Lg microwave oven... always got fused busted.. one by one i tried to trace the main problem i disconnect hv. diode. hv cap and the transformer. if i start it , it works ok but if i install back transformer suddenly fuse blows out. i double check transformer resistance. it is working normally. why it always blows fuse if i install this transfo

The transformer is probably drawing too much power by a low resistance or a short somewhere on the secondary's side. The components on that side include the HV cap, the HV diode, and the magnetron all of which should be tested. I would try the cap first but make sure you discharge it before operating on it.

did u check plug n internal fuse..lol. my diode that went to earth was the culprit coz my microwave leaked into base and diode to stop leakage to earth had blown

Forget the control board as that has nothing to do with the hv fuse and if that is blowing then the board is turning the magnetron on. The only things that will cause that to go are the hv cap, HGV transformer, hv diode or magnetron. Usually I would look at the cap and diode first as likely culprits but when they go they normally just go open circuit on the side and short on the cap all the time although could be breaking down under load. The most likely things that break under load intermittently in my opinion would be the magnetron which is just like an old fashioned valve in a magnetic field or the insulation layers in the transformer.


Please be extremely careful as the parts you are playing with are high voltage and the cap can kill you even when unplugged as if the discharge resistor had gone will remind charged and ready to discharge through you at high current like an electronic ignition system does on a car so always discharge the cap but touching an insulated handled screwdriver or pliers across its terminals before touching them. It will make a loud pop sound when it discharges. Only do so when unplugged.

Sorry to read about your problem, I hope this helps you out.

here are some issues, but you still maybe under warranty.

1. Blown fuse in HV transformer primary circuit or HV fuse (if used).
2. Bad connections (particularly to magnetron filament).
3. Open thermal protector or thermal fuse.
4. Open HV capacitor, HV diode, HV transformer, or magnetron filament.
5. Shorted HV diode, HV capacitor (will blow a fuse), or magnetron.
6. Damaged protective VDR from filament to chassis (not commonly used).
7. Defective HV relay (not commonly used).

Microwaves must be serviced by technicians due to the inherent dangers involved in repairing these appliances. Internal capacitors can retain a lethal electrical charge even though the unit is completely unplugged. A microwave radiation leakage test must be performed on the unit following any internal component repair.

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Sorry to read about your problem, I hope this helps you out.

You can many different issue's

1. Blown fuse in HV transformer primary circuit or HV fuse (if used).
2. Bad connections (particularly to magnetron filament).
3. Open thermal protector or thermal fuse.
4. Open HV capacitor, HV diode, HV transformer, or magnetron filament.
5. Shorted HV diode, HV capacitor (will blow a fuse), or magnetron.
6. Damaged protective VDR from filament to chassis (not commonly used).
7. Defective HV relay (not commonly used).

Microwaves must be serviced by technicians due to the inherent dangers involved in repairing these appliances. Internal capacitors can retain a lethal electrical charge even though the unit is completely unplugged. A microwave radiation leakage test must be performed on the unit following any internal component repair.

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Sorry to read about your problem, I hope this helps you out.

No heat but otherwise normal operation.
Possible causes:

1. Blown fuse in HV transformer primary circuit or HV fuse (if used).
2. Bad connections (particularly to magnetron filament).
3. Open thermal protector or thermal fuse.
4. Open HV capacitor, HV diode, HV transformer, or magnetron filament.
5. Shorted HV diode, HV capacitor (will blow a fuse), or magnetron.
6. Damaged protective VDR from filament to chassis (not commonly used).
7. Defective HV relay (not commonly used)

Microwaves must be serviced by technicians due to the inherent dangers involved in repairing these appliances. Internal capacitors can retain a lethal electrical charge even though the unit is completely unplugged. A microwave radiation leakage test must be performed on the unit following any internal component repair.

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All of your measurements sound good, quite possibly bad mag.

No heat but otherwise normal operationA shorted HV diode, magnetron, or certain parts of the HV wiring would probably result in a loud hum from the HV transformer but will likely not blow the main fuse. (However, the HV fuse - not present on most domestic ovens - might blow.)
If the main power fuse is located in the primary of the high voltage transformer rather then at the line input, the clock and touchpad will work but the fuse will blow upon initiating a cook cycle. Or, if the fuse has already blown there will simply be no heating action once the cook cycle is started. There are other variations depending on whether the cooling fan, oven light, and so forth are located down stream of the fuse.

Some models may have a separate high voltage fuse. If this is blown, there will be no heating but no other symptoms. However, high voltage fuses are somewhat rare on domestic ovens.

A number of failures can result in the fuse NOT blowing but still no heat:

• Bad connections - these may be almost anywhere in the microwave generator or the primary circuit of the HV transformer. A common location is at the crimp connections to the magnetron filament as they are high current and can overheat and result in no or intermittent contact. See the section: See the section: Testing the magnetron.
  
• Open thermal protector - usually located on magnetron case. Test for continuity. It should read as a dead short - near zero ohms. See the section: Testing thermal protectors and thermal fuses.
  
• Open thermal fuse - some ovens have one of these in the primary circuit. It may be in either connection to the HV transformer or elsewhere. Test for continuity. It should read as a dead short - near zero ohms.
  
• Open HV capacitor - see the section: Testing the high voltage capacitor. A shorted HV capacitor would likely immediately blow the fuse.
  
• Open HV diode - see the section: Testing the high voltage diode.
  
• Open magnetron filament - This failure may also be due to loose, burnt, or deteriorated press (Fast-on) lugs for the filament connections and not an actual magnetron problem. See the section: Testing the magnetron.
  
• Open winding in HV transformer. See the section: Testing the high voltage transformer.
  
• Defective HV relay. A few models use a relay in the actual high voltage circuitry (rather than the primary) to regulate cooking power. This may have dirty or burnt contacts, a defective coil, or bad connections
  
• Shorted HV diode - see the section: Testing the high voltage diode.
  
• Short or other fault in the magnetron - see the section: Testing the magnetron.
  
• Short in certain portions of the HV wiring. See the section: Testing and repairing the wiring and connections.

Depending on design, a number of other component failures could result in no heat as well including a defective relay or triac, interlock switch(s), and controller.

If you are interested in doing it yourself the following link will help : Microwave Repair Manual

(**All the above references to tests are found here)

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

sounds to me like the magnatron has shorted out. i don't know of a way to test them unless you have some high dollar equipment. a continuity test prob wont tell you anything. what reading did you get on the cap when you tested it?