By process of elimination, the opening I've described is clearly #3 in
Oren's list of 4 possibilities. Rather than replace it, I may try to use a
very small piece of aluminized duct tape to reinforce the two corner points
so that the mica cover would fit snugly in the two bottom grooves and cover
the entire opening.

THANKS! digger

"Oren" wrote in message
...
On Mon, 19 Oct 2009 18:54:36 -0700 (PDT),
wrote:



diggerodell wrote:

Inside my microwave is a small 3x5" opening which is covered by a 3 x 5"
piece of thin shinny cardboard. At one time, the four corners of the
cardboard fit snugly into four small slots, so that it barely covered
the
full opening. Now those four corners have become broken and worn,
allowing
the cardboard to slip 1/4" below the top opening. Will this affect the
performance or safety of the microwave? If so, can I replace the
cardboard
with a piece of non-shinny cardboard?

I strongly suggest you post a photo because there are several openings
in a microwave cavity:

1. door

2. steam/fumes vent

3. magnatron duct: A metal duct running from the microwave-generating
magnatron to the interior of the oven. This is covered by a piece of
mica to keep the duct clean of food deposits. However the mica is
transparent to microwaves. The mica resembles cardboard and is
silvery-white when new but discolors from food as the oven is used.
Don't run the oven without this cover in place, and don't substitute
anything for mica.

4. interior lightbulb window: Usually a series of small holes (1/8"
to 1/4" diameter) to block microwaves, and there may be a metallic
reflector behind the bulb. If you replace the reflector with anything
but the original part, be sure it can withstand the heat (bulb can get
very hot) and will not short against the lightbulb metal bottom or the
wiring. Aluminum foil tape may be suitable here, but, contrary to
another post, it is not necessarily made to withstand high heat since
its adhesive may be rated for just 160 Fahrenheit. Some aluminum foil
tape has adhesive rated for about 250-300F. If aluminum foil tape
comes undone, it could slip and short.