1) sometimes RCA VHF tuners get so dirty they quit.
2) Check the UHF IF cable. They open especially if just crimped.
3) be sure UHF is getting B+. Prob 12 VDC.
4) In the UHF there are U shaped grounds along the rotor.
Put a few drops of WD40 on them & run back & forth.
You can pull, clean & bend them but thats risky.
You can put IF in to the UHF IF input to test the VHF tuner on UHF.
If you dont have an analyst you can make a tuner subber out of a scrap tuner.
BTW on RCA the tuners have a number KRK###. thats the part number.
they also had a six digit p#.
73 Zeno
LFOD !