That looks like interference, by something strong nearby.
When you have bad connections, interference "leaks" into the signal.
Bad connections(cut wires) ruin the impedance - ie the 75ohm for round cable(RCA and cable tv) and 300 for flat(really old antenna wire).
Impedance is NOT resistance! Its kinda of hard to explain, but its kinda like the resistance "difference" between the signal(inside wire) and the shielding/ground(woven metal under the outside rubber,plastic,pvc - etc).
That description is probably wrong somehow, but having bad connections is what causes that. Like cutting ends off and using lose wire.
Use just a SHELLED RCA(either one attached to the original 2600 or a known good cable for the atari jr) and a RCA to coax(type f) adapter.
I'm assuming that by "1 fat and two smaller ones", you mean one original and two Atari jrs.
Using just a one of those plain red,white or yellow cables will have interference.
You can even use a RCA style sub woofer/digital audio cable if have one
- basically any RCA cable that at least 3 - 4mm thick will be shielded.
And almost all RCA cables are 75ohm - I never seen one that wasn't specially made in a different impedance.
Take one of you untouched, unopened Atari with the coax adapter, good cable and SHUT OFF ALL ELECTRONICS within 60 feet.
Then try it on channel three and four - one may have less static than the other.
That should tell whats causing the static after you know the connection to the tv is correct.
Computers, fans, motors, lights, AC, central heating, heaters could all be problems.
If ALL of your Atari do this, than its NOT any of the Atari!
It almost certain that the difference between your two videos was a slightly better connection and some piece of electronics was shut off.
Also, auto switchers wont work will the Atari - they need power that the VCS doesn't put out. (using a nes rf box will make a picture like yours)
Edited by H454, Thu Jun 1, 2017 12:18 AM.