Hey guys, great info here. I dusted off my old SL-7 that I have not had plugged in for about 15 years. I have kept it in a plastic bag on the basement shelf for many moons.
I had the same issue with no tone arm movement. to fix it, I had great success without doing much, I wanted to share my story.
I took the plexiglass cover off according to the service manual (thanks for that). I replaced the drive belt--(the little "O" ring rubber drive belt) with a 45 cent O ring from hardware store. this was a No Go. Next, I removed the drive belt, wound out the tone arm with the power off. I turned the power on and flicked the motor pulley ever so slightly, and viola! it started spinning like crazy. It was just frozen in place.
I put my 45 cent O ring drive belt on, and it works perfectly! woo hoo.
Thanks, I could not have done this without reading some of your posts.