Heating up a heat pack has caused the small

The black marks are burnt debris caused by splashing inside the oven. (Cling film usually stops this).
The waveguide cover, refered to as mica, has nothing to do with the actions of the magnetron, it is there to stop particles of food from entering the waveguide.

yes, it should not be doing this

The small plastic plate on the microwaves door with the spring it it needs to be replaced.

Place a glass pie plate in the oven with about 1/8" deep water in it. Turn the oven on high for about 30 seconds. If you get one small spot with intense boiling, the stirring antenna at the top of the oven is not rotating. It's usually driven by a small 60 RPM synchronous motor. To get at it, look for a pull-out plastic locking pin that holds the plastic dome or plate to the tope of the oven cavity. You may need to rotate the cover counter-clockwise a fraction of a turn to unlatch it. Sometimes greasy food vapor gets in there and gums up the bearings. Clean very thoroughly, but be careful not to bend the antenna blades. When you reassemble, make sure the antenna is properly seated in its bushing and engaged on its drive shaft. Repeat the test to see if you have even heating.

If you see no evidence of heating anywhere in the oven, check to see if the door switch in series with the transformer is malfunctioning. If the door switch is OK, you'll need a high-voltage measurement probe to determine whether the magnetron has failed, or (more likely) the high-voltage capacitor or diode is bad.

Hi

To locate the inlet cover, look towards the interior side of the microwave next to the microwave display. The inlet is typically located near the back of the microwave, in the middle of the wall.
Unplug your microwave from the electrical outlet and remove any food or containers.

Wipe the inlet cover, typically located on the right side of the microwave in the middle of the wall, with a damp cloth. The cover must be kept clean or it will not protect the microwave.

Dry the inlet cover with a soft towel.

Leave the microwave door open for several hours to air out the inlet cover.

Examine the inlet cover for wear and tear. If the cover has too much damage in the form of scratches or holes, it may need replacement.

Remove the inlet cover, if desired. Insert a putty knife under the plastic nail that holds the inlet cover in place. Pry out the nails and set them aside. Pull out the micro plate in front of the inlet and set it aside. Remove the inlet plate.

Install a new inlet plate. Slide the new inlet cover into its place. Replace the nails back into the side of the microwave by aligning them with the holes of the side.

Please get back to us if you have further query else please accept the suggestion.

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Check the door safety interlock switches.

Do you regularly shut the machine off by opening the door? Each event causes a small arc on the tiny contacts and will burn them. When they can no longer make contact, your microwave doesn't know if the door is open or closed. The machine must know the door is closed to turn in the magnatron (that thing that makes the heat).

Remove the cover and check the microw switches with a volt/omh meter.

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you need a new microwave. There are no heating elements in mocrowaves just micro-waces. It only heats moisture it does not heat objects. It is to dangerous to replace the actual device that heats liquids.

Although the carousel is usually driven by a small motor that's unrelated to the heating function of the microwave, the microwave cabinet needs to be opened to repair it. So we recommend that you seek the assistance of a qualified appliance repair technician. Inspect the plastic drive coupling that the tray sits on, as well as the ring-and-roller assembly, to make sure that they're working properly.

Go to repairclinic.com and type in your model number. I suspect you may be talking about item 1063610, Turntable Coupler. You may also try searspartsdirect.com. If this isn't the part you need, please post back and provide more information, so that I may assist you better. I hope this helps.

The magnetron tube which makes the microwaves is behind that cover plate, and it has gone bad. It is a replaceable part, but it's not worth the cost. Part and labor would be more than a new oven. It's time to treat yourself to a new oven.