Hi Bizman,
Check your tone arm can to see if it is broken.
Somtimes a piece plastic on the atm may break off (this is usually on the very thin section) also the locating peg that fits in the collar can break off.
I have even seen that one of the two pegs on the rear of the guide plate assembly can break off (these two pins follow the tone arm cam).
Regards
Alan Hood
ami-man
UK