Antique Radio Safety: Understanding the Dangers of a Hot ChassisAuthor: Warren Parks
Many antique radio chassis are "hot", meaning that they can have dangerously high voltages exposed externally. This presents a serious shock hazard, unless you understand what precautions to take.
In the environment of relaxed electrical safety codes of yesteryear, many radios were manufactured with the metal chassis electrically connected to one side of the power cord. This can give you a strong shock if you happen to touch the radio and anything else that is at ground potential at the same time. The danger is minimal if the complete cabinet, back, and knobs are in place and no metal parts are exposed. However, these things tend to get lost or become damaged, so exposed metal parts are common. As modern electrical codes introduce many more earth grounded elements into our households, there are now many more opportunities to touch them.
You also have to be extremely careful when trying to repair your set, as lots of test equipment is now earth grounded. It is very easy to be badly shocked unless you fully understand the problem, and keep it in mind at all times.
To begin, let's make sure you know clearly the difference between "hot", "neutral", and "ground" potentials:
NOTE: The electrical system described here is used in the United States. It can be quite different for other countries.
The electricity for your home comes from a transformer located on the powerpole outside, which has a center tapped 240 VAC secondary winding. The center tap is usually connected to earth ground there at the powerpole, and comes into the house through the "neutral" lead. The two sides of the secondary winding come into the house as two "hot" leads, that each have a 120 VAC potential with respect to the neutral lead. These two hot leads also have a 240 VAC potential between them.
The 120V outlets are connected between one of the hot leads and the neutral. The number of outlets is evenly divided up between the two hot leads, to try to keep the load on each hot balanced. The much larger 240V outlets for ovens and dryers are connected directly across the two hot leads. If the 120V loads on each hot are balanced well, then the neutral lead carries almost no current and is at ground potential. But if this load is unbalanced (as it always is to some extent) the neutral lead will be carrying enough current to make up the difference. This current can be many amps, so the small resistance of the neutral wire is enough to cause the voltage to increase above ground by at least a few volts, but usually not enough to be dangerous. In standard modern residential wiring, the hot leads are usually black, the neutral is white, and the ground is either green or bare.
This is fine if the only other things you might be touching are other similarly grounded or insulated modern appliances. But if that other appliance is your hot chassis radio, YOU become a return path to ground, and you'll get a bad shock.
Older cords are not polarized, and may be plugged into the outlet either way. If the chassis side of the cord is plugged into hot, much of the exposed metal on the radio chassis will be at 120VAC with respect to anything in the room that is at ground potential. Turning the cord over would tie the chassis to the safe neutral side, keeping the outer chassis within a few volts of ground.
It is very important to keep this fact in mind if you are replacing a power cord, or if you plan on connecting your radio to any modern appliance. Whenever possible you should fit a new polarized cord that connects the common side of the circuit to the wide blade. While it may not be historically authentic, it will be much safer.
Keep in mind that the chassis may not be directly connected to one side of the cord. It may be tied through a capacitor, which will fool an ohmmeter. A good example of a hot chassis connected in this way is the famous "All American 5" (tube) chassis, which was manufactured for decades. In this chassis, the metal frame is connected to the neutral lead through a 22M resistor and a .1uF cap. This capacitor has a low enough resistance to the AC voltage that you can recieve a shock through it.
You can usually trace the cord wires through the circuit and determine fairly easily which one has been treated as hot, and which one is common. The hot side usually goes to the rectifier tube plate, either directly or through a dial lamp. The other side of the cord usually goes to the negative side of the filter capacitor, and is eventually tied to the chassis either directly, or through a high ohm resistor in parallel with a small capacitor. While you're poking around in there, it is a very good idea to install an in-line fuse holder in series with the common side of power cord. The fuse should be rated at about twice the working line current, so a 1 to 2 Amp fuse is usually appropriate.
There are several simple ways to reduce the hot chassis shock hazard. The easiest way is to make sure that all insulating knobs are in place to cover exposed shafts, and to keep the cabinet and back in place to prevent anyone from touching the chassis. Always keep in mind which things around you might be grounded and don't touch them and the radio at the same time. Water pipes, outlet cover screws, modern stereo equipment, test equipment, and appliances are usually grounded. Don't try to connect the chassis to earth ground to improve radio reception unless there is a ground terminal on the chassis that is clearly marked for that purpose. As a precaution you should also install GFI (Ground Fault Interrupt) outlets on your radio workbench, because these will respond much faster than a fuse, and may also save your life.
If you need to connect any test equipment to your radio, such as an oscilloscope or signal generator, an "isolation transformer" is an absolute must to prevent damaging yourself and the equipment. An isolation transformer has a 1:1 voltage ratio, and eliminates the problem because the secondary is electrically isolated and can "float". This means that one side of the secondary can safely be connected to the modern earth grounded equipment with no danger. Please note that many variable "variac" transformers used in service work are not electrically isolated, and will not protect you. Antique Electronic Supply carries several true isolation transformers which can be ordered on-line. The power rating of the transformer in VA (Volt-Amps) should be at least 1.5X greater than the load (in Watts) you plan to connect to it. AES offers a 90VA model for $36.82 (US) that is good for radios up to the medium tabletop size. It might just save your life.
To operate a hot chassis antique radio safely, please remember: