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R-212M 5

The circuits to the 

high voltage

 transformer, fan motor

turntable motor, and the oven lamp are cut off when the
monitored latch switch and stop switch are made open.

6. MONITOR SWITCH CIRCUIT

The monitor switch (SW2) is mechanically controlled
by oven door, and monitors the operation of the moni-
tored latch switch (SW1).

6-1 When the oven door is opened during or after the

cycle of a cooking program, the monitored latch
switch (SW1) and stop switch (SW3) must open their
contacts first. After that the contacts (COM-NC) of the
monitor switch (SW2) can be closed.

6-2. When the oven door is closed, the contacts (COM-

NC) of the monitor switch (SW2) must be opened first.
After that the contacts (COM-NO) of the monitored
latch switch (SW1) and stop switch (SW3) are closed.

6-3. When the oven door is opened and the contacts

(COM-NO) of the monitored latch switches (SW1)
remain closed, the fuse F1 will blow, because the
contacts (COM-NC) of the monitor switch are closed
and a short circuit is caused.

When the microwave oven is preset for variable cooking
power, the line voltage is supplied to the high voltage
transformer intermittently within a 27-seconds time base
through the relay contact which is coupled with the cur-
rent-limiting relay RY2. The following levels of microwave
power are given.

SETTING;

NOTE: The ON/OFF time ratio does not exactly corre-

spond to the percentage of microwave power,
because approx. 3 seconds are needed for heat-
ing up the magnetron filament.

27 sec. ON

Approx. 70%

100%

HIGH

MEDIUM HIGH

MEDIUM

MEDIUM LOW

LOW

21 sec. ON

6 sec. OFF

12 sec. OFF

18 sec. OFF

22 sec. OFF

Approx. 50%

15 sec. ON

Approx. 30%

9 sec. ON

Approx. 10%

5 sec. ON

Closing the door activates all door interlock switches
(monitored latch switch and stop switch).

IMPORTANT
When the timer knob is at "0" the oven is in the OFF
condition.
When the oven door is closed, the monitor switch contacts
(COM-NC) are open.
When the microwave oven is plugged in a wall outlet (230
- 240V 50Hz), the noise filter and the control panel are
energized.

HIGH COOKING
When the door is closed, the contacts (COM-NO) of the
monitored latch switch (SW1) and the monitor switch
(SW2) are closed. When the microwave power level is set
to 800W by pressing the microwave power level button
and the cooking time is selected by turning the timer knob,
the following components are energized: H.V. transformer,
Magnetron, Oven lamp, Turntable motor and Fan motor,
H.V. capacitor, H.V. rectifier.

CONNECTED COMPONENTS

RELAY

Oven lamp, Fan motor, Turntable motor

RY1

High voltage transformer

RY2

1. The line voltage is supplied to the primary winding of

the 

high voltage

 transformer. This voltage is converted

to approx 3.3 volts A.C. output on the filament winding
and high voltage of approximately 2000 volts A.C. on
the secondary winding.

2. The filament winding voltage (3.3 volts) heats the

magnetron filament and the high voltage (2000 volts
A.C). is sent to the voltage doubling circuit, where it is
doubled to negative voltage of approximately 4000
volts D.C..

3. The 2450 MHz microwave energy produced in the

magnetron generates a wave length of 12.24 cm. This
energy is channelled through the waveguide (transport
channel) into the oven cavity, where the food is placed
to be cooked.

4. When the cooking time is up, a signal tone is heard and

the relays RY1+RY2 open. The circuits to the oven
lamp, 

high voltage

 transformer, fan motor and turntable

motor are cut off.

5. When the door is opened during a cook cycle, the

switches come to the following condition.

          CONDITION

DURING

DOOR OPEN

SWITCH

CONTACT

COOKING

(NO COOKING)

Monitored switch

COM-NO

Closed

  Open

COM-NO

Closed

  Open

Monitor switch

COM-NC

Open

  Closed

Stop switch

COM-NO

Closed

  Open

R-212M 6

The door can be opened by pulling the door.

1. When the oven door is closed, the contacts (COM-NO)

must be closed.

1. When the oven door is closed, the contacts (COM-NC)

must be opened.

2. When the oven door is opened, the contacts (COM-

NC) must be closed.

3. If the oven door is opened and t he contacts (COM-NO)

of the monitored latch switch (SW1) fail to open, the
fuse F1 blows immediately after closing the contacts
(COM-NC) of the monitor switch (SW2).

CAUTION: BEFORE REPLACING A BLOWN FUSE  F1.

TEST MONITORED LATCH SWITCH (SW1)
AND  MONITOR SWITCH (SW2) FOR
PROPER OPERATION.  (REFER TO CHAP-
TER “TEST PROCEDURE”).

1. The fuse F1 blows when the contacts (COM-NO) of the

monitored latch switch (SW1) remain closed with the
oven door open and when the contacts (COM-NC) of
the monitor switch (SW2) are closes.

2. If the wire harness or electrical components are short-

circuited, the fuse F1 blows to prevent an electric shock
or fire hazard.

The thermostat protects the high voltage transformer
against overheating. If the temperature goes up higher
than 150˚C because the fan motor is interrupted or the
ventilation openings are blocked, the thermostat will cycle,
line voltage to the high voltage transformer will also cycle.
(If operated, check the magnetron for damage.)

The thermal cut-out located on the top of the oven cavity
is designed to prevent damage to the oven, if the food in
the oven catch fire due to over heating produced by
improper setting of cook time or failure of control unit.
Under normal operation, the thermal cut-out remains
closed. However, when abnormally high temperatures are
reached within the oven cavity, the thermal cut-out will
open at 125˚C, causing the oven to shut down. The
defective thermal cut-out must be replaced with a new
one.

The turntable motor drives the turntable roller assembly to
rotate 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 at the rear of the oven
cavity.

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

Latch hook

SW3: Stop switch

Door

SW2: Monitor switch

SW1: Monitored 

latch switch

Latch heads

R-212M 7

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 proce-
dure letter which will be found in the “Test Procedure”
section.

IMPORTANT: If the oven becomes inoperative because

of a blown fuse F1. Check the monitored
latch switch and monitor switch before
replacing the F1 fuse.

CONDITION

PROBLEM

A

B

C

D

E

E E

F G

H

I

I

J

K

L

L

M

Home fuse blows when power cord 
is plugged into wall outlet.

Fuse F1  blows when power cord 
is plugged into wall outlet. (Timer 
off)

Oven does not operate at all.

Timer is not set but when door 
closed, oven starts operation.

Oven does not start when timer 
knob is turned, door closed.

Oven lamp does not light, when in 
operation. 

Fan motor does not operate. 

Turntable motor assembly does 
not operate. 

Oven or electrical parts does not 
stop when timer knob is at "0". 

Oven seems to be operating but 
little or no heat is produced in oven 
load. (Microwave power level is set 
at HIGH.)

Oven does not operate properly during 
the variable cooking condition except 
HIGH cooking condition.

Oven goes into cook cycle but 
shuts down before end of cooking 
cycle.

Oven stops as soon as the oven is 
started.

Fan motor, oven lamp and 
turntable motor do not operate at 
the same time.

OFF

CONDITION

COOKING

CONDITION

TEST PROCEDURE

POSSIBLE CAUSE

AND

DEFECTIVE PARTS

MAGNETRON

HIGH VOLTAGE  TRANSFORMER

H.V. RECTIFIER

HIGH VOLTAGE CAPACITOR

MONITORED LATCH SWITCH

STOP SWITCH

MONITOR SWITCH

THERMAL CUT-OUT 125˚C

FUSE F8A

NOISE FILTER

FAN MOTOR

TURNTABLE MOTOR

POWER SUPPLY CORD

OVEN LAMP OR SOCKET

SHORTED WIRE HARNESS

OPENED WIRE HARNESS

MIS-ADJUSTMENT SWITCHES

NO POWER AT WALL OUTLET

HVT THERMOSTAT

CONTROL UNIT

TACT SWITCH

RELAY RY1

RELAY RY2

FOIL PATTERN ON P.W.B.

R-212M 8

A

MICROWAVE OUTPUT POWER

The power output of this oven is rated using the method specified by IEC-705.  Full details of how
to carry out this
procedure can be found in the Sharp Technical Training notes which is available from Sharp
Parts Centre
(part number SERV-LITMW01).

The IEC-705 procedure must be carried out using laboratory-type procedures and equip
ment.
These requirements make the procedure unsuitable for routine performance checks.

Note: The following test method gives an indication of the output power only, it cannot be used to
establish the actual/rated output power.  If the true output power is required, then the IEC-705
test method must be used.

Alternative simplified method:
1.

Place 2 litres of cold water (between 12

°

C and 20

°

C) in a suitable container.

2.

Stir the water and measure the temperature in 

°

C.  Note temperature as T1.

3.

Place the container in the microwave and heat the water for 2 minutes on full power.

4.

When the 2 minutes is completed, remove the container and stir the water.  Note the water
temperature as T2.

5.

Calculate the output power using the following formula:
R.F. Power Output = (T2 - T1) x 70.

MICROWAVE LEAKAGE TEST

This oven should be tested for microwave leakage on completion of any repair or adjustment,
following the procedure described in the Sharp Technical Training notes (part number SERV-
LITMW01).  The maximum leakage permitted in BS EN 60335-2-25 is 50W/m

2 

(equivalent to

5mW/cm

2

), however it is not normal to detect any significant leakage, therefore, any leakage

which is detected should be investigated.

It is essential that only leakage detectors with current calibration traceable to the National Physical
Laboratories are used.

Suitable leakage detectors :

CELTEC A100
APOLLO X1