Zenith TVBR1342Z Service Manual page 3

SERVICING GUIDELINES FOR X-RADIATION PROTECTION
X-RAY PROTECTION CIRCUIT DESCRIPTION
If the High Voltage on Anode off (FB401) F.B.T. is increased, the
Heater voltage will increase at the same time. The voltage from
Heater is rectified by D412, and then the dc voltage is divided by
R420 and R442. At normal operation, the voltage V1 on pin 85
of IC-I1001 is 5.9V. If HV exceeds its safety level, Q1023 is
switched on, resulting in V1 loss on pin 85 this in turn causes a
loss of horizontal drive to q405, thereby shutting down the unit,
activating H.V. Shut Down. The X-Ray protect line also monitors
the B+ line via D408 which is fed by a voltage divider circuit
consisting of R441,R440 and R439.
CRT ANODE HIGH VOLTAGE MEASUREMENT PROCEDURE
Each screen size has its own safe Anode Voltage and shutdown
voltage. Critical Safety Components (designated with shading in the
component parts list) are designed to operate the CRT at a safe
operating Anode voltage and provide proper shutdown thresholds.
If replacement of any of these components are deemed necessary, it
is important to use original Zenith replacement components. After
replacement is made, confirm proper Anode Voltage using the fol-
lowing procedure.
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 the CRT is discharged, connect a high impedance high
voltage meter at the distribution point. Turn the television on
CM152
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. If the voltage reading is higher than the maximum,
verify circuit component values and proper operation.
SCREEN B+ VDC
SIZE (0 BEAM)
COMPONENT LEVEL REPAIR
13 128.5+/-2%
19 128.5+/-2%
25 128.5+/-2%
EQUIPMENT REQUIRED
a) Video Generator
b) HV DC meter (0 to 40kv, high Z)
TEST PROCEDURE
To verify the Shutdown Circuit is working properly, proceed with the
following procedure.
1. Istall a 100K ohm variable resistor across R529.
2. Turn the unit on and set the input A.C. Voltage to 130VAC.
3. Adjust the variable resistor from maximum to minimum. During this
adjustment monitor the anode voltage to the C.R.T. this voltage
should not exceed the maximum H.V. listed in the chart above. If
this voltage exceeds the maximum H.V. for the size C.R.T. listed,
the X-Ray protect circuit is not working properly and must be
repaired prior to returning the unit to the customer.
ii
HV NOM HV MAX
KV KV
26kv
29kv
28kv 33kv
31.5kv 34kv
H13C01L - SAFETY