Service manual
CB Chassis Safety Guidelines
X-RADIATION
To prevent possible exposure to radiation caused by excessive CRT
Anode voltage, the CB Chassis incorporates a High Voltage Shut-
down circuit. This circuit senses the level of flyback pulse from
Flyback Transformer representative of the actual high voltage on
the CRT anode. When this level exceeds a predetermined voltage,
the circuit shuts down the horizontal drive, preventing further gen-
eration of anode voltage. In this condition, the horizontal drive is
latched off. The drive will remain off until the microprocessor is
reset.
SHUTDOWN CIRCUIT OPERATION
This shutdown circuit operates by means of sample HV taken from
pin 6 of the sweep transformer TX3204. This sample of voltage is
rectified, filtered and compared with a reference voltage(fixed by
ZDX3004 by the transistor QX3002. When the HV reaches the maxi-
mum permitted voltage, the transistor QX3002 enters in conduction
mode like consequence the transistor QX3001 also enters in conduc-
tion. When QX3001 enters in conduction, the flyback pulses at the
entrance of the video processor are 1 volt or less,the the TV goes out
and enters in shutdown mode.
When the shutdown circuit is operating, the microcontroller will
prevent the TV from being turned on again unless the microcontroller
has been restarted by disconnecting the TV from the AC line.
CRT ANODE HIGH VOLTAGE MEASUREMENT PROCEDURE
Each CRT screen size has its own safe operating Anode Voltage and
shutdown voltage. Critical Safety components (designated with an
X in the component designator) are designed to operate the CRT at
a safe operating Anode voltage and provide proper shutdown thresh-
olds . If replacement of any of these components are deemed neces-
sary, it is important to use original type Zenith replacement compo-
nents. After replacement is made, confirm proper Anode voltage
using the following procedure.
NOTE: The CB chassis does not have a bleeder resistor to discharge
the Anode voltage. High voltage can remain on the CRT
Anode long after power is turned off. Before removing the
CRT anode connection, turn off and unplug the television,
then discharge the CRT Anode to DAG ground.
Measurement of the CRT Anode voltage must be performed using a
high impedance high voltage meter, with no visible raster on the
screen, and operating at nominal horizontal scanning frequency.
Connect a strong broadcast signal (or TV signal generator operating
at 15.734kHz horizontal scanning rate) to the RF input.
After discharging the CRT, connect a high impedance high voltage
meter to the CRT anode. Turn the television on and confirm a good
signal is being displayed . Reduce Brightness and Contrast settings
until the picture is well extinguished.
Observe the Anode voltage meter reading and compare with the
table below for the proper CRT screen size. If the voltage reading is
higher that the maximum, verify circuit component values and proper
operation.
7'6))2 &:(' ,:231 ,:1%<
7->) &)%1 /: /:
'31432)280):)06)4%-6
13(90)0):)06)4%-6320=
SAFETY CIRCUIT TEST PROCEDURE
This shutdown circuit operates by means of a sample of HV taken
from pin 6 of the sweep transformer TX3204. This sample of voltage
is rectified, filtered and compared with a reference voltage ( fixed by
ZDX3004) by the transistor QX3002. When the HV reaches the maxi-
mum permitted voltage, the transistor QX3002 enters in conduction
mode; like consequence the transistor QX3001 also enters in con-
duction.
When QX3001 enters in conduction, the flyback pulses at the en-
trance of the video processor (pin 18) are attenuated. If this
atenuattion is so that the flyback pulses at the entrance of the video
processor are 1 volt or less, then the TV goes out and enters in
shutdown mode.
When the shutdown circuit is operating, the microcontroller will
avoid that the TV is turned on again unless the microcontroller has
been restarted by disconnecting the TV from the AC line.
Equipment required:
a) Video Generator.
b) HV DC meter (0 to 40 KV, high Z).
c) External variable power supply (0V to 200 VDC @ 5Amps mini-
mum).
d) 1 Giga ohm, 5%, 2W film resistor.
Note: The external power supply may require a diode for blocking
voltage from the chassis power supply to the external power
supply. The diode should be connected between the positive
output of the external power and B+ of the chassis. The
cathode should be facing the B+ of the chassis. The
recommended diode is Part No. 103-00339-04A (400 V of
VRRM @ 3 Amps of average rectified forward current).
SHUTDOWN SAFETY CIRCUIT
CB-i
RX3211
1
470 pfd
CX3002
DX3002
5.6K
RX3005
1.5K
RX3003
PROV
RX3006
PROV
RX3004
QX3002
RX3002M
33K
CX3203
47uf
RX3007M
8.2K
ZDX3400
DX3001
RX3001M
10K
CX3001M
0.01
QX3001
+9VSW
0.1 / 25v
C2241M
PROV
R2223M
VIDEO PROCESSOR
D2205
R2224M
8.2K
R2222M
4.7K
PROV
C2215M
PROV
C2214M
IC2200
FROM SWEEP TRANSFORMER
FROM FLYBACK
PULSE
HP / XRAYPOT