I'm renting an apartment with an ancient Admiral electric oven range
in it. This range has just one oven knob: a temperature control whose
last choice is "Broil." Recently, when I set the oven for a non-broil
temperature, such as 400 degrees, the broiler element comes on instead
of the bottom element, which remains cold. The oven heat cycle
indicator light still comes on too, though it doesn't come on (and
isn't supposed to) when the knob is set to Broil. Because the heat is
coming from the top element, the oven never shuts off, just keeps
heating and heating. Obviously I can't bake anything like this.

Any ideas what causes this and how I can fix it?

Thanks.