Issue with GE "Portacolor III"

[kf4rca]

I think that set has a bridge rectifier power supply. So its ground is ground to that set only.
Those sets cannot have an external video input for that reason. Modern sets have a switching supply and the transformer in it provides the isolation for external grounded video inputs.
There are video transformers out there but they are kind of pricey ($22).
Those ground loop hum eliminators that are sometimes used by security camera installers do NOT provide ground isolation.
You can verify that with an ohmmeter.
Again congratulations!