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KDF-42WE655/50WE655
SAFETY CHECK-OUT
After correcting the original service problem, perform the following safety checks before releasing the set to the customer: 1. Check the area of your repair for unsoldered or poorly soldered connections. Check the entire board surface for solder splashes and bridges. 2. Check the interboard wiring to ensure that no wires are �pinched� or touching high-wattage resistors. 3. Check that all control knobs, shields, covers, ground straps, and mounting hardware have been replaced. Be absolutely certain that you have replaced all the insulators. 4. Look for unauthorized replacement parts, particularly transistors, that were installed during a previous repair. Point them out to the customer and recommend their replacement. 5. Look for parts which, though functioning, show obvious signs of deterioration. Point them out to the customer and recommend their replacement. 6. Check the line cords for cracks and abrasion. Recommend the replacement of any such line cord to the customer. 7. Check the B+ and HV to see if they are speci�ed values. Make sure your instruments are accurate; be suspicious of your HV meter if sets always have low HV. 8. Check the antenna terminals, metal trim, �metallized� knobs, screws, and all other exposed metal parts for AC leakage. Check leakage as described below. Leakage Test 2. A battery-operated AC milliampmeter. The Data Precision 245 digital multimeter is suitable for this job. 3. Measuring the voltage drop across a resistor by means of a VOM or battery-operated AC voltmeter. The �limit� indication is 0.75 V, so analog meters must have an accurate low voltage scale. The Simpson�s 250 and Sanwa SH-63TRD are examples of passive VOMs that are suitable. Nearly all battery-operated digital multimeters that have a 2 VAC range are suitable (see Figure A). The AC leakage from any exposed metal part to earth ground and from all exposed metal parts to any exposed metal part having a return to chassis, must not exceed 0.5 mA (500 microamperes). Leakage current can be measured by any one of three methods. 1. A commercial leakage tester, such as the Simpson 229 or RCA WT-540A. Follow the manufacturers� instructions to use these instructions.
How to Find a Good Earth Ground A cold-water pipe is a guaranteed earth ground; the cover-plate retaining screw on most AC outlet boxes is also at earth ground. If the retaining screw is to be used as your earth ground, verify that it is at ground by measuring the resistance between it and a cold-water pipe with an ohmmeter. The reading should be zero ohms. If a cold-water pipe is not accessible, connect a 60- to 100-watt trouble- light (not a neon lamp) between the hot side of the receptacle and the retaining screw. Try both slots, if necessary, to locate the hot side on the line; the lamp should light at normal brilliance if the screw is at ground potential (see Figure B).
To Exposed Metal Parts on Set
Trouble Light AC Outlet Box Ohmmeter Cold-water Pipe
0.15 F
1.5 K �
AC Voltmeter (0.75 V)
Earth Ground
Figure A. Using an AC voltmeter to check AC leakage.
