Why won't the alarm bell work on the G4 optima alarm system?

The bell will not work on our G4 optima alarm system. I have checked the wiring and there is a positive and negative feed to the strobe but only a black positive feed from the bell feed is it possible that the negative for the bell is using the strobe's negative as a return therefore the fault must be in the wiring of the box as the strobe works

Garry Ingham
December 2013