| rcgbob44,
I have a simillar prob with mine (M357T). It's been caused by a short circuit and resulted in the small 10amp fuse blowing. This stops the alarm from charging up it's backup battery. Trying to get a new fuse.
May not be what you're experiencing cos my alarm goes mental approx. 10 seconds after setting, but that is a trait of the flat alarm battery.
The fuse is in a small holder (the size of you middle finger nail-ish). Mine is located just to the right of the battery. It looks like a special type of fuse so am having problems replacing it at present. | 
Guido
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