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1 Safety Precautions
1.1.
General guidelines
1. When servicing, observe the original lead dress. If a short circuit is found, replace all parts which have been overheated or
damaged by the short circuit.
2. After servicing, see to it that all the protective devices such as insulation barriers, insulation papers shields are properly
installed.
3. After servicing, make the following leakage current checks to prevent the customer from being exposed to shock hazards.
1.1.1.
Leakage current cold check
1. Unplug the AC cord and connect a jumper between the
two prongs on the plug.
2. Measure the resistance value, with an ohmmeter,
between the jumpered AC plug and each exposed metal-
lic cabinet part on the equipment such as screwheads,
connectors, control shafts, etc. When the exposed metal-
lic part has a return path to the chassis, the reading
should be between 1MΩ and 5.2MΩ.
When the exposed metal does not have a return path to
the chassis, the reading must be
1.1.2.
Leakage current hot check
(See Figure 1.)
1. Plug the AC cord directly into the AC outlet. Do not use
an isolation transformer for this check.
2. Connect a 1.5kΩ, 10 watts resistor, in parallel with a
0.15μF capacitors, between each exposed metallic part
on the set and a good earth ground such as a water pipe,
as shown in Figure 1.
3. Use an AC voltmeter, with 1000 ohms/volt or more
sensitivity, to measure the potential across the resistor.
1.2.
Caution for fuse replacement
.
5
4. Check each exposed metallic part, and measure the
voltage at each point.
5. Reverse the AC plug in the AC outlet and repeat each of
the above measurements.
6. The potential at any point should not exceed 0.75 volts
RMS. A leakage current tester (Simpson Model 229 or
equivalent) may be used to make the hot checks, leakage
current must not exceed 1/2 milliampere. In case a mea-
surement is outside of the limits specified, there is a pos-
sibility of a shock hazard, and the equipment should be
repaired and rechecked before it is returned to the cus-
tomer.
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