Question about Sharp Dial Microwave R-21LCF Microwave Oven

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Why would the high voltage capacitor go bad? - Sharp Dial Microwave R-21LCF Microwave Oven

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Hi and welcome to FixYa. I am Kelly. High voltage capacitors can and do fail when a microwave is used for long periods of operation, when the unit is operated with no internal load and sometimes for no reason at all other than the capacitor had an internal failure such as corrosion or a break down of the metal strips inside of it where the strips can make contact with the the case. I have been working on microwaves since the 70's and often there is no apparent reason for the failure of the HV cap.. They just sometimes fail.
However...... there is also a power diode in the microwave that is connected directly to one terminal of the HV Cap. If the power diode has failed it can cause subsequent failure of the HV Cap. The Sharp Power diodes can be found here:

You use and ohm meter on resistance to check the diode AFTER removing the terminal of the diode from the capacitor. NOTE: the capacitor must be shorted across the terminals at least 3 times with a screwdriver while holding the handle only to prevent a horrible shock.
Once the diode terminal is disconnected from the HV Cap read resistance in BOTH directions by reversing the test leads on the terminals of the diode. One direction will read resistance and the other direction it will read open / infinity. Any other reading is a failed diode.

You check the capacitor using an analog (scaled vs digital display) ohm meter on R X 1,000 or R X 10,000 and watch for a jump on the meter just as you touch the terminals EACH time you reverse the leads. No jump or... steady resistance = bad Cap.

Sharp Capacitors can be found here:

Just note that either a CAP or Diode failure can cause the main fuse to blow at the end of the power cord in the fuse holder.

Fuses are normally either 15 or 20 A Slo-Blo fuses and can be found here:

Thanks for choosing FixYa


Posted on Sep 29, 2010


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You don't need a wiring diagram if you have basic knowledge of how a microwave works. The gadget responsible for heating is the magnetron just google to have an overview. How ever it must be powered by high voltage from a transformer. If the microwave indicates it is on yet not powering, just open the cover only don't touch capacitors inside without discharging first to discharge just short the capacitor with an insulated screw driver check for continuity with digital multi meter on the primary of the high voltage transformer if ok change your magnetron and you will have your microwave back and functioning just make sure not to plague on the A.C Mains while you are working on it.

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see this causes and fix it. God bless you
Diode The high voltage diode converts the A/C power output of the transformer to D/C, doubling the voltage to nearly 5,000 volts. This high voltage powers the magnetron which emits the energy that cooks the food. If the diode burns out, a lower A/C voltage reaches the magnetron, which is not sufficient to power the magnetron. When the diode fails it is often visibly burned out. If it appears to be good, it can be tested with a volt-Ohm meter capable of testing diodes. High voltage diodes frequently fail and are one of the most common points of failure in a microwave oven.

Door Switch If the microwave does not heat, one of the door switches might be defective. Microwave ovens normally have three door switches, if any of them fail the microwave does not turn on and does not heat. Check the switches for continuity with an Ohm meter.

Magnetron If the microwave oven doesn't heat the magnetron tube might have burned out. The magnetron uses high voltage, high current DC power to generate the microwave frequency that cooks the food. If the microwave oven is turned on when it is empty this can cause the magnetron to burn out. Once it is burned out it has to be replaced, it's not repairable.
High Voltage Capacitor If the microwave doesn't heat the high voltage capacitor might be defective. The high voltage capacitor works with the high voltage diode to convert the output of the transformer to DC voltage and to double the output voltage. If the capacitor is burned out the entire high voltage circuit stops working properly. The high voltage capacitor can be checked with a special VOM meter which has a capacitance testing capability. Be aware that the high voltage capacitor can retain a charge of more than 3,000 volts and can injure or kill a person if not handled properly. Only trained technicians should perform this type of testing.

High Voltage Transformer If the microwave does not heat, the high voltage transformer might be burned out. Microwave ovens produce a very high voltage in order to power the magnetron antenna, which emits the energy that cooks the food.

Thermal Fuse If the microwave doesn't heat the thermal fuse may have blown. It can be tested for continuity. Watch our fuse testing video for more information.

Thermoprotector If the microwave doesn't heat the thermoprotector may have tripped. This is a safety device to prevent the microwave from overheating. It can be tested for continuity to see if it's blown.

Main Control Board If the microwave doesn't heat the main control board may be defective. This is not common. When a microwave doesn't heat the problem is usually within the high voltage circuit; either the high voltage capacitor, diode, transformer or magnetron.

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I have a microwave that is not working, I started it and a loud pop and smell happened. Everything is still working, the lights, the motor, the clock, the ventahood, but doesn't cook anymore.

Hello & Welcome to FixYa

The loud pop sound is caused when either it is a bad fuse or the high voltage capacitor itself has gone bad and needs replacement. As everything is working fine so the problem seems to be either with the door switch or the high voltage capacitor that is bad. Please check the door switch if that appears to be fine then to replace the high voltage capacitor you'll have to take it apart completely. Please get back to me if you have any other questions.


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 Confirm 110-120 volts AC is entering the primary coil on the high voltage transformer....meter works but can be fooled, I usually rig up a light socket and household light bulb and hook that to the 2 input wires for the transformer and see if the light works or not when you run the microwave....can even set a power level and watch the light go on and off. Discharge the capacitor first before attempting any checks or tests.

Scary but works!....

If ok, put those 2 wires back on the transformer. Discharge the capacitor .
Remove the 2 wires going to the the microwave a few seconds ( 2-3 ) and remove power....again discharge the high voltage capacitor . If you got a good crack&spark when you discharge the cap the second time you are creating high voltage but not using it ( bad magnetron or loose High Voltage wire somewhere )....if you get no crack&spark you are not creating high voltage.

How to check a common high voltage diode - click here. Check the diode with one end off/disconnected....should show continuity one way, reverse your meter leads and none the other way.....many meters are not high/good enough and often show no continuity ( infinity ) in both directions. A full continuity reading in both directions  (0 ohms) on the diode may be shorted/bad one!
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How to test high voltage transformer

#1 Turn off and un-plug the unit for 20 minutes

#2 There is as real risk of electrocuting yourself.

#3 Short the capacitor wit a screwdriver. BE READY for an arc or pop.

#4 You need a special high voltage probe to measure the high voltage. We're talking THOUSANDS of VOLTS here DANGER -DANGER - DANGER!

#4 If you want to try, you first have to DISCONNECT the capacitor from the circuit.

#5 DON'T re connect the power. Measure the resistance of the SECONDARY of the transformer (one lead goes to the capacitor). TELL ME WHAT THE MEASUREMENT IS..

#6 The rectifier could be bad (which is connected to the Capacitor) Usually - if one of th components is bad, it'll blow one of the internal fuses.

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This is usually an indication that the magnetron has gone bad, or something in the high voltage network controls the magnetron. Any one of the following components could be at fault: 1. Magnetron 2. High Voltage Transformer 3. High Voltage Diode 4. Magnetron Thermal Cut-out 5. High Voltage Capacitor 6. Interconnecting wiring I do not advocate taking any voltage measurements inside the High Voltage section of a microwave since the voltages can be as much as 14,000 volts or higher. If you are savvy enough to attempt yourself, WITH THE MICROWAVE UNPLUGGED, you will need to disassemble the unit to get to the high voltage section and take resistance readings. The magnetron or diode are likely suspects. If the magnetron or the transformer are bad, the repair costs can meet or exceed the price of the oven. Let me know if you need further assistance.

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