Sharp 20LK31M, Service Manual

Page: 2 / 30

2
20LK31M
20LK61M
IMPORTANT SERVICE SAFETY PRECAUTION
Ë
Service work should be performed only by qualified service technicians who are throughly
familiar with all safety checks and the servicing guidelines which follow:
X-RADIATION AND HIGH VOLTAGE LIMITS
1. Be sure all service personnel are aware of the
procedures and instructions covering X-radiation.
The only potential source of X-ray in current solid
state TV receivers is the picture tube. However, the
picture tube does not emit measurable X-Ray
radiation, if the high voltage is as specified in the
"High Voltage Check" instructions.
It is only when high voltage is excessive that X-
radiation is capable of penetrating the shell of the
picture tube including the lead in the glass material.
The important precaution is to keep the high voltage
below the maximum level specified.
2. It is essential that servicemen have available at all
times an accurate high voltage meter.
The calibration of this meter should be checked
periodically.
3. High voltage should always be kept at the rated value
-no higher. Operation at higher voltages may cause
a failure of the picture tube or high voltage circuitry
and;also, under certain conditions, may produce
radiation in exceeding of desirable levels.
4. When the high voltage regulator is operating properly
there is no possibility of an X-radiation problem.
Every time a color chassis is serviced, the brightness
should be tested while monitoring the high voltage
with a meter to be certain that the high voltage does
not exceed the specified value and that it is regulating
correctly.
5. Do not use a picture tube other than that specified
or make unrecommended circuit modifications to the
high voltage circuitry.
6. When trouble shooting and taking test
measurements on a receiver with excessive high
voltage, avoid being unnecessarily close to the
receiver.
Do not operate the receiver longer than is necessary
to locate the cause of excessive voltage.
WARNING
1. For continued safety, no modification of any circuit
should be attempted.
2. Disconnect AC power before servicing.
3. Semiconductor heat sinks are potential shock
hazards when the chassis is operating.
4. The chassis in this receiver has two ground systems
which are separated by insulating material. The non-
isolated (hot) ground system is for the B+ voltage
regulator circuit and the horizontal output circuit. The
isolated ground system is for the low B+ DC voltages
and the secondary circuit of the high voltage
transformer.
To prevent electrical shock use an isolation
transformer between the line cord and power
receptacle, when servicing this chassis.
SERVICING OF HIGH VOLTAGE SYSTEM
AND PICTURE TUBE
When servicing the high voltage system, re-
move the static charge by connecting a 10k
ohm resistor in series with an insulated wire
(such as a test probe) between the picture
tube ground and the anode lead. (AC line cord
should be disconnected from AC outlet.)
1. Picture tube in this receiver employs integral
implosion protection.
2. Replace with tube of the same type number for
continued safety.
3. Do not lift picture tube by the neck.
4. Handle the picture tube only when wearing
shatterproof goggles and after discharging the high
voltage anode completely.