It may be the fuse or the thermal cutout / thermostat associated with the magnetron.
You say you have the service manual? If you mean the folded-up "mini-manual" that should show the relative locations of the parts in a perspective view.
Also, you can find exploded views at the Sears parts site
Looking at the "interior parts ii" exploded view here
, it shows the fuse (drawing reference # 5001) on the small RF noise filter board located on top of the air duct assembly.
You'll need to remove the oven from the wall to get to it for testing & replacement.
The installation instructions are handy to pull the oven from the wall.
If you've misplaced yours, the Sears site may have them free.You can just enter your full
model number at the Sears
On the resulting page, look to the right for the blue "View manuals"
button. Click there.
That Sears-related site has many free manuals. While the manuals they
list there are all free, they may ask you to register.
Many times, the owner's manual will have the installation instructions
If yours is not listed there for free, you would have to order it
through the Sears
We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.