Zenith C27A25, Service Manual

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CM154 XC - SAFETY
PRODUCT SAFETY SERVICING GUIDELINES FOR AUDIO-VIDEO PRODUCTS
ii
SHUTDOWN CIRCUIT THEORY OF OPERATION
The HV shutdown network feeds the shutdown or X-ray
protection input, pin 29, of ICX2200 video processor
IC. To perform the shutdown or activate the X-ray pro-
tection requires g3.5 VDC or more (threshold is
3.5 VDC) on pin 29.
A pulse voltage is derived from the sweep transformer
(TX3201 pins 5 & 6) and applied to a half wave rectifier
circuit (DX3001 & CX3001). This DC voltage is supplied
to a precision resistor divider, which sets the HV shut-
down point.
This voltage is applied to the Zener diode ZDX3002 12.0V.
The resulting voltage from the Zener diode and the gen-
eral-purpose diode is supplied to a DC filter (CX3000
RX3002) and finally to pin 29
through RX3000M.
The Zener Diode’s function is to keep a constant voltage
on its terminals. Since the Zener is in series with the
ICX2200 (see the figure below) an excess of voltage will
appear on pin 29. So, if the voltage coming from the
sweep increases drastically (as the HV does), the thresh-
old voltage on pin 29 of ICX2200 should be reached and
the TV set will turn off immediately.
NOTE: If a SHUTDOWN test is performed, the
microcontroller will turn the TV off. The set will
not turn on until it is unplugged and then
plugged back in (resets the microcontroller).
The table below contains specifications for the Shut-
down Circuit:
XC CHASSIS
SHUT DOWN CIRCUIT
Video Processor
ICX2200
Rl2
RX3001M
RIN
P29
C
CX3000
GND
ZDX3004
DX3005
RL1
RX3002
FROM_RESISTOR_DIVIDER
SCREEN
SIZE
HV NOM
KV
HV MAX
KV
PHASE III HV
SHUTDOWN LIMIT
ZERO BEAM
27 28.0 30.0 36.0
32 30.0 32.0 37.0
36 30.0 32.0 37.0