Panasonic NN-T221MB, Service Manual, Page: 9 / 21

9
7 Cautions to be Observed When Troubleshooting
Unlike many other appliances, the microwave oven is a high
voltage, high current device. Though it is free from danger in
ordinary use, extreme care should be taken during repair.
Caution
Servicemen should remove their watches whenever work-
ing close to or replacing the magnetron.
7.1. Check the grounding
Do not operate on a two wire extension cord. The microwave
oven is designed to be used when grounded. It is imperative,
therefore, to ensure the appliance is properly grounded before
beginning repair work.
7.2. Inverter Warnings
DANGER, HIGH VOLTAGE AND HIGH TEMPERATURE
(HOT/LINE) OF THE INVERTER POWER SUPPLY (U)
This high voltage inverter power supply handles very high
voltage and 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 fly-
back transformer, however, the current is extremely large
and is therefore, dangerous due to this high current and
high voltage.
The aluminium heat sink is also energized with high voltage
(HOT), so do not touch when the AC input terminal is con-
nected. The power devices (Collector) is directly connected
to the aluminium heat sink.
The aluminium heat sink may be (HOT) due to heat energy,
therefore, extreme care should be taken during servicing.
HV Inverter warning
Figure 1
WARNING FOR INVERTER POWER SUPPLY (U) GROUND-
ING
Check the high voltage inverter power supply circuit ground-
ing. The high voltage inverter power supply circuit board
must have a proper chassis ground, the inverter grounding
bracket must be connected to the chassis. If the inverter
board is not grounded it will expose very high voltages and
cause extreme DANGER! Be sure that the inverter circuit is
properly grounded via the inverter earth bracket.
Grounding of the inverter circuit board
Figure 2
WARNING! DISCHARGE THE HIGH VOLATGE CAPACI-
TORS
For about 30 seconds after the oven is turned off, an elec-
tric 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 be sure
to touch the chassis ground side first and then short to the
output terminals.
Discharging the high voltage capacitors
Figure 3
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 the oven energized.
DO NOT measure the voltage in the high voltage circuit
including the filament voltage of the magnetron.
WARNING
Never touch any circuit wiring with your hand nor with an
insulated tool during operation.