Hi!

I remember way back on an old Norelco microwave, the controller
failed, so I got out the old mechanical timer just like the one
on my first Heathkit microwave. I can't stress how this simple
and easy to use feature is missing on todays ovens.

I'm not sure I follow what you are saying. Are you saying that the
microwave, when its controller went bad, failed in an "on" position?
If I understand you correctly, I think it would be a far better idea
to have the oven fail in an "off" position.

I think on better models they used aother timer to cycle
the defrost.

Older microwaves sometimes had multiple timers. My grandfather has a
Litton microwave oven with two timers. One is the main timer, which is
an analog "knob" on the front. The other "timer" is engaged and used
in conjunction with the main timer when DEFROST mode is turned on. It
consists of a slow turning motor that drives an irregular cam. When
the cam rises up, it turns on a microswitch that enables the high
voltage and magnetron.

Today, the controller board in a "typical" oven has two relays...one
that runs the fan, turntable and light and another that can be turned
on and off to cycle the magnetron.

I'm not sure how long that technique was used, as I have a slightly
newer Litton microwave oven that is largely the same internally but
has a "rolling digits" timer and a solid-state control board for the
defrost and low power modes.

Those older models had at least 3 fail safe microswitches on
them, but the switch took the full current.

Newer microwaves also have multiple microswitches. (At least I have
yet to see one that does not, and that's on ovens manufactured as
recently as a year ago.) However, in normal operation, the microswitch
that interrupts the oven's operation when the door opens is actually a
low voltage signal to the controller board.

William