DIY Appliance Repair: How to Test the High Voltage Components of a Microwave Oven

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The high-voltage section of a commercial or domestic microwave oven consists of four basic components—the high-voltage oil-filled capacitor, the high-voltage rectifier, silicon diode, the high-voltage filament transformer, and the magnetron tube. T

Once you have removed the outer shell of the microwave oven, you will find a schematic diagram similar to this one affixed either to the inside top or end pieces of the wraparound shell piece.

At first glance most of the symbols used on this diagram will probably mean nothing to you, unless you have had experience reading schematics, but not to worry, you will learn what each of them mean as we proceed in this series. You will learn what the symbol means and what the actual part it represents looks like in the real world.

Specialized test equipment.

The professional use some very expensive, specialized test equipment when diagnosing and repairing microwave ovens, test instruments that no DIY Electrician can afford to invest in. The good news is that you really do not need this specialized testers to repair the microwave oven. I will show you how to do it with nothing more than an inexpensive Analog Multimeter (VOM) and a few testing jigs that you can make yourself from inexpensive parts bought from your local Radio Shack store or some other electronic parts supplier. You may think that the old analog meters are inferior to the Digital Multimeter, and in most cases they are, but you really need to use a cheap analog meter when testing capacitors. You will see why in a moment.

Other tools and supplies.

Besides a VOM you will need:

  • Flat-blade screwdrivers

  • Phillips Head screwdrivers

  • Long-nose pliers

  • Diagonal pliers/Wire cutters

  • Wire strippers

  • Nut drivers

  • Homemade High-voltage capacitor discharging tool

First things first.

The first thing that you need to do as soon as you have removed the microwave oven's wraparound shell, is to discharge the high-voltage oil-filled filter capacitor. If you have not read my article “DIY Appliance Repair: Safety Precautions to Take When Working on Microwave Ovensand built a discharging tool, read it now. Read it, build the high-voltage capacitor discharging tool and then use it to discharge the high-voltage capacitor before proceeding with the troubling shooting tests described in this article.

The high-voltage capacitor and silicon rectifier are a team.

The high-voltage silicon diode rectifier, represented by the schematic symbol

converts the high-voltage AC coming from the secondary winding of the high-voltage transformer to high-voltage DC. The voltage on the secondary of the high-voltage transformer, represented by the schematic symbol,

is 2,000 Volts AC. The HV Diode rectifies the 2,000 volts AC and feeds it to the high-voltage filter capacitor, which is represented on the schematic with this symbol.

The broken line rectangle represents the physical can of the oil-filled capacitor, the plates of the capacitor are shown as

and an internal bleeder resistor is indicated with this symbol.

Some capacitors have an external bleeder resistor soldered across its terminal or connected to the terminals using push-on connectors.

Together, the high-voltage diode and filter capacitor act as a voltage doubler, increasing the 2,000 Volts AC to 4,000 Volts DC, which powers the magnetron tube, shown on the schematic with this symbol.

The magnetron tube is the source of the energy that actually cooks your food.

How to test the high-voltage capacitor.

  • Double check to make sure that the high-voltage capacitor has been properly discharged. Connect one of the alligator clips of your homemade discharging circuit to the shaft of a screwdriver and the other alligator clip to the lip of the oil-filled capacitor. Be careful to keep your fingers away from the capacitor's terminals. To discharge the capacitor, touch the blade of the screwdriver to each of the capacitor's terminals and maintain contact with each terminal for at least 10 to 15 seconds.

  • If there is no schematic diagram affixed to the wraparound shell, make a sketch of how the wires connect to the capacitor's terminals.

  • Using your long-nose pliers, carefully pull the capacitor leads free of their push-on terminals. Remove the external bleeder resistor if there is one connected across the capacitor terminals.

  • Set up your analog Volt-Ohm-Miliampere (VOM) meter to read on its highest Ohms Range. In the case of my old Simpson 260, that is the R X10,000 Ohms range. As a side note, if your budget allows it and you are buying test equipment, Simpson is known for quality instrument that will last a lifetime with proper care. I bought this Simpson 260 back in the late 1960s and its still going strong.

  • Place the test probes on the capacitor terminals, the meter's pointer should slowly drift all the way over to a reading of infinite Ohms or to the resistance of the bleeder resistor if the capacitor that you are testing has an internal bleeder resistor.

  • Reverse the meters on the capacitor's terminals and the meter's pointer should deflect to zero and then slowly drift over to infinity or the internal bleeder resistor's resistance again.

  • Repeat this step one more time and if the same results occur, the capacitor has passed its first test.

  • Before the capacitor can be deemed viable, it must pass one more test that checks for a short between the capacitor's plates and the metal case. Check the reading between each of the capacitor's terminals and its case and you should get an infinite resistance reading if the capacitor is a viable capacitor. Some capacitors have an internal high-voltage diode and if you are testing such a capacitor for plate to case grounds, the meter should display the forward-bias resistance of the internal diode.

  • Finally inspect the capacitor for any signs of physical damage. Looks for signs of arching, the smell of oil, a oily deposit on the outside of the capacitor, or for a bulging case.

  • If the capacitor fails any of the meter test or displays any signs of physical damage, replace the capacitor with one of the same physical and electrical characteristic.

How to Test a High-voltage Diode.

  • If this is the first test being performed on the oven make sure that all the high-voltage capacitors have been properly discharged.

  • Disconnect all the wires from the capacitor just as explained in the preceding capacitor testing steps.

  • Once again set your VOM at the R X 10,000 or higher scale if your meter has a higher scale.

  • Depending on the make of diode that you are testing, your meter should display a resistance between 50,000 and 200,000 Ohms when it is forward biased by the meter's probes ans it should display infinite resistance when the diode is being reversed biased by the meter probes.

  • If the meter indicates infinite resistance for both forward and reverse polarity, the diode is open and needs to be replaced. If the meter indicates 50,000 to 200,000 Ohms whem both forward and reverse biased, the diode is shorted and needs to be replaced.

  • If you have a diode that is located inside the capacitor, determine which capacitor terminal it connects to and then perform these tests between that terminal and the capacitor's metal case.

How to test the High-Voltage (HV) Transformer.

WARNING: Make sure that the microwave oven is unplugged and the high-voltage capacitor(s) have been discharged before beginning these test. The filament winding on the high voltage transformer supplies 3 to 5 Volts AC to the magnetron but you should never attempt to measure primary voltage or any other voltage on the HV Transformer. You should confind your testing to taking resistance reading.

  • Using your long-nose pliers, pull the HV lead from the HV tap on the HV transformer.

  • Set the function switch on your analog VOM to its lowest Ohms scale, which should be an R X 1 scales. Take a reading between the HV tap on the transformer and the oven's chassis which is the HV sections ground. If you have an older oven, there may several HV taps on the transformer and you will have to check each tap to ground.

  • Depending on which tap you are testing you should geta reading somewhere between 55 and 70 Ohms to ground. Any reading substantially higher or lower than those readings is indicative of a defective transformer and you need to replace it.

  • Disconnect the wires from the primary input taps on the transformer and take a resistance reading across them. The primary should have a resistance reading of less than 1.0 Ohms because it is wound with very big wire. A typical reading would be about 0.21 Ohms. A substantially higher reading indicates an open primary and the transformer needs to be replaced.

  • Set your VOM to its highest scale and read between each primary tap and the chassis ground. A good transformer will read infinite resistance to ground. Any indication of a resistance reading means that the transformer is shorted and needs to be replaced.

  • Return the function switch on your VOM to its lowest range and check across the filament leads. A good filament winding will display a normal resistance of less than 1.0 Ohms.

  • Set the meter once again to it highest resistance scale and check the filament taps to chassis ground. Anything less than an infinity reading indicates a short to ground and the transformer needs to be replaced.

How to Test the Magnetron Tube.

Make sure that the oven is unplugged and the high-voltage capacitor discharged correctly before performing these tests. You may be starting to think that I am overdoing it with the “Make sure the oven is unplugged and the high-voltage capacitor's discharged” bit but safety cannot be over emphasized when working on a microwave oven. They can and will kill you if you become sloppy or careless when working on them.

The tests that you can perform on a Magnetron Tube are limited to testinth filaments continuity and for shorts to ground.

 

  • Set the VOM to its R X 1 scale and take a reading between the filament terminals. There should be less than 1 Ohms resistance indicated, a significantly higher reading indicates a defective Magnetron tube.

  • Set the VOM to it highest scale and take a reading from each of the filament terminal and the chassis ground. Anything less than infinite resistance indicates a shorted out magnetron and it needsto be replaced.

3 comments

carol roach
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Posted on Apr 21, 2012
Roberta Baxter
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Posted on Apr 20, 2012
Charlene Collins
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Posted on Apr 19, 2012