Panasonic CT-20SL15 - 20" CRT TV Manual de servicio - Página 3
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Special components are used in this television set which are important for safety. These parts are identified on the
schematic diagram by the symbol
these critical parts are replaced with the manufacturer's specified replacement part to prevent X-ray radiation, shock,
fire or other hazards. Do not modify the original design without the manufacturer's permission.
General Guidelines
An Isolation Transformer should always be used
during the servicing of a receiver whose chassis is not
isolated from AC power line. Use a transformer of
adequate power rating as this protects the technician
from accidents resulting in personal injury from electrical
shocks. It will also protect the Receiver from being
damaged by accidental shorting that may occur during
servicing.
When servicing, observe the original lead dress,
especially in the high voltage circuit. Replace all
damaged parts (also parts that show signs of
overheating.)
Always Replace Protective Devices, such as
fishpaper, isolation resistors and capacitors, and shields
after servicing the Receiver. Use only manufacturer's
recommended rating for fuses, circuits breakers, etc.
High potentials are present when this Receiver is
operating. Operation of the Receiver without the rear
cover introduces danger for electrical shock. Servicing
should not be performed by anyone who is not
thoroughly familiar with the necessary precautions when
servicing high-voltage equipment.
Extreme care should be practiced when Handling the
Picture Tube. Rough handling may cause it to implode
due to atmospheric pressure. (14.7 lbs per sq. in.). Do
not nick or scratch the glass or subject it to any undue
pressure. When handling, use safety goggles and heavy
gloves for protection. Discharge the picture tube by
shorting the anode to chassis ground (not to the cabinet
or to other mounting hardware). When discharging
connect cold ground (i.e. dag ground lead) to the anode
with a well insulated wire or use a grounding probe.
Avoid prolonged exposure at close range to unshielded
areas of the picture tube to prevent exposure to X-ray
radiation.
The Test Picture Tube used for servicing the chassis at
the bench should incorporate safety glass and magnetic
shielding. The safety glass provide shielding for the tube
viewing area against X-ray radiation as well as
implosion. The magnetic shield limits the X-ray radiation
around the bell of the picture tube in addition to the
restricting magnetic effects. When using a picture tube
test jig for service, ensure that the jig is capable of
handling 50kV without causing X-ray radiation.
Before returning a serviced receiver to the owner,
the service technician must thoroughly test the unit to
ensure that is completely safe to operate. Do not use a
line isolation transformer when testing.
Leakage Current Cold Check
Unplug the AC cord and connect a jumper between the
two plug prongs.
Important Safety Notice
and printed in BOLD TYPE on the replacement part list. It is essential that
Safety Precautions
A2-1
Measure the resistance between the jumpered AC plug
and expose metallic parts such as screwheads, antenna
terminals, control shafts, etc. If the exposed metallic
part has a return path to the chassis, the reading should
be between 240k
and 5.2M . If the exposed metallic
part does not have a return path to the chassis, the
reading should be infinite.
Leakage Current Hot Check
Plug the AC cord directly into the AC outlet. Do not use
an isolation transformer during the check.
Connect a 1.5k
10 watt resistor in parallel with a
0.15µF capacitor between an exposed metallic part and
ground. Use earth ground, for example a water pipe.
Using a DVM with a 1000 ohms/volt sensitivity or higher,
measure the AC potential across the resistor.
Repeat the procedure and measure the voltage present
with all other exposed metallic parts.
Verify that any potential does not exceed 0.75 volt RMS.
A leakage current tester (such a Simpson Model 229,
Sencore Model PR57 or equivalent) may be used in the
above procedure, in which case any current measure
must not exceed 0.5 milliamp. If any measurement is out
of the specified limits, there is a possibility of a shock
hazard and the Receiver must be repaired and
rechecked before it is returned to the customer.
AC VOLTMETER
TO INSTRUMENT'S
1500
EXPOSED METAL
PARTS
Figure 1. Hot Check Circuit
X-ray Radiation
WARNING: The potential source of X-ray radiation in
the TV set is in the High Voltage section and the picture
Note:
It is important to use an accurate, calibrated high
voltage meter.
Set the brightness and picture controls to Minimum.
Measure the High Voltage. The high voltage should be
±
30.0
1.5kV. If the upper limit is out of tolerance,
immediate service and correction is required to insure
safe operation and to prevent the possibility of
premature component failure.
(Fig. 1)
COLD
WATER
PIPE
(GROUND)
0.15µF
,10 W