Sharp 14V-L70M Service Manual - Page 2

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14V-L70M
Service work should be performed only by qualified service technicians who are thoroughly familiar
with all safety checks and the servicing guidelines which follow:
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, remove 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.
IMPORTANT SERVICE SAFETY PRECAUTION
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 no use a picture tube other than that specified or
make unrecommended circuit modifications to the
high voltage circuitry.
6.
When
trouble
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.
2
shooting
and
taking
test