1976 RCA picture problem

[Electronic M]

My thoughts based on the recent pics:

Seller pic shows some snow. The tuner was probably dirty then and has gotten worse.

Your snow picture shows that the raster almost completely fills the screen which points to two things: the small bit of wavy underscan on the side is probably one or more power supply lytics failing. And all your current signal sources are cropping the image...Your DTV box should have crop/zoom settings try playing with them...Also try different channels as some channels lock the zoom/crop settings such that a box can't fix it.

One more thought there is an analog channel 6 in the Chicago market (they are pretending to be an FM radio station, but have to keep their video carrier on to send audio) you may want to try tuning them.