I happen to have a "trial" access to Ancestry.com and took a look for Samuel Robinson.You may be on to something with the TNN units. Also take a look at the NPS Soldiers & Sailors site at: http://www.itd.nps.gov/cwss/http://www.itd.nps.gov/cwss/
I'll list what I found.
Samuel Robinson 3rd Tenn Mounted Infy.Co. K (?) Wife: Sarah B. Robinson Appliyed for pension in 1888 State that he appiled from was North Carolina.
Samuel Robinson 2nd NC Inf.Co. (L)? Wife:Mary Robinson Applied for pension in 1919 State that he appiled from was North Carolina.
Samuel C. Robinson (First_Last) Regiment Name 2 Tennessee Mounted Inf. Side Union Company K Soldier's Rank_In Pvt. Soldier's Rank_Out Sergt. Alternate Name Notes Film Number M392 roll 13
Here is a good site for information on wrtting the National Archives in DC for pension records. ww.oz.net/~cyndihow/pensionshttp://w.htm