Can you return the oven? You can use the argument
that it's not fit for its intended purpose. Which it isn't.

Nope, my inability to forecast the consequences is not
the fault of the seller.

If your description is correct, the oven is grossly misdesigned. You do not
implement variable power by turning the magenetron on for 15 seconds, then
letting it sit for a minute! I've /never/ seen a microwave oven that works
that way. My home GE works fine, as do all those I've seen where I've
worked.


I always assumed variable power was simple duty-cycle variation --
pulse-width-modulation -- over a fraction of a second. That the "on" time
would be fixed at 15 seconds (!!!), with the off time varied, is absurd.
It
would produce exactly the effect you see.

Yep, that's the way most of 'em work. The problem is the filament in
the magnetron. Much shorter and you don't get any power out cause
the filament ain't hot yet. With enough mass inside the oven, it
averages out pretty well. For a single frozen hamburger at 1100W, not
so much.

I've never heard of varying a magnetron's power by adjusting its filament
voltage! I've always ASS+U+MEd there was some way of turning the tube on and
off by varying an electrode voltage. (Simply pulsing the anode voltage would
produce variable output.)