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 essen- tial that these critical parts should be replaced with manu- facturer’s specified parts to prevent
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 acci- dents resulting in personal injury from electrical shocks. It will also protect AC Adaptor from being damaged by acci- dental 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 over- heated 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 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 1 MΩ and 5.2 MΩ. When the exposed metal does not have a return path to the chassis, the reading must be infinity.
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 volt- age 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 equiva- lent) may be used to make the hot checks, leakage cur- rent must not exceed 1/2 mA. In case a measurement is outside of the limits specified, 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
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