Panasonic CT-25G5B Service Manual

Also see for CT25G5B - 25" COLOR TV: Operating instructions

Panasonic CT-25G5B Service Manual Manual pdf 2 page image

- 2 -Important Safety NoticeSpecial components are used in this television set which are important for safety. These parts are identified on theschematic diagram by the symbol and printed in BOLD TYPE on the replacement part list. It is essential thatthese critical parts are replaced with the manufacturer’s specified replacement part to prevent X-ray radiation,shock, fire or other hazards. Do not modify the original design without the manufacturer’s permission.Safety PrecautionsGeneral GuidelinesAn Isolation Transformer should always be usedduring the servicing of a receiver whose chassis is notisolated from AC power line. Use a transformer ofadequate power rating as this protects the technicianfrom accidents resulting in personal injury fromelectrical shocks. It will also protect the Receiver frombeing damaged by accidental shorting that may occurduring servicing.When servicing, observe the original lead dress,especially in the high voltage circuit. Replace alldamaged parts (also parts that show signs ofoverheating.)Always Replace Protective Devices, such asfishpaper, isolation resistors and capacitors, andshields after servicing the Receiver. Use onlymanufacturer’s recommended rating for fuses, circuitsbreakers, etc.High potentials are present when this Receiver isoperating. Operation of the Receiver without the rearcover introduces danger for electrical shock. Servicingshould not be performed by anyone who is notthoroughly familiar with the necessary precautionswhen servicing high-voltage equipment.Extreme care should be practiced when Handling thePicture Tube. Rough handling may cause it to implodedue to atmospheric pressure. (14.7 lbs per sq. in.). Donot nick or scratch the glass or subject it to any unduepressure. When handling, use safety goggles andheavy gloves for protection. Discharge the picturetube by shorting the anode to chassis ground (not tothe cabinet or to other mounting hardware). Whendischarging connect cold ground (i.e. dag ground lead)to the anode with a well insulated wire or use agrounding probe.Avoid prolonged exposure at close range to unshieldedareas of the picture tube to prevent exposure toX-ray radiation.The Test Picture Tube used for servicing the chassisat the bench should incorporate safety glass andmagnetic shielding. The safety glass provide shieldingfor the tube viewing area against X-ray radiation aswell as implosion. The magnetic shield limits the X-rayradiation around the bell of the picture tube in additionto the restricting magnetic effects. When using apicture tube test jig for service, ensure that the jig iscapable of handling 37.69kV without causingX-ray radiation.Before returning a serviced receiver to the owner,the service technician must thoroughly test the unit toensure that is completely safe to operate. Do not use aline isolation transformer when testing.Leakage Current Cold CheckUnplug the AC cord and connect a jumper between thetwo plug prongs.Measure the resistance between the jumpered AC plugand expose metallic parts such as screwheads,antenna terminals, control shafts, etc. If the exposedmetallic part has a return path to the chassis, thereading should be between 240kΩ and 5.2MΩ. If theexposed metallic part does not have a return path tothe chassis, the reading should be infinite.Leakage Current Hot Check (Fig. 1)Plug the AC cord directly into the AC outlet. Do not usean isolation transformer during the check.Connect a 1.5kΩ 10 watt resistor in parallel with a0.15μF capacitor between an exposed metallic partand ground. Use earth ground, for example awater pipe.Using a DVM with a 1000 ohms/volt sensitivity orhigher, measure the AC potential across the resistor.Repeat the procedure and measure the voltagepresent with all other exposed metallic parts.Verify that any potential does not exceed 0.75 voltRMS. A leakage current tester (such a Simpson Model229, Sencore Model PR57 or equivalent) may be usedin the above procedure, in which case any currentmeasure must not exceed 1/2 milliamp. If anymeasurement is out of the specified limits, there is apossibility of a shock hazard and the Receiver must berepaired and rechecked before it is returned to thecustomer.X-ray RadiationWARNING: The potential source of X-ray radiation in theTV set is in the High Voltage section and the picture tube.Note: It is important to use an accurate, calibratedhigh voltage meter.Set the brightness, picture, sharpness and colorcontrols to Minimum. Measure the High Voltage. Thehigh voltage should be 29.25kV ± 1.25kV. If the upperlimit is out of tolerance, immediate service andcorrection is required to insure safe operation and toprevent the possibility of premature component failure.Horizontal Oscillator Disable Circuit TestThis test must be performed as a final check before theReceiver is returned to the customer. See HorizontalOscillator Disable Circuit Procedure Check inthis manual.0.15μFCOLDWATERPIPE(GROUND)TO INSTRUMENT’SEXPOSED METALPARTS1500Ω,10 WAC VOLTMETERFigure 1. Hot Check Circuit