If you choose to work with vintage electronic equipment, use an isolation transformer is a MUST!
A NOTE CONCERNING SAFETY! non power transformer radios can be a SHOCK HAZARD and even be lethal. Take care in operating such radio and you test equipment as serious damage can result to your equipment without the use of an isolation transformer. Some vintage radios were made in metal cabinets and can be DEADLY if the AC line is at case or chassis potential. Such radios and some early televisions used what is known as a HOT chassis, typical of those made for AC/DC power
The proper way to work on or service an AC/DC type radio or other electronic equipment is to always have it powered from an isolation transformer. WHY?, your test equipment will have it's case (if metal) and ground leads at negative or neutral potential (negative and neutral are essentially the same as GROUND potential. Connecting the leads to an non-isolated chassis is a 50/50 gamble as 50% of the time, the radio's chassis (depending on he polarity of the AC plug may be at the same potential as ground and nothing may happen. The other 50% (figuratively speaking) when you connect your equipment's test leads to the radio (or whatever you are servicing) you will create a big spark that will very likely damage you radio, and your test equipment. Plus you are subject to a severe shock.
DO NOT! plug both your equipment and the radio or device you are servicing to the isolation transformer (as I have seen done) as this will provide no isolation between the two pieces. Only plug the device that's being serviced to the isolation transformer.
my companion article: Operating your small AC/DC vintage radio