A few notes.
When you go to UHF the VHF osc is turned off & the VHF
RF amp & mixer are turned into two more stages of IF amps.
This is how you can get band related fails. You should get strong
snow & hiss on all channels.
Be sure the FT is actually engaging the slug in the tuner.
Its normal to be able to FT up or down & hit the adjacent channel
if in the same band ( put on ch5 & tune to ch4 or 6 ).
You can try a little WD40, it can work miracles.
Video can usually be injected just after the video detector diode,
more on that later.
BTW watch the module sockets on these RCA's . They crack & loosen
with age.
73 Zeno
LFOD !