Test Procedures - Sharp SMD2499FS Service Manual

SMD2499FS
PROCEDURE
LETTER
A MAGNETRON ASSEMBLY TEST
HIGH VOLTAGES ARE PRESENT DURING THE COOK CYCLE, SO EXTREME CAUTION SHOULD BE
OBSERVED.
DISCHARGE THE HIGH VOLTAGE CAPACITOR BEFORE TOUCHING ANY OVEN COMPONENTS OR
WIRING.
To test for an open filament, isolate the magnetron from the high voltage circuit. A continuity check across the
magnetron filament 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 should indicate an infinite resistance. If there is little or no resistance the magnetron
is grounded and must be replaced.
Power output of the magnetron can be measured by performing a water temperature rise test. This test should
only be used if above tests do not indicate a faulty magnetron and there is no defect in the following components
or wiring: silicon rectifier, high voltage capacitor and power transformer. This test will require a 16 ounce (453cc)
measuring cup and an accurate mercury thermometer or thermocouple type temperature tester. For accurate
results, the following procedure must be followed carefully:
1. Fill the measuring cup with 16 oz. (453cc) of tap water and measure the temperature of the water with
a thermometer or thermocouple temperature tester. Stir the thermometer or thermocouple through the
water until the temperature stabilizes. Record the temperature of the water.
2. Place the cup of water in the oven. Operate oven at POWER HI(HIGH) selecting more than 60 seconds
cook time. Allow the water to heat for 60 seconds, measuring with a stop watch, second hand of a watch
or the digital read-out countdown.
3. Remove the cup from the oven and again measure the temperature, making sure to stir the thermometer
or thermocouple through the water until the maximum temperature is recorded.
4. Subtract the cold-water temperature from the hot water temperature. The normal result should
be 22 to 43°F (12.2 to 23.8°C) rise in temperature. If the water temperatures are accurately
measured and tested for the required time period the test results will indicate if the magnetron tube has
low power output (low rise in water temperature) which would extend cooking time or high power output
(high rise in water temperature) which would reduce cooking time. Because cooking time can be adjusted
to compensate for power output, the magnetron tube assembly should be replaced only if the water
temperature rise test indicates a power output well beyond the normal limits. The test is only accurate
if the power supply line voltage is 117 volts and the oven cavity is clean.
B POWER TRANSFORMER TEST
DISCHARGE THE HIGH VOLTAGE CAPACITOR BEFORE TOUCHING ANY OVEN COMPONENTS OR
WIRING.
Disconnect the primary input terminals and measure the resistance of the transformer with an ohmmeter. Check
for continuity of the coils with an ohmmeter. On the R x 1 scale, the resistance of the primary coil should be
less than 1 ohm and the resistance of the high voltage coil should be approximately 79.5 ohms; the resistance
of the filament coil should be less than 1 ohm.
(HIGH VOLTAGES ARE PRESENT AT THE HIGH VOLTAGE TERMINAL, SO DO NOT ATTEMPT TO
MEASURE THE FILAMENT AND HIGH VOLTAGE.)
C HIGH VOLTAGE RECTIFIER TEST
DISCHARGE THE HIGH VOLTAGE CAPACITOR BEFORE TOUCHING ANY OVEN COMPONENTS OR
WIRING.
Isolate the rectifier from the circuit. Using the highest ohm scale of the meter, read the resistance across the
terminals and observe, reverse the leads to the rectifier terminals and observe meter reading. If a short is
indicated in both directions, or if an infinite resistance is read in both directions, the rectifier is probably defective
and should be replaced.

TEST PROCEDURES

COMPONENT TEST
16