Beogram RX issue

Hello, I'm new here and to vinyl in general so I'm hoping someone can help me out.

I picked up a very nice looking Beogram RX sans cartridge from a friends basement a few weeks ago. When I got it home I plugged it in to test functionality. Everything seemed to work except for when I pressed 33 or 45, the tonearm would swing out and drop as expected, but then immediately swing back to it's resting position, leaving the platter spinning. 

Being an engineer I examined the inner workings and found nothing glaringly wrong, but unable to find a service manual I decided a free turntable wasn't worthy of much time and moved on.

Last week I snagged another RX off eBay cheap, claiming to be working but again, sans cartridge. I bit and ordered a smmc4 from soundsmith for it the same day. This morning it arrived, so I plugged it in to try it out. 
It's doing the same thing as the basement one! Tonearm immediately swings back to resting position.

So what I'm wondering is, does it somehow know there's no cartridge installed and is swinging back to prevent
damage to the LP, is this a common point of failure in these players, or am I just incredibly unlucky?
I'm more motivated than ever to get this up and running, having invested $200 in a new cartridge, so any advice would be be very appreciated.

Thanks
Brendan