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sharp microwave oven

09/26/2009 6:22 AM

Hello There,

Just wondring about micreowave technology & how does it work, Technically ?, I have Sharp R200K model & not working, I checked basic things i.e. electricty supply & fueses, all fine its spins exhaust fan work but not heating up the object in side the compartment , what could go wrong?

Please explain,

Regards

Mateen

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#1

Re: sharp microwave oven

09/26/2009 9:50 AM

There's a device inside that generates radio frequency waves that excite water molecules in the food to be cooked. The water gets hot and cooks the food. The problem with this is that water boils at 212F so the food never gets any hotter than that. That's why microwave ovens can't "brown" food.

The Klystron generator is dead.

What is a Klystron Tube?

Chunk it in the trash and buy a new one!

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#2

Re: sharp microwave oven

09/26/2009 10:23 PM

Microwave ovens actually use Magnetron tubes (cheaper and easier to mass produce). BE CAREFUL as the power supply for this tube supplies LETHAL voltages and currents! These tubes are pretty reliable, but it is possible for them to fail. If everything else is confirmed working, the Magnetron is probably toast. I would just buy a new oven. Cost to repair/replace is usually close to or more than a whole new oven.

http://en.wikipedia.org/wiki/Microwave_oven

http://en.wikipedia.org/wiki/Cavity_magnetron

Just scrap the old one. There are some nice tinker parts inside, but only someone who understands the dangerous voltages and currents should be allowed to take it for parts. DON'T EVER play with & power up an exposed Magnetron!

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#3

Re: sharp microwave oven

09/26/2009 10:54 PM

Your magnetron is kaput. Careful trying to service this. Have a professional fix it otherwise your children might all be born naked from here after !

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#17
In reply to #3

Re: sharp microwave oven

09/28/2009 7:16 AM

it is not necesseraly the magnetron itself, it could

be the high voltage power supply which is broken.

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#4

Re: sharp microwave oven

09/26/2009 11:36 PM

The magnetron requires a DC voltage of several thousand volts. As others have said, This voltage is at LETHAL currents, and there is a capacitor that can store this voltage long after the unit has been unplugged from the power lines, so don't open the covers unless you know what you are doing!

That said, I disagree with the others: I have not seen a blown-out magnetron since the early '60s. I have repaired several microwave ovens by replacing the diode (a very special high-voltage one, around $10 last time I bought one, which was some years back) in the power supply that provides that high voltage to the magnetron.

Microwave ovens have gotten so cheap, that for most people, the best solution is to recycle the old oven and buy a new one!

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#6
In reply to #4

Re: sharp microwave oven

09/27/2009 6:27 AM

Thanks Mate!

I do appreciate your technical response & thanks to all of the guys who warned about high voltage capacitive current, I am glad to know peoples do care of each other at this platform, I wish & pray we should all be having the same feelings for every one & world would be nice / peace full place to live, (Ameen).

Back to the subject! I did discharged the capacitor at first attempt, kindly explain the location of this suspected DIODE, I appreciate if you can send snap short & advice some quick checks to confirm the integrity of the components involve, i.e. magnetron, capacitor, diode etc; & test procedure.

I do realized it's relatively cheep now a days, its not money But matter/hobby of my achievement that I can fix things!!

Thanks & regards

Cheers!

mateen

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#8
In reply to #6

Re: sharp microwave oven

09/27/2009 9:17 AM

If you MUST attempt a repair, please start here and be CAREFUL!

http://www.repairfaq.org/sam/micfaq.htm

Good Luck!

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#12
In reply to #8

Re: sharp microwave oven

09/27/2009 5:06 PM

Excellent Link!

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#9
In reply to #6

Re: sharp microwave oven

09/27/2009 4:34 PM

Ok, since you have already taken the thing apart and are still alive, I guess I'm not liable if I give you a little more info. My safety caveat still stands!

I do actually have a Sharp microwave, and I have had it apart to repair a broken shaft on the rotating table, but it's harder to take apart than my old one. So all the following are from a 1976 Sears Kenmore unit that I keep in the patio. Here is the circuit diagram:

Everything looks normal except the switch just above the transformer labeled 'Interlock Monitor switch'. It appears to be a normally-closed switch, which will guarantee blowing the fuse if the oven is somehow forced to operate with the door open. I couldn't quickly trace the circuit to verify that.

Here is a photo showing most of the dangerous stuff: The ckt diagram indicates 6.1kV on this unit.

At the top left is the overheat switch, which is mounted on the cooling fins of the magnetron. Below that are the two filament/anode connections of the magnetron, and at bottom left is the transformer. At bottom right is the capacitor/resistor/diode assembly. Above that is the cooling fan, with its motor visible between the magnetron and the fan housing.

Here is a detail of the c/r/diode:

The 6MΩ resistor ( the brown-striped cylinder) is intended to discharge the capacitor shortly after power-off, but never trust the resistor! Always discharge the capacitor using a well-insulated tool. The diode is the black rectangular box mounted on the rear wall of the unit, connected between the capacitor and ground. It is labeled HVPR15-4, and since it has a 1981 date code (5 yrs newer than the unit), must be one of those I replaced. If I recall correctly, at least one of the diodes I replaced (in a different oven) was a black cylinder about 0.3" in diameter and around 1.5" long, plus wires.

Be safe and good luck!

Dick

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#16
In reply to #9

Re: sharp microwave oven

09/28/2009 12:40 AM

Dear Dick,

Thanks indeed for real technical & moral support, I will follow the schematic & graphics & let you know the development, either side!!

Cheers!

Mateen

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#20
In reply to #16

Re: sharp microwave oven

09/28/2009 10:44 AM

You're quite welcome!

My internet connection has been intermittent, so I couldn't add a few other things:

1. All the switches, the fuse, and the thermal protector can be checked by simple resistance measurements (NO power, of course). Since your fan runs, these will presumably all be OK.

2. Once the capacitor is fully discharged, you can check the magnetron filament continuity by resistance, but you must disconnect one lead to the transformer first.

3. You can NOT check this diode with the diode setting of an ordinary multimeter! In order to withstand the 6+ kV, it is actually several diodes in series, so it will take considerably more voltage to make it conduct in the forward direction. I don't remember how much voltage it takes to make one conduct. If you elect to try a variable DC voltage to check for conduction, there MUST be a relatively high series resistance to prevent blowing the diode when it does conduct.

4. The capacitor is far too dangerous for me to describe any testing method here.

5. I gather you are relatively young. Keep up the learning! The ability to repair a wide range of both mechanical and electrical devices has kept me doing interesting work for nearly 60 years, and I don't see any reason to change! I only did a real job search once; all the rest of the time, the jobs found me.

Good luck!

Dick

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#5

Re: sharp microwave oven

09/27/2009 12:05 AM

I would strongly suggest, DO NOT ATTEMP TO REPAIR THIS YOUR SELF! If you do not understand the theory, then you don't have the technology to do the repairs yourself, which could lead to serious bodily injury, if not a nice WARM FEELING inside you. As previously stated, the magnetron is probably bad and they do have High Voltage inside the cabinet. So save your self some money on hospital bills and buy a new one.

If you do insist on opening it up to see how it ticks,and you get pass the high voltage lead,(discharged), there is usually some pretty cool magnets you can play with. Just be sure not to get any fingers or skin in between the magnets because they too will cause bodily injuries.

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#7

Re: sharp microwave oven

09/27/2009 7:45 AM

Every equipment has component so does Microwave. The component burned out is Megatron which delivers Microwaves energy.

Mirowave energy at room temperature heats water and other substance. Some time one can force it to heat metal, glass and ceramic by preheating so frequency resonates with microwave frequency.

Now since microwave is so cheap it is not worth of buying megatron and replacing unless some sentiment value attached

Masyood

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#11
In reply to #7

Re: sharp microwave oven

09/27/2009 4:51 PM

"The component burned out is Megatron (sic) which delivers Microwaves energy."

Are you some kind of psychic? How do you know it is the magnetron (correct spelling) that is burned out? I have worked on magnetrons since the early '60s, and the last time I saw one burned out was early in that decade. That of course does not mean that they don't burn out, but rather that there are several other devices that have a higher likelihood of failing, such as: the diode, capacitor, wiring, and transformer. Although the resistor can also fail, it will always fail to open condition (essentially infinite resistance). A failed resistor would not affect the operation of the microwave - it just wouldn't protect a careless explorer/serviceperson.

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#10

Re: sharp microwave oven

09/27/2009 4:48 PM

Since you seem to have checked the obvious problems, I will guess that the source of radiation which is the magnetron has used up its useful life. If the unit is old, for example, five to ten years old, then I recommend replacing the entire microwave unit. I just bought one from Wal-Mart for about 80 dollars and it is a GE. Nice and big and powerful, over 1000 watts. It is not practical in most cases to attempt a repair or replacement of the magnetron. Bon chance, good luck.

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#13

Re: sharp microwave oven

09/27/2009 6:02 PM

Im an appliance repair tech., the magenetron is usually the last to go.

See dkwarners reply and photos , the bottom photo shows a circular device in the upper left with two connections on it , it is described as a thermal protector and is simply a non resettable thermal fuse. Remove one or both connections and check for continuity across the fuse , if its open replace with same .

Another culprit is the door interlock switches , there could be three or four of them.

They are mounted around the door area and shut down the unit when the door is opened. Check all swiches for proper operation . Theses are all safe to check with power OFF.

Best of luck and stay away from the high power capacitor and magnetron circuits.

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#14
In reply to #13

Re: sharp microwave oven

09/27/2009 8:07 PM

Thanks for backing me up on the magnetron durability!

"the bottom photo shows a circular device in the upper left with two connections on it , it is described as a thermal protector and is simply a non resettable thermal fuse."

Actually, its the middle photo, and in this old unit, it is resettable (off @ 260°F, on @ 190°F). I'm sure that you would be right on newer units, since non-resettable ones are cheaper, and failure would in most cases lead to another unit (microwave oven) sold.

IF newer units continue to be wired in a similar fashion to the one shown, we can rule out the interlock switches, as his table rotates and the fan runs.

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#19
In reply to #14

Re: sharp microwave oven

09/28/2009 8:58 AM

Thats a nasty trick from the manufacturers using non resettable [fuse?] I thought that these round units all the ones I have looked at were resettable bi metal trips. Thermal fuses that I know of look like small resistors with pointy ends which blow when designated temperature is reached. Have they started putting these inside the little round cans?

The fact that the poster said no heat was being produced the culprits have been covered fairly well but would like to point out if it was heating very slowly I usually found the magnetron was suffering old age with low output.

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#15

Re: sharp microwave oven

09/27/2009 9:03 PM

Recycle, like I did. The Postie loves it and I haven't had wet mail since. Possums can't breed in there either which helps around here.

Good luck, Ky.

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#18
In reply to #15

Re: sharp microwave oven

09/28/2009 8:30 AM

I like that ky - you got it made over there.......heck, that thing wouldn't last 3 days in my neck of the woods, before something tears into it - it's just too tempting!

And I didn't know you have possum's there too.....younger days we could run 'em all night long

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#23
In reply to #18

Re: sharp microwave oven

09/28/2009 5:25 PM

CUTiger

you got it made over there. (?)

I made it my self because I couldn't repair it. To make sure there was no power stored (high voltage capacitive current) I chucked it in the ocean and then retrieved it. I was told to use a big fat screw driver (insulated) to discharge any stored power but thought it would be safer that way. Gutting it was easy after that. The door mechanism was still in tact and it is strong enough to repel even cute tigers, so far.

Since our neighbor has a Jack Russel, Possums are only on the roof, some times. It's been there for 6 years now and so far the young bloods have left it alone. We don't have cracker nights here but I agree that could be deadly for my little Sputnik.

Wish you were here, Ky.

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#21

Re: sharp microwave oven

09/28/2009 12:12 PM

Recycle it and buy a replacement one with the accompanying manufacturer's warranty. For safety's sake, it is the only recommendable way forward.

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#22

Re: sharp microwave oven

09/28/2009 5:00 PM

Hello,

Just a couple more tricks of the trade that I learned long ago.

1) If the Cap Arched when you discharged it. It "probably" ok.

2) use a 1M 1/2watt resistor and a 9V battery to check the diode. There are between 4 and 6 diodes in that pack at .7 V for each. Measure the voltage across the diode if its close then its "probably" ok.

3) You can check the transformer by turning the unit on and holding a screw driver near the core. If its working you will feel a buzz in the screw driver. about two inches or so from the core.

4) To check the maggie discharge cap, and disconnect both filament wires. You should all but a dead short across the filament. and infinite resistance from the filament to the Maggie case (or ground)

I would guess that if the unit turns on and the fans all run that you have an open diode or it is possible that the Maggie is just worn out. It will check ok all day but may be weak.

You can still get maggie's pretty cheap through places like MAT Electronics. I am not familiar with the model you have but if its one of the under counter fancy ones. its is worth fixing yourself. If its a table top. Chuck it you can buy a new one at wally world for under a 100 bucks! A maggie will cost you 30-60! and my rule of thumb has been if its gonna cost you half the cost of a new one. Your better off moving on.

Good luck...Remeber to discharge that cap EVERYTIME! and don't make any measurements with it turned on. Or you and your meter will be toast! hahaha!

bill12780

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#24
In reply to #22

Re: sharp microwave oven

09/28/2009 5:41 PM

Some Good points there!. Special thanks for the diode info.

Of course if the bleed resistor across the cap is doing its job, there won't be any arcing even on a good capacitor, so the lack of a spark does NOT necessarily indicate a bad capacitor. As you say, the presence of an arc would almost certainly indicate a good cap (and possibly a bad resistor, unless the discharge was pretty quick after power-off).

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#25

Re: sharp microwave oven

09/29/2009 2:30 PM

Significantly out of warranty?

Assuming no loose/disconnected wires, and no obviously-defective door switch/interlock...don't trouble yourself. Get a new replacement. Ovens that "run" but don't cook (don't produce waves) would generally be more costly to repair than replace.

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