Craftsman GDO's are made by Chamberlain and very similar in design to Chamberlains. The main difference is cosmetics.
Here is a manual for a current model. Based on the age and the fact that yours is a Secutiry Plus System and uses a late model logic board I think that the operation is similar.
Look at the trouble shooting on pages 25 & 26
If that does not help you can call Craftsman Tech Support at 1-800-528-5236
You can get replacement parts here.
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