Sharp R-890SLM Service Manual download pdf (Page 15)

R-890SLM

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CHAPTER 8. TEST PROCEDURES

[1] A: MAGNETRON TEST

NEVER TOUCH ANY PART IN THE CIRCUIT WITH YOUR HAND OR AN INSULATED TOOL WHILE THE OVEN IS IN OPERATION.

CARRY OUT 3D CHECKS.

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 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.

1. MICROWAVE OUTPUT POWER

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 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 1 minute and 36 seconds on full power.

NOTE: The operation time of the microwave oven includes “3” sec. (3 sec. is magnetron ﬁlament heat-up time.)

4. When the 1 minute and 36 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. (±10%).

2. 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

(equivalent to 5W/m

), 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 National Physical Laboratories are used.

Suitable leakage detectors: CELTEC A100

APOLLO X1

[2] B: HIGH VOLTAGE TRANSFORMER TEST

WARNING: High voltages and large currents are present at the secondary winding and ﬁlament winding of the power

transformer. It is very dangerous to work near this part when the oven is on. NEVER make any voltage

measurements of the high-voltage circuits, including the magnetron ﬁlament.

1. CARRY OUT

3D CHECKS.

2. Disconnect the leads to the primary winding of the high voltage 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 2.1 Ω

b.Secondary winding --------------- approximately 133 Ω

c.Filament winding --------------------------- less than 1 Ω

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 150°C. 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

restricted air ﬂow to the high voltage transformer, especially the ventilation opening.

4. CARRY OUT

4R CHECKS.

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Sharp R-890SLM Service Manual Page 15

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