Test Procedures - Sharp R-27STM Service Manual

R-27STM
A: MAGNETRON TEST
NEVER TOUCH ANY PART IN THE CIRCUIT WITH YOUR HAND OR AN INSULATED TOOL WHILE THE OVEN IS
IN OPERATION.
1. CARRY OUT 3D CHECKS.
To test for an open filament, isolate the magnetron from the high voltage circuit. A continuity check across the magnetron filament
2.
leads should indicate less than 1 ohm.
To test for a shorted magnetron, connect the ohmmeter leads between the magnetron filament leads and chassis ground. This test
3.
should indicate an infinite resistance. If there is little or no resistance the magnetron is grounded and must be replaced.
4. Reconnect all leads removed from components during testing.
5. Reinstall the outer case (cabinet).
6. Reconnect the power supply cord after the outer case is installed.
Run the oven and check all functions.
7.
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 IEC 60705 test method must be used.
Alternative simplified method:
1. Place 2 litres of cold water (between 12ºC and 20ºC) in a suitable container.
2. Stir the water and measure the temperature in ºC. Note temperature as T1.
3. Place the container in the microwave and heat the water for 1 minute and 48 seconds on full power.
NOTE: The operation time of microwave oven includes "3" sec. (3 sec. is magnetron filament heat-up time.)
4. When the 1 minute and 48 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 80.
NOTE: The result from this test should be within the allowance of 2000cc alternative method. (±10%).
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
), however it is not normal to detect any significant leakage, therefore, any leakage which is detected should be
2
(equivalent to 5W/m
investigated.
It is essential that only leakage detectors with current calibration traceable to National Physical Laboratories are used.
Suitable leakage detectors: CELTEC A100
APOLLO X1
B: HIGH VOLTAGE TRANSFORMER TEST
WARNING: High voltage and large currents are present at the secondary winding and filament winding of the power trans-
former. It is very dangerous to work near this part when the oven is on. NEVER make any voltage measure-
ments of the high-voltage circuits, including the magnetron filament.
1. CARRY OUT 3D CHECKS.
Disconnect the leads to the primary winding of the high voltage transformer. Disconnect the filament and secondary winding con -
2.
nections 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 - - - - - - - - - 2.8 ohms approximately
b. Secondary winding - - - - - - - 204 ohms approximately
c. Filament winding - - - - - - - - - - - - - - less than 1 ohm
If the readings obtained are not stated as above, then the high voltage transformer is probably faulty and should be replaced.
3. Also, the high voltage transformer has the thermal cut -out in the primary coil. The thermal cut -out will open when the temperature
of the thermal cut -out in the primary coil reaches approximately 150ºC. The thermal cut -out resets automatically at 130°C. If an
ohmmeter indicates an open circuit under normal condition, replace the high voltage transformer because the primary coil (thermal
cut-out) has opened. An open primary coil (thermal cut-out) indicates overheating of the high voltage transformer. Check for re -
stricted air flow to the high voltage transformer, especially the ventilation opening.
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TEST PROCEDURES

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