If you have good clean strong snow the tuner is the cause
99% of the time.
Another easy test is turn the channels. You should see crashing in the
pix & noise from audio from the switches moving.
Try the mix/osc tube 1st even if it tests good.
Check the B+ to the tuner.
Check the AGC. Can be subbed to test if way off. AGC is RARE problem.
Usually a dead RF amp will still let a strong signal pass but snowy.
Last thing is a VERY dirty tuner. Try a good shake of the knob & see
if the channel pops in.
DO NOT spray clean an RCA Nuvistor tuner ! It can throw off the RF amp
& cause it to oscilate. Take time to pull the tuner down, remove
usually 2 shields & just put cleaner on the contacts, spin, & let dry.
73 Zeno
LFOD !