Zenith C27A25 Service Manual page 3

PRODUCT SAFETY SERVICING GUIDELINES FOR AUDIO-VIDEO PRODUCTS
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 getting +3.5 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.
Video Processor
ICX2200
P29
RX3001M
CM154
SHUT DOWN CIRCUIT
RIN
DX3005
C
Rl2
CX3000
GND
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:
SCREEN HV NOM
SIZE KV
27 28.0
32 30.0
36 30.0
XC CHASSIS
ZDX3004
FROM_RESISTOR_DIVIDER
RL1
RX3002
ii
PHASE III HV
HV MAX
SHUTDOWN LIMIT
KV
ZERO BEAM
30.0 36.0
32.0 37.0
32.0 37.0
XC - SAFETY