HOW TO CHECK THE MAGNATRON OF MICROWAVE OVEN - Testing Procedure
Sharp R1900J Microwave oven _ Testing Procedure
MAGNETRON
TEST
NEVER TOUCH
ANY PART IN THE CIRCUIT WITH YOUR HAND OR AN INSULATED TOOL WHILE THE OVEN IS
IN OPERATION
When the
magnetron MG1 and/or MG2 is replacing, the relays (RY3 and RY4) on control unit
must be replaced at the same time. Because if the magnetron's life has been
over, the relay's life may also be over.
Isolate the magnetron from high voltage circuit by removing all leads connected
to filament terminal.
To test for an open circuit filament use an ohmmeter to make a continuity test
between the magnetron filament terminals, the meter should show a reading of
less than 1 ohm.
To test for short circuit filament to anode condition, connect ohmmeter between
one of the filament terminals and the case of the magnetron (ground).
This test should be indicated an infinite resistance. If a low or zero
resistance reading is obtained then the magnetron should be replaced.
MICROWAVE
OUTPUT POWER (1 litre water load)
The following test procedure should be carried out with the microwave oven in a
fully assembled condition (outer case fitted). Microwave output power from the
magnetron can be measured by IEC test procedure, i.e. it can be measured by
using water load how much it can be absorbed by the water load. To measure the
microwave output power in the microwave oven, the relation of calorie and watt
is used. When P(W) heating works for t (second), approximately P x t/4.187
calorie is generated. On the other hand, if the temperature of the water with
V(ml) rises T (°C) during this microwave heating period, the calorie of the
water is v x T.
Measuring condition:
1) Container
The water container must be a cylindrical borosilicate glass vessel having a
maximum material thickness of 3 mm and an outside diameter of approximately 190
mm.
2) Temperature of the oven and vessel. The
oven and the empty vessel are at ambient temperature prior to the start of the
test.
3) Temperature of the water
The initial temperature of the water is (10±1)°C
4) Select the initial and final water temperature so that the maximum
difference between the final water temperature and the ambient temperature is
5°C.
5) Select stirring devices and measuring instruments in order to minimize
addition or removal of heat.
6) The graduation of the thermometer must be scaled by 0.1°C at minimum and an
accurate thermometer.
7) The water load must be (1000±5) g.
8) “t” is measured while the microwave generator is operating at full power.
Magnetron filament heat-up time is not included.
NOTE: The operation time of the microwave oven is “t + 3" sec. (3 sec. is
magnetron filament heat-up time.)
Measuring method:
1) Measure the initial temperature of the water before the water is added to
the vessel.
(Example: The initial temperature T1 = 11°C)
2) Add the 1 litre water to the vessel.
3) Place the load on the centre of the shelf.
4) Operate the microwave oven at HIGH until the temperature of the water rises
by a value T of (10±2)K.
5) Stir the water to equalize temperature throughout the vessel.
6) Measure the final water temperature. (Example: The final temperature T2 =
21°C)
7) Calculate the microwave power output P in watts from above formula.
POWER
TRANSFORMER TEST
Disconnect the leads to the primary winding of the
power transformer. Disconnect the filament and secondary winding connections
from the rest of
the HV circuitry. Using an ohmmeter, set on a low range, it is possible to
check the continuity of all three winding. The following readings should be
obtained:
a. Primary winding -------------------- approximately 1.3 Ω
b. Secondary winding --------------- approximately 76 Ω
c. Filament winding --------------------------- less than 1 Ω
If the readings obtained are not stated as above, then the power transformer is
probably faulty and should be replaced.
HIGH VOLTAGE
RECTIFIER (1) AND/OR (2) TEST
HIGH VOLTAGE RECTIFIERFTEST
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 100k ohms in the other direction.
ASYMMETRIC RECTIFIER TEST
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 power transformer is shorted.
NOTE: Be sure to use an ohm HIGH
VOLTAGE CAPACITOR (1) AND/OR (2) TEST
If the capacitor is open, no high voltage will be available to the magnetron.
Disconnect input leads and check for short or open between the terminals using
an ohmmeter.
Checking with a high ohm scale, if the high voltage capacitor is normal, the
meter will indicate continuity for a short time and should indicate an open
circuit once the capacitor is charged. If the above is not the case, check the
capacitor with an ohmmeter to see if it is shorted between either of the
terminals and case. If it is shorted, replace the capacitor.
CARRY
OUT SECONDARY INTERLOCK SWITCH (1)
AND/OR (2) TEST
Isolate the switch and connect the ohmmeter to the common (COM.) and normally
open (NO) terminal of the switch. The meter should indicate an open circuit
with the door open and a closed circuit with the door closed. If improper
operation is indicated, replace the secondary interlock switch.
Isolate the switch (SW1) and connect the ohmmeter to the common (COM.) and
normally close (NC) terminal of the switch (SW1). The meter should indicate a
close circuit with the door open and an open circuit with the door closed. If
improper operation is indicated, replace the switch (SW1).
CARRY OUT 4R CHECKS.T 4PRIMARY INTERLOCK SYSTEM TEST
1. DOOR SENSING SWITCH
2. Isolate the switch and connect the ohmmeter to the common (COM.) and
normally open (NO) terminal of the switch. The meter should indicate an open
circuit with the door open and a closed circuit with the door closed. If
improper operation is indicated, replace the door sensing switch.
3. CARRY OUT 4R CHECKS.
2. PRIMARY INTERLOCK RELAY (RY1)
1. CARRY OUT 3D CHECKS.
2. Disconnect two (2) wire leads from the male tab terminals of the Primary
Interlock Relay RY1. Check the state of the relay contacts using a ohmmeter.
The relay contacts should be open. If the relay contacts are closed, replace the
circuit board entirely or the relay itself.
MONITOR
SWITCH (1) AND/OR (2) TEST
Before performing this test, make sure that the secondary interlock switches
(1), (2) and the primary interlock relay RY1 are operating properly, according
to the above Switch Test Procedure. Disconnect the wire lead from the monitor
switches (1), (2) (COM) terminals. Check the monitor switches (1), (2)
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 upper 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
switches reconnect the wire lead to the monitor switch (COM) terminals and
check the continuity of the monitor circuit.
WEAK POINT
TEST.
If the weak point (1) or (2) is blown when the door is opened, check the
primary interlock relay RY1, secondary interlock switches (1), (2) and monitor
switches (1), (2) according to the “TEST PROCEDURE” for those switches before
replacing the blown weak point (1) or (2).
CAUTION: BEFORE REPLACING A BLOWN WEAK POINT (1) OR (2), TEST THE
PRIMARY INTERLOCK RELAY RY-1, SECONDARY INTERLOCK SWITCHES (1), (2), DOOR
SENSING SWITCH AND MONITOR SWITCHES (1), (2) FOR PROPER OPERATION.
If the weak point is blown, check the magnetron, high voltage capacitor, high
voltage rectifier assembly, according to the “TEST PROCEDURE” before replacing
the blown weak point. Then replace the weak point.
MAGNETRON
THERMISTOR TEST
Disconnect the connector of the magnetron thermistor from the harness. Measure
the resistance of each thermistor with an ohmmeter. Connect the ohmmeter leads
to the connector of each thermistor.
20°C > Approx. 750KOHms.
If the meter does not indicate above
resistance, replace the thermistor.
POWER UNIT
TEST
1. Check the out put voltage of power unit at CN21 as follows.
i) Disconnect the power supply cord, and then remove outer case and rear cover.
ii) Open the door and block it open.
iii) Discharge two high voltage capacitors.
iv) Disconnect the leads to the primary of the power transformer.
v) Ensure that these leads remain isolated from other components and oven
chassis by using insulation tape.
vi) Check the voltage between Pin No1 and No2 of CN21.
vii) The output voltage should be approx. 24V.
viii) If not so, the power unit is defective.
2. Check the continuity of the fuse F1 and protector F21 on the power unit as
follows.
i) Disconnect the power supply cord, and then remove outer case and rear cover.
ii) Open the door and block it open.
iii) Discharge two high voltage capacitors.
iv) Remove the power unit from the oven, referring to “POWER UNIT REMOVAL”.
v) Check the continuity of the fuse F1 and the protector F21.
vi) If they are open, the power unit is defective.
RELAY UNIT
TEST
2. Remove the relay unit from the oven, referring to “RELAY UNIT REMOVAL”.
3. Check the continuity of the FUSE1 on the relay unit.
1) If it is open, the relay unit is defective.
4. Check the open and /or short of the transformer primary coil on the relay
unit.
1) If it is open and /or short, it is defective.
5. Check the state of the relays contacts RY-1, RY-2 and RY-3 using an
ohmmeter.
1) The relay contacts should be open. If not so, the relay unit is defective.
MICROWAVE SENSOR TEST
There is no test procedure for the microwave sensor.
When the error history display shows EE18, EE28 or EE38, if the magnetrons,
control unit and wiring are normal, the microwave sensor may be defective.
CONTROL UNIT
TEST
The following symptoms indicate a defective control unit. Before replacing the
control unit, perform the Key units test (Procedure O) to determine if control
unit is faulty. Connect the power supply cord. And check for followings.
1. 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.
2. In connection with display
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.
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.
3. Other possible problems caused by defective control unit.
1) Buzzer does not sound or continues to sound.
2) Cooking is not possible.
4. If the protectors F1 and/or F2 on the control unit PWB are open, the control
unit is defective. To check, carry out following procedures.
i) Disconnect the power supply cord, and then remove outer case and rear cover.
ii) Open the door and block it open.
iii) Discharge two high voltage capacitors.
iv) Remove the control unit from the control panel.
v) Check the continuity of the KEY UNIT
TEST
1. CARRY OUT 3D CHECKS.
2. Check Key unit ribbon connection before replacement.
3. If key unit ribbon connection is normal, carry out the following procedures.
4. Re-install the rear cover and the outer case (cabinet).
5. Reconnect the power supply cord.
6. The following symptoms indicate a defective 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.
7. When you cannot judge if the key unit is normal or not by using the item 8
above, carry out following test procedures.
1) Disconnect the power supply cord, and then remove outer case and rear cover.
2) Open the door and block it open.
3) Discharge two high voltage capacitors.
4) Disconnect the leads to the primary of the power transformer.
5) Ensure that these leads remain isolated from other components and oven
chassis by using insulation tape.
6) Reconnect the po7) If the display fails to clear when the STOP/CLEAR pad is
depressed, first verify the flat ribbon cable is making good contact, verify
that the door sensing switch (stop 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 (stop
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/CLEAR pad 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 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.
8) For the door key unit, check the open/short and/or connection of the switch
unit and the harness, too.
8. If the Key unit is defective.
1) Disconnect the power supply cord, and then remove outer case and rear cover.
2) Open the door and block it open.
3) Discharge two high voltage capacitors.
4) Replace the Key unit.
5) Reconnect all leads removed from components during testing.
6) Re-install the rear cover and the outer case (cabinet).
7) Reconnect the power supply cord after the rear cover and the outer case are
installed.
8) Run the oven and check all functions. Protectors F1 and F2
on the control unit.
How to test a Microwave oven Magnetron
