DFF1 sounder no longer recognised

Hi
After 2 + years of working well my sounder is no longer recognised on start up.
I have a TZT serial no 4393-0870, DFF1, PSU012, Hub, Radar.
On start up two days ago, the unit, for unknown reason started in demo mode. On turning everything off and restarting it was no longer in demo mode but had lost the sounder. There is no longer an option of a sounder screen in the home key selection window. Only Plotter, Weather, Radar, Camera screens appear.
The home key - Menu - has no sounder option listed.
Current update version is 1950123-03.12
Please advise as to how I may troubleshoot this issue.
Thank You
Andrew

Power up the system and let it sit for 5 minutes, then check under MENU - INITIAL SETUP - SENSOR list and see if your sounder is showing up in the listing. If not, then it looks like your sounder is no longer talking on the network. Find the sounder unit and look at the status light (SB-TY). What is it doing? (Off, Flashing quickly, Flashing 1 sec on and 1 sec off, Lite Solid)

Hi Johnny
The green light on the DFF1 is off. The green light on the hub where the network cable from the DFF1 joins it is flashing green. The fuses are ok and there is power to the DFF1
Thanks
Andrew

If the DFF1 standby light is off, then it sounds like you might have the MODE switches setup incorrectly for the TZT. Check the mode switches are DWN, UP, UP, DWN. (Two outside ones down to circuit board and the middle two are up)

Hi Johnny
The switches are correctly set as you describe them. They have always been on the TZT setting and have not been changed.
Thanks
Andrew

It appears something is wrong with the sounder. I recommend you send it into the factory service center for evaluation. The ST-BY light should be lit with both power applied and the MODE switches setup as we discussed.

Ok - will do and thanks for the help