R-772M (serv.man2) - Sharp Home Appliance Service Manual (repair manual). Page 9

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voltage doubler circuit.

3. D2 of the rectifier is shorted.
4. The large electric currents flow through the high volt-

age winding of the high voltage transformer.

5. The large electric currents beyond 8A flow through the

primary winding of the high voltage transformer.

6. The fuse F8A blows by the large electric currents.
7. The power supplying to the high voltage transformer is

cut off.

The noise filter assembly prevents radio frequency inter-
ference that might flow back in the power circuit.

The turntable motor rotates the turntable.

The fan motor drives a blade which draws external cool air.
This cool air is directed through the air vanes surrounding
the magnetron and cools the magnetron. This air is chan-
nelled through the oven cavity to remove steam and
vapours given off from heating food. It is then exhausted
through the exhausting air vents of the oven cavity.

The grill heating element is provided to brown the food and
is located on the top of the oven cavity.

The grill heating element is provided to brown the food and
is located at the base of the oven cavity.

Food is cooked by the top and bottom heating elements.
And the temperature of the oven cavity can be controlled
by the thermistor circuit. When the temperature inside the
oven cavity reaches the selected temperature, the heating
elements are de-energized. When the temperature inside
the oven cavity drops below the selected temperature, the
heating elements are energized again. In this way, the
inside of the oven cavity is maintained at approximately
the selected temperature.

When the oven cooking time reaches “0”, the heating
elements are de-energized and the oven shuts off. At that
time if the cavity air temperature has rised above120˚C,
the fan motor remains rotating. Automatically the fan
motor will be shut down at low temperature (less than
105˚C).

The oven will stop its operation when there is a fire in the
oven cavity in microwave cooking condition.
LSI measures the voltage across the temperature meas-
urement circuit intermittently within 32-seconds time base
since the oven is started in microwave cooking condition.
The oven will stop its operation when the difference of the
voltage is more than 0.781 volts in microwave cooking
condition.

1. Within a 32-seconds base, the thermistor is energized

for 2 seconds.  At that time, the voltage across the
temperature measurement circuit is measured.

2. The oven carries out the procedure above again. If the

second voltage is 0.781V higher than first voltage, LSI
judges it  is a fire in the oven cavity and stop the oven.

3. When LSI judges it  is a fire in the oven cavity, LSI will

switch off the relays to high voltage transformer and fan
motor and LSI stops countring down.

1. If the temperature of the thermistor does not rise to

more than 40˚C after 4 minutes and 15 seconds from
when the oven is started in oven cooking mode, dual
cooking mode or grill (top and bottom) cooking mode,
the oven is turned off.

2. When the thermistor or the wire harness to the thermis-

tor is opened, the oven is turned off after 4 minutes and
15 seconds because this condition is same as above.

THERMISTOR

Sensing 

Voltage

ON

OFF

ON

OFF

30

32 (sec.)

Sensing the voltage across temperature measurement circuit.

When troubleshooting the microwave oven, it is helpful to follow the Sequence of Operation in performing the checks.
Many of the possible causes of trouble will require that a specific test be performed. These tests are given a procedure
letter which will be found in the “Test Procedure” section.

IMPORTANT: If the oven becomes inoperative because of a blown fuse (F8A) in the monitored latch switch - monitor

switch circuit, check the monitored latch switch and monitor switch before replacing the fuse (F8A).