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...al/Temp/Rar$EXa0.580/viewing/SGML_VIEW_DATA/ALL/HC-MDH3PP/SVC/VM1803004CE/doc/VM1803004CE_01.xml[11/12/2018 4:08:11 PM]
1 Safety Precautions
1.1 General Guidelines
1.
IMPORTANT SAFETY NOTICE
There are special components used in this equipment which are important for safety. These parts are marked by
in the Schematic
Diagrams, Circuit Board Layout, Exploded Views and Replacement Parts List. It is essential that these critical parts should be replaced with
manufacturer’s specifed parts to prevent X-RADIATION, shock, fre, or other hazards. Do not modify the original design without permission
of manufacturer.
2. An Isolation Transformer should always be used during the servicing of AC Adaptor whose chassis is not isolated from the 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 AC Adaptor from being damaged by accidental shorting that may occur during servicing.
3. 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.
4. After servicing, see to it that all the protective devices such as insulation barriers, insulation papers shields are properly installed.
5. After servicing, make the following leakage current checks to prevent the customer from being exposed to shock hazards.
1.2 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 metallic cabinet part on the
equipment such as screwheads, connectors, control shafts, etc. When the exposed metallic part has a return path to the chassis, the
reading should be between 1 MΩ and 5.2 MΩ. When the exposed metal does not have a return path to the chassis, the reading must be
infnity.
1.3 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 1.5 kΩ, 10 W resistor, in parallel with a 0.15 µF capacitor, between each exposed metallic part on the set and a good earth
ground, as shown in Figure. 1.
3.
Use an AC voltmeter, with 1 kΩ/V or more sensitivity, to measure the potential across the resistor.
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 V 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 mA. In case a measurement is outside of the limits specifed, there is a
possibility of a shock hazard, and the equipment should be repaired and rechecked before it is returned to the customer.
Figure. 1
