Mystery component in motor speed controller

[jimmyblacksmith]

Hi,
This circuit is a 240 volt motor speed controller. There's a missing component (blank space above the triac), which looked like a zener diode, but it melted, so I can't identify it. I've tried diodes, thermistors, and diacs, but the motor only runs at top speed, or not at all. Can anyone tell me what the missing component should be?

 

[Bluejets]

Diac...

 

[jimmyblacksmith]

With a diac in place I have an open circuit.

 

[Bluejets]

With a diac in place I have an open circuit.

You need to be more specific.
Your reply could mean any number of things.

 

[Alec_t]

+1 for diac. If that hasn't solved the problem there must be some other component faulty. The triac is the most likely culprit.