It is possible to rebuild the tension. Is DH or a kid around who can help you read a repair manual? There are schematics on line how to rebuild a tension. For that matter if you have an owner's manual it will tell you how to rebuild it. Chances are though it just needs a good cleaning since it hasn't been in use for a while. you should always start making adjustments with the upper tension. Messing with the bobbin tension is very easy to mess up completely (and lose the screw). Usually the bobbin doesn’t come out of balance and changing the top tension is all you need to do. Also, if you use a fluffy dental floss, you can run that through your thread guides and tension disks to really get everything clean. You should be able to find more info in the vintage sewing machine section of this forum.