I have two of the subject microwaves, both manufactured in late 1980.
One has developed a serious problem where it makes a loud buzzing
noise, almost like a Jacob's ladder type sound, and yellow flashes are
visible in the cavity when this happens. I thought it was the
magnetron, changed it out with the one from the other oven, but the
problem continued. Any ideas on what could be wrong with this one?

On the other one, which has been in service since new, it developed a
problem with the turntable after I switched out the magnetron with the
other unit. Now it sometimes won't turn at all, or turns very slowly.
Is there a belt somewhere that I need to replace? Changing out the
revolving metal plate that the glass dish sits on hasn't made any
difference.

I really like this microwave. It's roomy, it's got enough power for
me, and I hate touchpads and love that all I need to do is turn the
knob and hit start, and I want to keep one good one of these going for
as long as I can.

I'll make whatever repair is cheaper and easier to whichever oven based
on feedback I receive here and am willing to sacrifice one of them for
parts if needed. Thanks for any advice!