Sony Betamax SL-HF400 won't eject tape

[ESigma25]

I have a Sony Betamax SL-HF400 (a Super Beta Hi-Fi unit from 1985) with a tape stuck in it.

https://pbs.twimg.com/media/EGxyUdaW...name=4096x4096

When you press the Eject button the tape retracts into the cassette like it should, the mechanism lifts the tape up like it's going to actually eject it...and then drops the tape back down almost immediately and pulls the tape back out again.

This thing looks really complicated internally so I don't really want to go poking around in it without an idea of where to start, what should I check to fix this eject problem?