Film or TV show about a Magic Marble

I am trying to identify a fantasy story that was either a movie (theatrical or made for TV) or show I saw as a young child. In it, there is a Magic Marble that can work magic and has a will of its own, but when held by someone, it has to obey them. I remember one key scene very vividly:

The marble has fallen into the hands of a child who is the villain after it had been in possession of a child who is the protagonist. The villain holds the marble and makes himself all-powerful. He is sitting on a throne or standing at the top of stone steps somewhere outdoors during the daytime. He is gloating over the protagonist, who has been apprehended by the soldiers he wished for. As he gloats, he begins tossing the marble up into the air and catching it. Thinking fast, the protagonist waits until the marble is in the air (i.e., not in the villain’s possession) and shouts: "Magic Marble help me!" The villain and all his soldiers disappear, and the protagonist catches the marble as it bounces down the steps. I can recall very few other details, but here they are:

• I seem to recall the entire cast was children.
• I think the actors all had American accents.
• The movie or show was in color.
• I think it was on PBS (New York Metro area Public Broadcasting Service)
• I seem to recall that the scene I described and perhaps the rest of the production was set in China (based on set design and costumes), though all the principle characters I recall were caucasian.
• I saw it some time between 1973 and 1979, though it could easily have been filmed earlier.

Would love it if someone could help me id this story.