I have a 50 yr old Sigma D-3 steel 6-string wood body acoustic guitar by Martin that I received for my 9th birthday. I'm not certain about composition but it appears to be a spruce top with sapele sides and back, rosewood finger board and a hardwood neck. The instrument is in pristine condition because it's rarely been out of it's case.
A few months ago I pulled it out, put on some new strings (10's) and tried to play it. Sadly, there was just way too much string buzz, seemingly from all 6 strings. After replacing the plastic nut and saddle with Tusq parts and adjusted them to produce my typical string action for acoustic guitars. The buzzing persisted. I then noticed that the neck was absolutely flat (perhaps even a very slight backward bend) so I slowly (over 2 weeks) started loosening the truss nut and tightening the strings, while storing it in a new humidified case. Unfortunately, I couldn't create any neck relief, even as the nut approached the end of the truss rod, to the point that I could hear and feel the rod rattling around slightly inside of the neck.
Although this guitar was never exactly taken care of properly, it's never been subjected to any extreme conditions. It's spent most of it's 50 years in house closets and basements in the US Midwest region. The neck does not appear to be twisted, cracked or separated from the finger board. I'm not certain what type of joint attaches the neck to the body of the guitar but it doesn't appear to be separating.
I took the guitar to a couple of local guitar shop "luthiers" but they just scratched their heads. I live in a depressed area where real luthiers are not in demand. From my description, is there something obvious that I'm missing here, or does that sound like a broken truss rod?