Look at the filaments of the tuner & IF tubes & see if one is
changing brightness. The other thing on these oldies is the video
detector diode. They are lots of trouble & can give you a negative pix.
Quick hints. Finding tuner vs IF problem. No equipment needed !
A good running set with no signal should give a strong clean snowy
pix & a nice waterfall noise from the sound.
Start by turning channels. You should see & hear crashing of the
switches. If not its probably IF but possibly tuner.
Pull the 1st IF tube. You should see the same thing. If not its the IF.
Do the same with the 2nd & 3rd IF's one at a time. That at least can narrow things down.
Tube testers. As pointed out dont trust them. We had a TOTL B&K
tester but rarely used it. It was for the customers. Behind the main
bench was about a 15 by 6 foot tube shelf. It had almost every modern
tube in multiples. On top was stored grosses of 6GH8, 6LB6 & many others
bought in larger quantity. So you just turn around & try new tubes.
enuf fer now
73 Zeno
LFOD !