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Important Safety Notice
Special components are used in this television set which are important for safety. These parts are identified on the
schematic diagram by the symbol
and printed in BOLD TYPE on the replacement part list. It is essential that
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.
Safety Precautions
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 50.0kV 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.
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
Ω
and 5.2M
Ω.
If the
exposed metallic part does not have a return path to
the chassis, the reading should be infinite.
Leakage Current Hot Check
(Fig. 1)
Plug the AC cord directly into the AC outlet. Do not use
an isolation transformer during the check.
Connect a 1.5k
Ω
10 watt resistor in parallel with a
0.15
µ
F 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 1/2 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.
X-ray Radiation
WARNING: The potential source of X-ray radiation in the
TV set is in the High Voltage section and the picture tube.
Note: It is important to use an accurate, calibrated
high voltage meter.
Set the brightness, picture, sharpness and color
controls to Minimum.
Measure the High Voltage. The high voltage should be
30.55kV ± 1.25kV for 25in models and 28.80kV ±
1.25kV for 20in models. 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.
0.15
µ
F
COLD
WATER
PIPE
(GROUND)
TO INSTRUMENT’S
EXPOSED METAL
PARTS
1500
Ω
,10 W
AC VOLTMETER
Figure 1. Hot Check Circuit
