Dryer is a Whirlpool GGQ9800LW1, about 20 years old. The belt broke, but the motor is also not spinning when it's turned on. No 60 hz hum either, and the armature spins freely when turned by hand.

The thermal fuse has continuity, and the door switch is fine -- turns on the light, and you can hear a relay click inside the control panel when you move the switch.

Where do I start troubleshooting? I can get parts diagram online, but not a circuit diagram, apparently.

Start with these:

Clothes Dryer Repair Manuals Table of Contents

https://www.whirlpool.com/digitalassets/MLPDF/Repair%20Part%20List%20-%208180012.pdf

Check to see if there is voltage to the motor when there should be.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Thanks - fixed it. New belt did it -- there was a switch in the tensioner arm that shut the motor off if the belt broke.

I didn't expect a dryer to be so overdesigned.

Not overdesigned...OLD SCHOOL!!! and not a Chinese POS!

Yep, made in Michigan.