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1 Safety Precautions
1.1.
General Guide
1. It is advisable to insert an isolation transformer in the AC supply before servicing this hot chassis.
2. When servicing, observe the original lead dress, especially the lead dress in the high voltage circuits. If a short circuit is found,
replace all parts which have been overheated or damaged by the short circuit.
3. After servicing, see to it that all the protective devices such as insulation barriers, insulation papers, shields, and isolation R-C
combinations, are properly installed.
4. When the receiver is not to be used for a long period of time, unplug the power cord from the AC cord outlet.
5. Potential, as high as 31.7 kV. is present when this receiver is in operation. Operation of the receiver without the rear cover
involves the danger of a shock hazard from the receiver power supply. Servicing should not be attempted by anyone who is not
thoroughly familiar with the precautions necessary when working on high voltage equipment. Always discharge the anode of the
picture tube to the receiver chassis before handling the tube. 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. Fig. 2.
2. Turn on the receiver's power switch.
3. Measure the resistance value, with an ohmmeter, between the jumper AC plug and each exposed metallic cabinet part on the
receiver, such as screw heads, aerials, connectors, control shafts, etc. When the exposed metallic part has a return path to the
chassis, the reading should be between 4 MΩ Ω Ω Ω and 20 MΩ Ω Ω Ω . When the exposed metal does not have a return path to the chassis,
the reading must be zero.
1.3.
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 2kΩ, 10W resistor, in series with an exposed metallic part on the receiver and an earth such as a water pipe.
3. Use an AC voltmeter, with high impedance type, to measure the potential across the resistor.
4. Check each exposed metallic part, and measure the voltage at each point. Fig. 3.
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 1.0V rms.
In the case of a measurement being outside of these limits specified, there is a possibility of a shock hazard, and the receiver
should be repaired and rechecked before it is returned to the customer. Fig. 4.
Fig. 1
Fig. 2
3
TX-25P20T
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