Vdc Supply; Rgb Drive And Cutoff Voltages; Composite Drive; Composite Vertical Sync - Zenith ZVM-131 Service Manual

+26 VDC Supply
Transformer T3201 located on the main board is used as
a start-up transformer to provide the necessary voltage to
get the horizontal oscillator operating. AC current from the
secondary winding of T3201 is rectified by diode CR3224.
Only the positive alternations through CR3224 produce
voltage as the negative alternations are shunted to ground
by CR3219. This half wave rectifier circuit supplies +26
VDC to the sync processor IC3401. An oscillator within the
sync processor then begins to operate, sending the hori-
zontal drive signal to pin 5 of IC3401.
+
12 VDC Supply
The regulated + 12 VDC supply is derived from the + 26
VDC supply and is used to power most of the transistors
found on the main board. Transistor 03200 is the active
regulator whose emitter supplies 12.7 VDC. Current flow
through CR3205 produces a proportional voltage drop
across the voltage divider comprised of resistors R3203
and R3202. The base of transistor 03201 is connected to
the middle of the voltage divider and therefore detects
changes in output voltage at the emitter of 03200. If the
output voltage decreases, the current through R3203 and
R3202 decreases and therefore the voltage at the base of
03201 decreases. When this occurs, conduction through
03201 decreases effectively raising the voltage at the
base of 03200. Regulator 03200 is driven harder, thereby
increasing its output voltage and current-delivering capabil-
ity.
If the output voltage rises above + 12 VDC, the current
through resistors R3203 and R3202 increases and causes
the voltage at the base of 03201 to increase. This transis-
tor is forced to conduct harder which lowers the voltage at
the base of 03200, and therefore its output voltage at the
emitter.
RGB Drive and Cutoff Voltages
The RGB signals are processed by the RGB gain control
board and are then interfaced to the main board through
connector E.
Circuit Descriptions
The RED signal is supplied to the base circuit of emitter
follower transistor 02501. During its quiescent state, the
base of 02501 is biased to approximately 11.3 volts as de-
termined by the voltage divider comprised of resistors
R2501 and R2502.
When an active-low RED signal drives the base of 02501,
the emitter of 02501 reproduces the signal and drives the
base of 02502 through potentiometer R2506. This poten-
tiometer establishes the bias voltage at the base of 02502
and therefore is used to vary the RED signal drive. The
RED signal cutoff voltage is adjusted by potentiometer
R2505 which is located in the emitter circuit of 02502.
The collector of 02502 conducts the inverted RED signal
to the base of emitter follower transistor 02507 which
drives the signal through resistor R2550 to pin 4 of con-
nector A.
The BLUE and GREEN signal drive circuits function in
exactly the same way as the RED signal circuits.
Composite Drive
The composite signal at pin 2 of connector C goes to the
base of emmitter follower transistor 03404. Capacitor
C3462, in conjunction with zener diode CR3409, protects
the base of 03404 from over-voltage conditions. Resistors
R3467, R3468, and R3470 determine the bias voltage on
the base. The composite signal goes from the emitter of
03404 to pin 27 of the sync processor (IC3401). Pin 16
of the sync processor drives the resulting signal to the
base circuit of 03405. The collector of 03405 supplies
each of the drive level adjustment potentiometers with the
signal.
Composite Vertical Sync
The sync processor adjusts the vertical signal with respect
to the horizontal signal, to provide properly synchronized
CRT scan.
If either positive or negative vertical sync is configured, the
Signal is supplied from pin 2 of connector E to the base
of common emitter transistor 03401. The inverted signal at
the base of 03401 goes to the base of common emitter
03402 and is then applied to pin 28 of the sync processor.
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