Had a Valmet 412 years ago the the mid adjustment did the same thing- solder broke on the fixed portion. It was repaired cannot remember by whom but believe factory service stateside. Beware of silver solder- lots of heat involved to get it to flow properly. Should be able to accomplish a repair with lower heat solder or possibly an epoxy. May not be traditional but certainly some adequate high strength adhesives out there. MGW would be a good place to start.