How to access underside of Perfection 1200 scanner glass?

After I locked the scan head and lifted the back, the scan head was lifting on the lock side, along with the top, although it was staying down on the other side.

I tried lifting the top with the head unlocked, but it lifted just the same.

So I plugged the unit back in (I had unplugged it, and I would recommend doing so) started a scan and then pulled the plug when the head was about 3/4 of the way to the front which stopped the head down there. I was then able to lift the top off without moving the scan head.

(I am assuming the scan head is the big black mechanism that moves back and forth on rails when doing a scan.)

When I put the top back, a bump on the side of the top interfered with the scan head. I manually moved the head just a little bit, and was able to get the top back down.

What I did doesn't sound right; but I think the scan locking mechanism wasn't working properly.