sengsational wrote:

I've got an old convection microwave that's probably seen it's better
days, but I thought I'd throw this out here and see if there are any
ideas. It's actually on it's second magnetron tube, which I replaced
like 8 years ago. The symptoms (way back 8 years ago) were when you
tried to nuke something, it made a loud hum (loud enough to say "oh
sh*t" and turn it off).

This time, the controls seem to work fine (set it to nuke or to
"convect" something, the timer counts down as long as the door is
shut... just exactly like it's working), but the fan motor doesn't
start, there's no gentle hum of nuking. Set to convection, the fan
motor doesn't start (I didn't leave it on long enough to see if the
heating element heated).

Anyway, it sounds like maybe the momma transformer has given up the
ghost. Yes? The baby transformer is putting out it's 13, 3, and 8,
like it should, which is why the control panel is so happy.

www.repairfaq.org has lots o' information about microwave ovens.

Is there some kind of interlock that prevents the magnetron from
nuking if its cooling fan doesn't spin? I think I saw a sail switch
for this in ours.

www.microwavedisplays.com has lots of oven repair manuals available
online at no charge.