PANASONIC NNH744
Service Manual

CAUTIONS TO BE OBSERVED WHEN TROUBLESHOOTING
Unlike many other appliances, the microwave oven is high-voltage, high current equipment. Though it is free from danger in ordinary use, extreme care should be taken during repair.

CAUTION Servicemen should remove their watches whenever working close to or replacing the magnetron. 1. Check the grounding
Do not operate on a 2-wire extension cord. The microwave oven is designed to be used when grounded. It is inoperative, therefore, to make sure it is grounded properly before beginning repair work.

WARNING Never touch any circuit wiring with your hand nor with an insulated tool during operation. 3. When parts must be replaced, remove the power plug from the outlet. 4.When the 18 Amp fuse is blown due to the operation of short switch: WARNING When the 18 Amp. fuse if blown due to the operation of short switch, you must replace Primary latch switch and short switch. Also replace power relay B (RY1) when the continuity check reads shorted contacts (1-2).
(A) This is mandatory. Refer to �Measurements and Adjustments� for these switches. (B) When replacing the fuse, confirm that it has the appropriate rating for these models. (C) When replacing faulty switches, be sure mounting tabs are not bent, broken or otherwise deficient in their ability to hold the switches.

2. Inverter Warnings

NEW H.V.

DANGER OF HIGH VOLTAGE AND HIGH TEMPERATURE (HOT/LIVE) OF THE INVERTER POWER SUPPLY (U)
This High Voltage Inverter Power Supply circuit supplies very high voltage and very high current for the magnetron tube. Though it is free from danger in ordinary use, extreme care should be taken during repair. As you can see, it looks like a TV flyback transformer, however the current is extremely large and so danger exists because of its high current and high voltages. The aluminum heat sink is also energized with high voltage (HOT), so do not touch when AC input terminal is connected to the power line because one of the IGBT switching power devices (Collector) is directly connected to the Aluminum heat sink. The Aluminum heat sink may be HOT from heat energy; therefore, extreme care should be taken during servicing and replacing. WARNING OF INVERTER POWER SUPPLY (U) GROUNDING Check the High Voltage Inverter Power Supply circuit grounding. This High Voltage Inverter Power Supply circuit board must have a proper chassis ground by the grounding bracket to the chassis ground; otherwise, this H.V. Inverter circuit board will expose very high voltage and cause extreme DANGER! Be sure to have proper grounding by the grounding plate and screws.

5.Avoid inserting nails, wire, etc. through any holes in the unit during operation.
Never insert a wire, nail or any other metal object through the lamp holes on the cavity or any other holes or gaps, because such objects may work as an antenna and cause microwave leakage.

WARNING OF DISCHARGING HIGH VOLTAGE CAPACITORS
Warning about the electric charge in the high voltage capacitors. For about 30 seconds after the oven is turned off, an electric charge remains in the high voltage capacitors in the inverter power supply circuit board. When replacing or checking parts, remove the power plug from the outlet and short the Inverter output terminal of the magnetron filament terminals to the chassis ground with an insulated handle screwdriver to discharge. Please make sure to touch chassis ground side first then short to the output terminals. WARNING
There is high-voltage present, with high-current capabilities in the circuits of the primary, and secondary windings, choke coil and heat sink of the Inverter. It is extremely dangerous to work on or near these circuits with oven energized. DO NOT measure the voltage in the high voltage circuit including filament voltage of magnetron.
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MAGNETRON INSULATED HANDLE FILAMENT TERMINAL SCREWDRIVER
Touch chassis side first then short to the terminal of the magnetron filament terminal.