3
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 specified parts to prevent X-RADIATION, shock, fire, 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.
1.1.1.
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 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 1M
Ω
and 5.2M
Ω
.
When the exposed metal does not have a return path to the chassis, the reading must be
1.1.2.
Leakage Current Hot Check
1. Plug the AC cord directly into the AC outlet. Do not use an isolation transformer for this check.
2. Connect a 1.5k
Ω
, 10 watts resistor, in parallel with a 0.15
μ
F capacitors, between each exposed metallic part on the set and a
good earth ground such as a water pipe, as shown in Figure 1-1.
3. Use an AC voltmeter, with 1000 ohms/volt 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 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 milliamp. 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 1-1
