Panasonic AG-7500A: Can't load tape

[ESigma25]

I acquired a Panasonic AG-7500A S-VHS VTR off eBay recently, and it has a problem I don't really understand. When you power the machine on, the head drum starts spinning on its own and none of the transport buttons on the front panel do anything. You also cannot load a tape, and if you load one manually the machine will not thread it or eject it, it has to be ejected manually by operating the loading mechanism in reverse.

Here's a picture showing the head drum spinning:

https://i.imgur.com/84C2gkR.jpg

Where do I even start with this? I have a service manual, thankfully, but I don't even know what's causing this problem.