microwave & rf pcb 101

Microwave Oven Magnetron Power Output Test

The magnetron’s performance can be tested for proper operation by performing a power output test in the following manner:

• The oven cavity must be cool and clean. Inspect the waveguide and stirrer blade.
• The voltage to the oven should be around 120 volts (±10 percent).
• Place an 8-ounce glass of water heated to about 75 degrees Fahrenheit in the middle of the cavity floor (remove the metal rack).
• Take a thermometer and stir the water for 30 seconds. Record the water temperature.
• Set the oven controls to 100 percent power and the timer to one minute, and press the start button.
• When the oven turns off, remove the glass, and stir the water with the thermometer for 30 seconds. Record the water temperature.
• Subtract the two recorded water temperatures to determine the temperature rise.
• The temperature rise should be between 19 and 26 degrees Fahrenheit at 120 volts (±10 percent), indicating that the magnetron output power is operating properly.

Repair Procedures

Each of the following repair procedures is a complete inspection and repair process for a single microwave oven component, containing the information you need to test a component that might be faulty and to replace it, if necessary.

Any person who cannot use basic tools should not attempt to install, maintain, or repair any microwave ovens. Any improper installation, preventative maintenance, or repairs will create a risk of personal injury, as well as property damage. Call the service manager if installation, preventative maintenance, or the repair procedure is not fully understood.

Electronic Control Board, Touchpad Panel, or Display Board

The electronic control board, touchpad, or display board operate the microwave oven functions. The electronic control board also displays the cooking or defrost settings, the time of day, timer, preprogrammed functions, and any error or fault codes. The electronic control board, touchpad, or display board are located in the control panel.

The typical complaints associated with the electronic control board, touchpad panel, or display board are:

• The microwave oven will not run or power up.
• Unable to program the touchpad panel functions.
• The display board will not display anything.
• One or more key pads will not accept commands. Unusual display readouts and/or error codes.

To handle these problems, perform the following steps:

• 1.Verify the complaint.Verify the complaint by operating the microwave oven. On electronic models, turn off the electricity to the appliance and wait for two minutes before turning it back on. If a fault code appears, look up the code. If the microwave oven will not power up, locate the technical data sheet behind the control panel for diagnostics information. On some models you will need the actual service manual for the model you are working on to properly diagnose the microwave oven. The service manual will assist you in properly placing the microwave oven in the service test mode for testing the microwave oven functions.
• 2. Check for external factors. You must check for external factors not associated with the appliance. Is there electricity to the microwave oven? The voltage at the receptacle is between 108 volts and 132 volts during a load on the circuit. Do you have the correct polarity?
• 3.Disconnect the electricity.Before working on the microwave oven, disconnect the electricity. This can be done by pulling the plug out of the wall receptacle. Or disconnect the electricity at the fuse panel or circuit breaker panel. Turn off the electricity.

WARNING Before you begin to service any high-voltage components within the microwave oven, you must discharge the high-voltage capacitors or inverter board first. Failure to do so will cause injury or death.

WARNING Some diagnostic tests will require you to test the components with the power turned on. When you disassemble the control panel or remove the outer cabinet, you can position the panel in such a way that the wiring will not make contact with metal. This will allow you to test the components without electrical mishaps.

• 4. Remove the outer cabinet or control panel to gain access. You can gain access to the electrical components, touchpad panel, or electronic control board (some models have multiple circuit boards) by removing the access panel or the outer cabinet on a microwave oven or combination oven. If you are repairing a wall oven or an eye-level combination range, the control panel can be removed by opening the door and removing the screws that secure the panel. These might be underneath the front of the exhaust hood or just below the control panel. Some control panels are hinged; just tilt the control panel toward you for servicing.
• 5.Test the electronic control board, touchpad panel, or display board.If you are able to run the microwave oven’s diagnostic test mode, check the different functions of the oven. Use the technical data sheet for the model you are servicing to locate the test points on the wiring schematic. Check all wiring connections and wiring. Using the technical data sheet, test the electronic control and display board, input voltages, and output voltages. On some models, fuses are soldered to the printed circuit board (PCB). These fuses must be tested before condemning the component. The touchpad panel can also be tested for proper operation by checking the continuity for each keypad function.
• 6.Remove the defective component.To remove the defective component, remove the screws that secure the boards to the control panel or oven frame. Disconnect the connectors from the electronic control board, touchpad membrane, or display board.
• 7.Install the new component.To install a new component, just reverse the disassembly procedure, and reassemble. Reinstall all panels or the console panel, and restore the electricity to the oven. Test the microwave oven operation. Perform the microwave performance and leakage test. Make sure to take the microwave oven out of the service test mode when the repair is completed.

Magnetron

The magnetron produces the microwave energy that will cook the food. The magnetron is attached to the outer oven cavity.

Figure 24-14 Location of the high-voltage components.

The typical complaints associated with the magnetron are:

• Unable to cook the food.
• Failed magnetron power output test.
• Noisy operation.
• Unusual display readouts and/or error codes.

To handle these problems, perform the following steps:

• 1. Verify the complaint. Verify the complaint by operating the microwave oven with a glass of water and the microwave test light bar centered on the oven floor. The lights in the light bar will light up if the magnetron and high-voltage system are working correctly. At the same time, place an ammeter on one wire of the service cord—the meter should indicate approximately 15 amps at 100 percent power. It may be necessary to turn off the electricity to the appliance and wait for two minutes before turning it back on. If a fault code appears, look up the code. If the microwave oven will not power up, locate the technical data sheet behind the panel or cabinet for diagnostics information. On some models you will need the actual service manual for the model you are working on to properly diagnose the microwave oven. The service manual will assist you in properly placing the microwave oven in the service test mode for testing the microwave oven functions.
• 2. Check for external factors. You must check for external factors not associated with the appliance. Is there electricity to the microwave oven? With over-the-range models, is the vent system blocked? The voltage at the receptacle is between 108 volts and 132 volts during a load on the circuit. Do you have the correct polarity?
• 3.Disconnect the electricity.Before working on the microwave oven, disconnect the electricity. This can be done by pulling the plug out of the wall receptacle. Or disconnect the electricity at the fuse panel or circuit breaker panel. Turn off the electricity.

WARNING Before you begin to service any high-voltage components within the microwave oven, you must discharge the high-voltage capacitors and/or inverter board first. Failure to do so will cause injury or death.

WARNING Some diagnostic tests will require you to test the components with the power turned on. When you disassemble the control panel or remove the outer cabinet, you can position the panel in such a way that the wiring will not make contact with metal. This will allow you to test the components without electrical mishaps.

• 4. Remove the outer cabinet or control panel to gain access. You can gain access to the magnetron by removing the access panel or the outer cabinet on a microwave oven or combination oven. If you are repairing a wall oven or an eye-level combination range, the control panel can be removed by opening the door and removing the screws that secure the panel. These might be underneath the front of the exhaust hood or just below the control panel. Some control panels are hinged; just tilt the control panel toward you for servicing. On some combination models, you might have to uninstall the appliance to gain access.
• 5. Test the magnetron. Locate the magnetron tube within the high-voltage section of the microwave oven. Set your ohmmeter on the R × 1 scale. Carefully remove the wires from the magnetron filament terminals. Inspect the terminal seal for any damage. Take your ohmmeter probes, and place one probe on each filament terminal. The ohmmeter should read less than 1 ohm. Next, leave one probe on the filament terminal, and place the other probe on the chassis. The meter should read infinite ohms. Do the same procedure for the other filament terminal. The meter should read infinite ohms also. If the testing results differ, replace the magnetron.
• 6.Remove the defective magnetron.To remove the defective magnetron, remove the screws that secure the magnetron to the oven frame. Carefully remove the magnetron from the oven so as not to damage any other components. Inspect the magnetron tube antenna and embossed ring; replace the magnetron if there are signs of arcing, burn marks, or a broken antenna. Also, inspect the wave guide for burn marks. If there are burn marks, replace the microwave oven.
• 7.Install the new magnetron.To install a new magnetron, just reverse the disassembly procedure, and reassemble. Reinstall all panels or the console panel, and restore the electricity to the oven. Test the microwave oven operation. Perform the microwave performance and leakage test. Make sure to take the microwave oven out of the service test mode when the repair is completed.

Figure 24-15 Place a glass of water in the microwave cavity along with the microwave test light bar.

Figure 24-16 The microwave oven door is closed, the controls are set, and the oven is on. This illustration shows that the microwave test lights are on, indicating that the magnetron and the high-voltage system are functioning correctly.

Figure 24-17 The magnetron.

High-Voltage Transformer

The high-voltage transformer will step up the voltage to 2000 VAC on one part of the secondary coil. On the other secondary coil, the voltage will be stepped down to 3 to 4 VAC. The primary winding is connected to 120 VAC. The high-voltage transformer is located behind the control panel, mounted to the frame of the microwave oven.

The typical complaints associated with the high-voltage transformer are:

• The microwave oven operates but is unable to cook the food.
• Erratic cooking.
• The oven turns off the electricity or blows a fuse in the circuitry when the oven door is closed.
• You hear a constant buzzing noise coming from the high-voltage section.
• Unusual display readouts and/or error codes.

To handle these problems, perform the following steps:

• 1. Verify the complaint. Verify the complaint by operating the microwave oven with a glass of water and the microwave test light bar centered on the oven floor. The lights in the light bar will light up if the magnetron and high-voltage system are working correctly. At the same time, place an ammeter on one wire of the service cord; the meter should indicate approximately 15 amps at 100 percent power. It may be necessary to turn off the electricity to the appliance and wait for two minutes before turning it back on. If a fault code appears, look up the code. If the microwave oven will not power up, locate the technical data sheet behind the panel or cabinet for diagnostics information. On some models you will need the actual service manual for the model you are working on to properly diagnose the microwave oven. The service manual will assist you in properly placing the microwave oven in the service test mode for testing the microwave oven functions.
• 2. Check for external factors. You must check for external factors not associated with the appliance. Is there electricity to the microwave oven? The voltage at the receptacle is between 108 volts and 132 volts during a load on the circuit. Do you have the correct polarity?
• 3.Disconnect the electricity.Before working on the microwave oven, disconnect the electricity. This can be done by pulling the plug out of the wall receptacle. Or disconnect the electricity at the fuse panel or circuit breaker panel. Turn off the electricity.

WARNING Before you begin to service any high-voltage components within the microwave oven, you must discharge the high-voltage capacitors first and/or inverter board first.