When conncted to power plug the red indicator light comes on. However when the audio signal source is switched on the red light would normally changed to green after sensing the signal. This is not happenng now for one speaker. I swapped powerlink and power cord, thus suggesting sonething wroung in speaker instead of cables as the other speaker works fine.

The speaker started this suddenly while working fine, since then i cannot activate the green light. 

Is this a simple fix?

What could be wrong? I guess something wrong with the signal sensor trigger switch. All modes line,left or right are not switching the speaker. 

Than

Funny i have exactly the same problem on 1 of my BL4000 (mk1s), started just before Christmas. 

I had the LED go an orange colour on another LB4000 a few months ago and the circuit board was replaced and it only cost about $120.

How old are your LB4000s?  Mine are about 7 years old now and get alot of use connected to my sons xBox360 and PC, so i'm happy with them

They are about 8 years old. Almost looks like a power issue.