Probably one of worst feelings in the world has to be when you break the
truss rod in you guitar.  That's what happened when the owner tried to
tighten the rod on this FG-512 Yamaha 12 string. Hey, it happens to the
best of us.
This rod broke deep inside
the neck, so there is no
easy way to repair.  The
first step is to remove the
fingerboard, shown here.  
This is done with heat and
thin blade spatula.
Here's what a channel type
truss rod looks like.  The
top one is the original.  
Below it is the
replacement, which will be
cut to length and threaded.
Click on any
picture for a
larger view
After replacing the rod and
gluing the fingerboad
back, frets and nut, the
entire neck gets a
tough-up. Not a cheap
repair, but not a total loss
either.
 
A TIP FOR SAFELY ADJUSTING A TRUSS ROD

First, try to loosen the rod before tightening
it.  This help break the nut loose and
prevents over-stressing the rod.  If the nut
seems stuck, don't force it.   If you don't feel
comfortable with it, take it to repair shop.  It
may take some heat to break it loose.  Or
they may need to soak the guitar in WD-40
(kidding).