The details are from my childhood, so they may be vague, but any help would be appreciated on remembering what show this is.
It was live-action, aired in the US, late 80's early 90's, on maybe the PBS channel but I could be wrong. The premise was a man (and I think a male assistant) were scientists that ran a diner. In the back they had a time machine. The main cast of kids (maybe 3 of them?) would come hang out at the diner and go on time travel adventures. The scientists had to hide the time machine from the "bad guys," which I think were government agents or something like that. I think the agents would come to the diner to try to prove the owner was hiding something, but they could never find the time machine.
A specific episode I remember involved the cast traveling to medieval times. Their time machine was out in the woods and was disguised as a tree. They all left the machine to do... something, but knew that they had to meet back at the tree before the time machine left. I believe the side of the tree had a door that slid open to let one person in at a time (I think they could only travel through time one-by-one). One of the characters (I believe the assistant) was being chased by bad guys or something, and ended up either lost in the woods or late to the rendezvous. He ended up being trapped in the past. I think he also had a love interest that he met in the past. Or it's possible the love interest was the one who was left behind. Either way, someone was searching the woods for the time machine, but all of the trees were just ordinary trees because it had left.
I may be mixing up some of the details of that episode because I know there is a movie with a somewhat similar premise (Timeline). However I have very vivid memories of the kids visiting the diner with a hidden time machine, and the characters running through the woods and one being trapped in the past.
I have been searching for this show for years, any help is appreciated!