Service manual

Page 11
Procedure B - Simplified Power Test
You will need:
A thermometer capable of reading to ±0.1°C.
A Polypropylene tray approximately 200 mm x 200 mm.
A measuring jug.
A calculator.
Water which is at a temperature of 10°C ± 2°C.
1. Measure 1 litre of cold water into the tray using the measuring jug.
2. Measure the water temperature, and record it as T[s].
3. Place the tray on the turntable in the oven and close the door.
4. Turn the oven on.
5. Set the timer to 1:02.
6. Press the “100%” pad.
7. When the oven bleeps, open the door and remove the tray.
8. Stir the water thoroughly, and measure its temperature. Record this as T[e].
Calculation:
1. T[r] = T[s] - T[e].
2. Power = 70 x T[r]. Power is in Watts.
The power given by the above test should be within ±10% of the rated power.
Procedure C - Power Transformer Test
You will need:
A Digital Multi-meter (D.M.M.)
A Megger or similar resistance meter using 500V d.c.
1. Isolate the oven from the mains supply.
2. Ensure that the High Voltage Capacitor is discharged before commencing work.
3. Remove all connections from the Power Transformer.
4. Using a D.M.M., check the resistance of the windings.
Results should be as follows:
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 at the High
Voltage circuits, including the magnetron filament.
WARNING:
Even when the oven is not cooking, the Power Transformer has High Voltages
present because of the Soft Start circuit. Isolate the oven before testing.
Mains winding (between tags)
Approximately 1.1
Ω
High Voltage winding (between tag & chassis)
Approximately 60
Ω
Filament winding (between terminals)
Less than 1
Ω
Primary winding and chassis
Pass if over 10 M
Ω
Filament winding and chassis
Pass if over 10 M
Ω
5. Using a Megger, test the insulation resistance between:
One end of the High Voltage winding is connected to the chassis, so this is not tested.