KitchenAid Model: KHHS179L Serial No: TR3314099 Experiencing heavy sparking. Where is Wave Guide cover, and how to replace it if necessary. Oven is mounted above stove and too heavy for this old man to move. Thanks for your help.

You can't, you take it to a Sharp Service Centre. The magnetron has a poisonous heavy metal core and the capacitor holds a heart shopping 20,000 volts. The microwave emissions can make you blind in 8 to 10 seconds. The case has security screws to stop you getting inside the case.

At 17 years old, you should be replacing it. Having said that, "they don't make them like they used to".

Wave guide cover appears easy to get, the magnetron not so easy. My guess is that is is a dealer only spare.

https://www.google.com/search?q=sharp+r-510+magnetron+%26+wave+guide+cover&

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not until you have it checked out in a repair shop.
using a microwave oven where it is possible that the microwave generated by the magnatron can escape to outside, means that any person in the vicinity of the unit can suffer radiation burns and /or get cancer if allowed to be exposed a simple check is all that is necessary

Get rid of it and buy a new one. This sounds like a dangerous item to continue to use.

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Some ovens have a tuning stub in the wave guide. If it has become loose or rusted, it could cause the problem. If that's the cause, you would have to transfer all of the oven's components to a new cavity. This is usually not done due to the cost, although I did do this once on an oven that was still under warranty.

Make sure the wave guide is clean inside.

Also check for rust near the wave guide cover. Re painting damaged areas might help.

Sparks fly, too, if the ovens contents do not contain enough water to absorb enough of the microwave energy. Leftover energy converts itself into electrical current in any conductive material (such as cavity panels).

If you need further help, I’m available over the phone at https://www.6ya.com/expert/david_29ad5d1dd86564b0

the magnetron may have burnt itself up. it may be time to replace

If there is spark please do not switch on unless you have confirmed the fault. It is possible that the Magnetron is leaking and sparking to the ground. As you have mentioned it was stored, it is quite possible that insect can get into the oven and create soft spots. When you switch on these will create a short and make the voltage jump and in the long run damage the magnetron.
So Remove cover after disconnecting , check for dirt/moisture in the cabin, if so wipe clean and dry, check the magnetron and area clean and use an jet air to clean off.
Now switch the microwave power on and use a cup of water to test.
If you notice any sparks shut off as the magnetron or the main HT capacitor can be faulty. Replace with same type. If not check the waveguides within the cabin for food particles that can burn and spark, check for rust that can corrode and spark when the ceramic paint has worn out.
If there is heating without any issues, you can use the microwave safely

Click on replacing waveguides: http://www.espares.co.uk/advice/microwave/a/5/1146/how-to-replace-a-microwave-guide-cover.html

That "Cardboard" item is a wave guide cover and is critical part of the machine.
It is made of mica and needs replacing. The magnetron is OK as long as it does not spark too much or get any foreign items in there.
Replace the wave guide cover.

A dirty or damaged wave guide cover can cause sparks, fire and noises. You can order a replacement wave guide cover from many appliance parts websites for less than $15 in many cases. It should fix the problem.

Your problem is a common one. When you see arcing inside your microwave, it's because of food or liquid left in the cavity or on the wave-guide cover:

• If the wave guide has any burn spots, replace it and clean the unit well.
  
  
• If the cavity has any burn spots, spot-sand it smooth and touch it up with microwave interior paint.
  

Note… If there are any burn holes in the cavity, don't use the unit-seek help from a qualified appliance repair technician.

Try pressing "Clock" and then "Start"