The foot pedal that came with my Bernina 530-2 Record is a great big clumpy thing, rectangular and iirc around 8 and a half inches by about 6, although it is languishing in the back of a cupboard at the minute and has been for some time so I could be slightly mistaken about the size. But it is BIG. Well although it did turn the motor when I pressed it, it was stuttery and obviously something was wrong. When I opened it up it was broken inside - the carbon discs had crumbled away - so I removed it and attached a generic clamshell one that came with a YDK motor set up I had. I just cut the two wires where they joined the foot and wired them to the new one. The clamshell foot works great for me but it does move itself around on the floor a bit whilst being used which is a bit annoying, one day I'll screw it on to a piece of board or something to keep it still. I kept the old pedal in case it ever gets repaired.
My 717 came with a slightly less huge green metal pedal, rectangular, but narrower, around 3 inches or so. I use it on my 830 which came without a pedal because the end that connects to the machine is the same. I've never peeked inside that pedal it so I don't know what's in there...
What size is your pedal? Does it have wires coming straight out of it or is there a connection socket bit that plugs into the pedal itself?
I know the old pedals with carbon pile rheostats (... I may have got the name of the bits wrong there...) can start to give up the ghost when the discs start crumbling away with age and use. I've heard of people replacing them.
@Barnyard I think has mentioned the carbon discs in the past and would know of such things but he's not been around lately (.. I do hope he's ok).
Also the interference suppressors in older pedals can fail but they make the machine suddenly start running without you even being near the pedal, that happened to me with my 807, it was like a machine possessed, so if anyone has an old Bernina, never leave it plugged in and unattended.