hi, Gramps!
Sorry to hear of your trouble. I would ask this same question over on POTN. There are many, many very knowledgeable folks over there that are eager to help, and chances are that there is a thread there specifically for the 400mm f5.6 lens.
Whenever I've had an equipment-related issue, I ask on POTN and get very accurate, helpful info almost immediately.
Good luck!