I've just acquired a 1965 Gillette Slim. Pretty good condition and cleaned up very nicely, but there's an issue with the quarter turn to lock down the mechanism: it's just not there or discernible. The razor locks down okay, but there's no haptic feed for that final quarter turn. I've used some mineral oil on the mechanism, but made no difference. I have another Slim and a Fatboy, and no issues with either. Is this something I can fix myself, or does it require a professional tune-up? It's my birth year razor, and the OCD part of me wants it to perform as perfectly as it can.