JVC AV20NMG3B
Service Manual

AV-20N3 AV-20NMG3 AV-20NMG3B

SAFETY PRECAUTIONS
1. The design of this product contains special hardware, many circuits and components specially for safety purposes. For continued protection, no changes should be made to the original design unless authorized in writing by the manufacturer. Replacement parts must be identic al to thos e used in the original circuits. Service should be performed by qualified pers onnel only. 2. Alterations of the design or circuitry of the products should not be made. Any design alterations or additions will void the manufacturer's warranty and will further relieve the manufacturer of responsibility for personal injury or property damage resulting therefrom. 3. Many electrical and mechanical parts in the products have special safety-related characteristics. T hese characteristics are often not evident from visual inspection nor can the protection afforded by them necessarily be obtained by using replacement components rated for higher voltage, wattage, etc. Replacement parts whic h have these special s afety characteristics are identified in the parts list of Servic e manual. Electrical components having su ch features ar e identified by shading on the schematics and by (! ) on the parts list in Service ! manual. The us e of a substitute replacement which does not have the same safety characteristics as the recommended replac ement part shown in the parts list of Service manual may cause shock, fire, or other hazards . 4. Do n't shor t between the LIVE side ground and ISOL ATED (NEUTRAL) side ground or EARTH side ground when repairing. Some model's power circuit is partly different in the GND. The differenc e of the GND is shown by the LIVE : (") side GND, the ISOLATED(NEUTRAL) : (#) side GND and EARTH : ($) side GND. Don't short between the LIVE side GND and ISOLATED(NEUTRAL) side GND or EARTH side GND and never measure with a measuring apparatus (oscilloscope etc.) the LIVE side GND and ISOLATED(NEUTRAL) side GND or EARTH side GND at the s ame time. If above note will not be kept, a fuse or any parts will be broken. 5. If any repair has been made to the chassis, it is recommended that the B1 setting should be checked or adjusted (See ADJUSTMENT OF B1 POWER SUPPLY). 6. The high voltage applied to the picture tube must conform with that specified in Service manual. Excessive high voltage can cause an increase in X-Ray emission, arcing and possible component damage, therefore operation under excessive high voltage conditions should be k ept to a minimum, or should be prevented. If s evere arc ing occurs, remove the AC power immediately and determine the cause by visual inspection (incorrect installation, cracked or melted high voltage harness, poor soldering, etc.). To maintain the proper minimum level of soft X-Ray emission, c omponents in the high voltage circuitry including the picture tube must be the exact replacements or alternatives approved by the manufacturer of the c omplete product. 7. Do not c hec k high voltage by drawing an arc. Use a high voltage meter or a high v oltage probe with a VTVM. Discharge the picture tube before attempting meter connection, by c onnecting a clip lead to the ground frame and c onnecting the other end of the lead through a 10k� 2W resistor to the anode button. 8. When service is required, observe the original lead dress. Extra prec aution should be given to assure correct lead dress in the high voltage circuit area. W here a s hort circuit has occurred, those components that indicate evidence of overheating should be replaced. Always use the manufacturer's replacement components. 9. Isolation Check (Safety for Electrical Shock Hazard) After re-ass embling the product, always perform an isolation check on the exposed metal parts of the cabinet (antenna terminals, video/audio input and output terminals, Control knobs, metal cabinet, screwheads, earphone jack, control shafts, etc.) to be sure the product is s afe to operate without danger of electrical shoc k. (1) Dielectric Strength Test The isolation between the AC primary circuit and all metal parts exposed to the us er, particularly any expos ed metal part having a return path to the chass is should withs tand a voltage of 3000V AC (r.m.s.) for a period of one sec ond. (. . . . Withstand a voltage of 1100V AC (r.m.s.) to an applianc e rated up to 120V, and 3000V AC (r.m.s.) to an appliance rated 200V or more, for a period of one second.) This method of test requires a test equipment not generally found in the servic e trade. (2) Leakage Current Check Plug the AC line c ord directly into the AC outlet (do not use a line isolation transformer during this check.). Using a " Leakage Current Tester", measure the leakage current from each exposed metal part of the cabinet, particularly any expos ed metal part having a return path to the chassis , to a known good earth ground (water pipe, etc.). Any leakage current must not exceed 0.5mA AC (r.m.s.). However, in tropic al area, this must not exceed 0.2mA AC (r.m.s.). " Alternate Check Method Plug the AC line c ord directly into the AC outlet (do not use a line isolation transformer during this check.). Use an AC voltmeter having 1000 ohms per volt or more sens itivity in the following manner. Connec t a 1500� 10W res istor paralleled by a 0.15µF AC-type c apacitor between an exposed metal part and a known good earth ground (water pipe, etc.). Meas ure the AC voltage across the res istor with the AC voltmeter. Move the resistor connec tion to each exposed metal part, particularly any exposed metal part having a return path to the chassis, and measure the AC voltage ac ross the res istor. Now, reverse the plug in the AC outlet and repeat eac h measurement. Any voltage measured must not exc eed 0.75V AC (r.m.s.). This c orresponds to 0.5mA AC (r.m.s.). However, in tropical area, this must not exceed 0.3V AC ( r.m.s.). This corresponds to 0.2mA AC (r.m.s.).
AC VOLT MET ER (HAVING 1000 �/V, OR MOR E SENSIT IVITY)

0.15μF AC-T YPE PLACE T HIS PROBE ON EACH EXPOSED MET AL PART

1500 � 10W GOOD EARTH GROUND

No.52025

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