House alarm sounder fault

[ajstars]

My house alarm fitted in 2007 has started emitting a single tone rather than the dual tone it used to from the external bell box. I have checked the power supply and the battery, both good. I have cleaned out the alarm sounder itself and stripped it down, cleaned the metal disc and refitted but it's made no difference.
Nothing appears to be burnt or out of the ordinary.
Is this likely to be a circuit board fault in the bell box or just a new sounder?
Thanks

 

[Anon_LG]

Could you provide a picture/s (good quality, all the text must be readable) of the internal circuit board, both front and back? This would help a lot.