LM 800a lamp problem

[jgj]

I have a Liftmaster 800A, about 5 years old, and the lamp in the unit now stays on permanently. Pressing the lamp button on the wall switch causes the normal click from within the unit as though a relay is trying to operate, but the lamp stays on although very slightly dimmer. I've removed the bulb as a temporary solution, but I'd like to repair it if it's not too expensive. Is it a fault on the control board, or is there a separate relay that has stuck closed?

Regards, John