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Safe Power for Old Radios

by Mark Persons
Quarter Century Wireless Journal
May, 2021

This month’s cover
As a part of restoring radios from the 1930s through the 60s, I recently tackled a Hallicrafters S-40B communications receiver. It was a mostly enjoyable experience bringing the old piece of gear back to life. Let me suggest that old AC powered radios, especially those with metal cases, should be reworked to have three-wire power cords. This applies to radios that you might use, rather than a restoration where the radio is faithfully returned to factory new appearance before putting it on a shelf for display. I put a 3-wire power cord on the S-40B because the radio has a metal case and is used on a regular basis in my man cave. To be clear, I am talking about radios with power transformers. Radios that are AC/DC and/or have resistance power cords are not good candidates for this kind of modification. No manufacturer would build anything with a “hot chassis” nowadays. The liability is too great.

Power cords
Be safe rather than sorry. The third wire in a power cable is a safety ground. There were no ground wires on radios and other appliances before the 1960s. It was normal to occassionally get a tingle from a radio with leakage to its chassis. Radio enthusiasts today are in greater danger of being shocked or even electrocuted from these relics as insulation breaks down. A power transformer can develop a short from a winding to its core. That could put a full 120 VAC on the radio case. Ouch! My junk box is a source of power cords. The one shown (cover photo, right) is left over from an old computer. I cut off the plug, at the computer end, and used the rest. The wires are not always the traditional black for hot, white for neutral, and green for ground. When this happens, I ohm them out to be sure I am connecting/soldering to the correct wire. Replacing the original two-wire power cord, with a three-wire with ground, is a good choice. It will keep you and others safer. That assumes you plug the radio into a grounded power outlet and do not use a 3-wire to 2-wire cheater/adapter, as shown in Fig. 2 above. Adapters of that sort will defeat the ground and can create a dangerous situation.

If you do the update, you may find the radio now trips a Ground Fault Circuit Interrupter in your home wiring. Why is that? Well, many old radios used a capacitor to RF couple one side of the power line to the radio’s chassis. The idea was to create an RF ground connection so only a single wire was needed for an antenna. That capacitor might be shorted or draw enough current to trip the GFCI, which compares current traveling between the 120 VAC wire and the neutral wire in the same cord. Any difference between the two means AC is going to ground somewhere. The GFCI trips to keep people along with pets safe and can even prevent fires. I rigidly attach/solder the three-wire power cord’s ground wire to a radio chassis. It will provide the needed RF ground. I then disconnect the original RF capacitor because it is no longer needed.
Fuse it
Let me also encourage you to add a fuse in the hot side of the power cord. You can drill a hole in the chassis and install a fuse holder or you could use an inline fuse holder, as shown in on the left cover photo, under the chassis. The advantage here is that it will not damage the outward appearance of the vintage radio. To determine a proper fuse size, I generally start by measuring current drawn by the radio with the audio turned all the way up. Then I select a fuse of 1.5 to 2 times that value. Safety, safety, safety It is up to you to use good judgment when making modifications. Do not attempt anything if you are unsure of what all this means and/or your capabilities in doing it well.

Another tip
Turning an old radio upside down for service can present its own problem. The figure on the right shows a hardware spacer, also from a junk box. It was secured to the chassis with one screw and allows turning the chassis over for repair without damaging tubes or other hardware.
Submitted by: Mark Persons, W0MH.  Editor’s note: Mark Persons W0MH is a retired radio broadcast engineer, member number 36456 in the QCWA and life member of the ARRL. He was first licensed in as WN0AXD at the age 15 in 1962. Mark received the Engineer of the Year award from the Society of Broadcast Engineers in 2018 and their Lifetime Achievement Award in 2020. He currently mentors four radio broadcast engineers and is a member of the National Radio Systems Committee, which develops improved technical standards for radio broadcast and ultimate adoption by the FCC.


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