MICROWAVE OUTPUT POWER
The following test procedure should be carried out with the microwave oven in a
fully assembled condition (outer case fitted).
The power output of this oven is rated using the method specified by IEC 60705.
Full details of how to curry out this procedure can be found in the Sharp
Technical Training notes which is available from Sharp Parts Centre (part
number SERV-LITMW01).
The IEC60705 procedure must be carried
out using laboratory-type procedures and equipment.
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
IEC60705 test method must be used.
Alternative simplified method:
1. Place 2 liters 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 1 minute and 40
seconds on full power.
4. When the 1 minute and 40 seconds 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 90.
NOTE: The result from this test should
be within the allowance of 2000cc alternative method. (±15%).
HIGH VOLTAGE RECTIFIER TEST
CARRY OUT 3D CHECKS.
Isolate the high voltage rectifier assembly from the HV circuit. The high
voltage rectifier can be tested using an ohmmeter set to its highest range.
Connect the ohmmeter across the terminal B+C of the high voltage rectifier and
note the reading obtained. Reverse the meter leads and note this second
reading. The normal resistance is infinite in one direction and more than 100 kW in the other direction.
CARRY OUT 4R CHECKS.
ASYMMETRIC RECTIFIER TEST
CARRY OUT 3D CHECKS.
Isolate the high voltage rectifier assembly from the HV circuit.
The asymmetric rectifier can be tested using an ohmmeter set to its highest
range across the terminals A+B of the asymmetric rectifier and note the reading
obtained. Reverse the meter leads and note this second reading. If an open
circuit is indicated in both directions then the asymmetric rectifier is good.
If the asymmetric rectifier is shorted in either direction, then the asymmetric
rectifier is faulty and must be replaced with high voltage rectifier. When the
asymmetric rectifier is defective, check whether magnetron, high voltage
rectifier, high voltage wire or filament winding of the high voltage
transformer is shorted.
CARRY OUT 4R CHECKS.
NOTE: FOR MEASUREMENT OF THE RESISTANCE OF THE RECTIFIER, THE BATTERIES OF THE
MEASURING INSTRUMENT MUST HAVE A VOLTAGE AT LEAST 6 VOLTS, BECAUSE OTHERWISE AN
INFINITE RESISTANCE MIGHT BE SHOWN IN BOTH DIRECTIONS.
MONITOR SWITCH TEST
CARRY OUT 3D CHECKS.
Before performing this test, make sure that the monitored interlock switch and
the relay RY-4 on the control unit are operating properly, according to the
above Switch Test Procedure. Disconnect the wire lead from the monitor switch
(COM) terminal. Check the monitor switch operation by using the ohmmeter as
follows. When the door is open, the meter should indicate a closed circuit.
When the monitor switch actuator is pushed by a screw driver through the right
latch hole on the front plate of the oven cavity with the door opened (in this
condition the plunger of the monitor switch is pushed in), the meter should
indicate an open circuit. If improper operation is indicated, the switch may be
defective. After testing the monitor switch, reconnect the wire lead to the
monitor switch (COM) terminal and check the continuity of the monitor circuit.
CARRY OUT 4R CHECKS.
THERMISTOR TEST
CARRY OUT 3D CHECKS.
Disconnect the thermistor from the control unit. Measure the resistance of the
thermistor with an ohmmeter.
THERMAL CUT-OUT TEST
CARRY OUT 3D CHECKS.
Disconnect the leads from the terminals of the thermal cut-out. Then using an
ohmmeter, make a continuity test across the two terminals as described in the
below.
If incorrect readings are obtained, replace the thermal cut-out.
An open circuit thermal cut-out (MG) TC1 indicates that the magnetron has
overheated, this may be due to resistricted ventilation, cooling fan failure.
An open circuit thermal cut-out (OVEN) TC2 indicates that the oven cavity has
overheated, this may be due to no load operation or fire in the oven cavity.
An open circuit thermal cut-out (Engine) TC3 indicates that the engine unit has
overheated, this may be due to empty water tank or pump motor failure.
CARRY OUT 4R CHECKS.
BLOWN FUSE (F1) 20A
CARRY OUT 3D CHECKS.
If the fuse F1 20A is blown, there is a shorts or grounds in electrical parts
or wire harness. Check them and replace the defective parts or repair the wire
harness.
CARRY OUT 4R CHECKS.
CAUTION: Only replace fuse with the
correct value replacement.
BLOWN FUSE (F2) F8A (NOISE FILTER)
CARRY OUT 3D CHECKS.
1. If the fuse F2 F8A is blown when the door is opened, check the monitored
interlock switch and monitor switch.
2. If the fuse F2 F8A is blown by incorrect door switching replace the
defective switch(es) and the fuse F2 F8A.
3. If the fuse F2 F8A is blown, there could be shorts in the asymmetric
rectifier or there is a ground in wire harness. A short in the asymmetric
rectifier may be occurred due to short or ground in H.V. rectifier, magnetron,
high voltage transformer or H.V. wire. Check them and replace the defective
parts or repair the wire harness.
CARRY OUT 4R CHECKS.
GRILL HEATING ELEMENTS (TOP) AND ENGINE HEATING ELEMENT TEST
CARRY OUT 3D CHECKS.
Before carrying out the following tests make sure the heating element is cool
completely.
1. Resistance of heater.
Disconnect the wire leads to the heating element to be tested. Using ohmmeter
with low resistance range. Check the resistance across the terminals of the
heating element as described in the following table.
CONTROL UNIT TEST
The control unit consists of circuits including semiconductors such as LSI,
ICs, etc. Therefore, unlike conventional microwave ovens, proper maintenance
can not be performed with only a voltmeter and ohmmeter.
In this service manual, the control panel assembly is divided into two units,
Control Unit and Key Unit, and troubleshooting by replacement is described
according to the symptoms indicated.
1. Key Unit Note: Check Key unit ribbon
connection before replacement.
The following symptoms indicate a defective key unit. Replace the key unit.
1) When touching the pads, a certain pad produces no signal at all.
2) When touching a number pad, two figures or more are displayed.
3) When touching the pads, sometimes a pad produces no signal.
Control Unit
The following symptoms indicate a defective control unit. Before replacing the
control unit perform the key unit test (Procedure N) to determine if control
unit is faulty.
In connection with pads
1) When touching the pads, a certain group of pads do not produce a signal.
2) When touching the pads, no pads produce a signal.
In connection with indicators
1) At a certain digit, all or some segments do not light up.
2) At a certain digit, brightness is low.
3) Only one indicator does not light up.
4) The corresponding segments of all digits do not light up; or they continue
to light up.
5) Wrong figure appears.
6) A certain group of indicators do not light up.
7) The figure of all digits flicker.
Other possible troubles caused by defective
control unit.
1) Buzzer does not sound or continues to sound.
2) Clock does not operate properly.
3) Cooking is not possible.
4) Proper temperature measurement is not obtained.
KEY UNIT (MEMBRANE SWITCH) TEST
If the display fails to clear when the STOP key is depressed, first verify the
ribbon cable is making good contact, verify that the door sensing switch
operates properly; that is the contacts are closed when the door is closed and
open when
the door is open. If the door sensing switch is good, disconnect the flat
ribbon cable that connects the key unit to the control unit and make sure the
door sensing switch is closed (either close the door or short the door sensing
switch connector). Use the Key unit matrix indicated on the control panel
schematic and place a jumper wire between the pins that correspond to the STOP
key making momentary contact. If the control unit responds by clearing with a
beep the key unit is faulty and must be replaced. If the control unit does not
respond, it is a faulty and must be replaced. If a specific pad does not
respond, the above method may be used (after clearing the control unit) to
determine if the control unit or key pad is at fault.
CARRY OUT 4R CHECKS.
To be continued
