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1. 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 injuryfrom 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 fish paper, 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 precautionswhen 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, usesafety 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 wellinsulated 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 shieldlimits 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 A.C. 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 returnpath 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 0.15
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 0.5 milliampere.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.
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