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: http://www.repairclinic.com/Sharp-Microwave-Parts?s=t-FH-DZA119WRKZ-%3d%3db113a6c41

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:
http://www.repairclinic.com/Sharp-Microwave-Parts?s=t-FH-DZA119WRKZ-%3d%3db113a6c12

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:
http://www.repairclinic.com/Sharp-Microwave-Parts?s=t-FH-DZA119WRKZ-%3d%3db113a6c44

Thanks for choosing FixYa

Kelly

Posted on Sep 29, 2010

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Microwave pvm1870sm3ss not heating and have no wiring diagram


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.

Mar 23, 2016 | GE Microwave Ovens

1 Answer

Sharp micro doesn't heat


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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Oct 02, 2012 | Sharp Microwave Ovens

1 Answer

My JES636WK01 microwave heats up but it makes a loud humming noise. I suspected the Magnetron and disconnected it. The microwave was then operated and the n oiuse was the same. Do you have any ideas


Disconnect the HV transformer leads from the magnetron and the capacitor. Make sure none of the wires are touching the chasis or other parts. If the loud humming sound is still present, it is the High Voltage transformer making the noise. If the noise goes away, it is the High Voltage Capacitor going bad.

*MAKE SURE TO UNPLUG UNIT BEFORE TOUCHING ANY OF THE HIGH VOLTAGE LEADS! ALSO, DISCHARGE THE CAPACITOR BY SHORTING THE TWO SIDES AGAINST EACH OTHER!

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1 Answer

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.

Kevin

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1 Answer

Press to start microwave within 10 secs error 42 comes up on screen


E42 is a High voltage transformer (HVT-II) error. This can be caused by a bad HV transformer, a bad high voltage diode, a broken wire / burned terminal or a bad capacitor. The only way to confirm which one has failed is to read the circuitry with a meter and do a Visual inspection.

Kelly

Nov 16, 2010 | Samsung SMH7178STD Microwave Oven

1 Answer

Panasonic 1250 Watt genius microwave- will not cook. message refer to operators manual before use. Have owned for several years, has run afew times after unplugging for a few minutes, then nothing. clock,...


 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 magnetron....run 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!
Many  microwaves however have a defect in the microwave distribution system which causes the magnetron tube to overheat and cut out and/or damage the diode. 

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2 Answers

Kenwood CJ/AL28 microwave does not heat the food anymore...any reason why?


Check the high voltage fuse between the transformer and the capacitor, had the same problem with mineand this fixed it. Not an ordinary fuse though (5Kv 0.8a), cost me about a fiver.

Oct 16, 2008 | Microwave Ovens

1 Answer

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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1 Answer

GE microwave


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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2 Answers

Fuse is always blown


When does the fuse blow? If it blows the fuse as soon as you hit START, you probably have a high voltage capacitor shorting. Does it blow a few seconds after you press START? If so, then it may be a bad high voltage transformer. Does it blow when you open or close the door? If so, it may be a bad door swithc or a misaligned or damaged door or door switch mount. Please give more details.

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