The usual place would be just after the video detector diode. The
generator should have a 1V P-P output preferably adjustable.
Use an isolation transformer. Changing to a safe & effective way
is overly complex. Some sets did it with a photo optical isolators
like some 1980's NAP sets. Also you need an isolation transformer
anyways. Dont confuse isolation with a variac ! Some come together
in one package many dont.
73 Zeno
LFOD !