuit Description
High B
+
Indicating Circuit
If the B + voltage exceeds a certain level, an exces-
sively high voltage can develop and result in damage
to the monitor. Zener diode CR302 and associated
components prevent the vertical sync pulses from
being coupled through capacitor C316 if the excess
voltage condition develops. The lack of sync pulses
makes the monitor unusable.
Video and Highlight
Driver Circuits
The video signal is fed through resistor R212 to the
base of 0201, the video driver amplifier. The highlight
input Signal is fed through resistor R221 to the base
of 0203, the highlight amplifier. These are TTL-level,
positive-polarity input signals.
The two signals are amplified and inverted by 0201
and 0203 respectively. The signals are combined at
the junction of R222 and 0201 collector. The Contrast
control, R402, is connected to the base of 0204 to
vary the bias voltage, thus controlling the signal current
through 0204 and 0202. 0202, the video output
amplifier, drives the cathode of the CRT, VX201.
Horizontal Sweep
The operation of the horizontal processor, IC101 (221-
141-01), is the same as the 221-86-01, and they are
interchangeable in the ZVM-1240 and ZVM-1240-E
monitors. The integrated circuit has four distinct circuit
configurations; phase detector, oscillator, regulator,
and predriver.
Phase
Detector-The phase detector is comprised of
a differential amplifier and a gated current source. The
current source is strobed by a negative sync signal
that is AC coupled to pin 3 of IC1 01.
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