Many vintage AC/DC radios (or other gear) can cause a shock if one touches the radio’s metal chassis and “earth” (a radiator, faucet, water line, a heat or air-conditioning register, basement or garage floor) at the same time. This hazard can be mitigated.
This hazard would seem to be removed if one reverses the sense (left/right) in which the vintage AC cord is plugged into the receptacle on the wall; however, this is not a practical remedy for the electronically unsophisticated person. Further, there is a remnant risk if your housecleaner subsequently unplugs the radio and then re-plugs it with the reverse orientation after, say, putting that receptacle to use with a vacuum cleaner….
This is why today we have polarized plugs on all cords. If one is a purist who does not wish to remove this danger by replacing an unusually attractive original AC line cord plug with a modern-looking polarized plug –of the wrong color– then they can insert an 1:1 voltage isolation transformer between the wall receptacle and the AC/DC receiver. The wattage rating of this transformer should match that of the radio (or other kind of electronic device), typically 35 to 60 watts. Last, and very importantly, the chassis of this AC/DC radio must be grounded (to earth) before the isolation transformer is energized (“plugged in”).
Such an isolation transformer is not the same as used for European travel. This latter kind is a 2:1 voltage autotransformer that would send your device to electronics heaven! Not only is the output voltage incorrect, but an “autotransformer” provides no ground-isolation safety feature whatsoever. This use of an isolation transformer is a more secure solution than is use of a polarized plug; for, there is always a risk that the receptacle was installed by your electrician “backwards.”
Again, the safest way to enjoy your AC/DC electronic device is to use of 1:1 voltage isolation transformer. I do my electronic work in a basement and use such an isolation transformer for all pieces of gear –even modern ones that might have a “leaky” built-in (isolation) transformer.