Background

In an effort to sell radios to the masses, manufacturers set their sites on producing inexpensive radios that could be assembled quickly. To this end, they minimized the tube and component count required to operate, manufactured and utilized cheaper tubes, and omitted the input transformer and regulated power supply by connecting one side of the A/C supply voltage to the radio chassis. These chassis are easily recognized by the conspicuous absence of the large input transformer.

These early sets have non-polarized plugs and present a serious peril to both the radio technician/restorer and to the radio user. Any exposed metal part can present a serious, even deadly shock hazard.

First - Home Wiring Fundamentals

Note 1: The wiring arrangement discussed here is for standard United States house wiring and differs for other countries.

Note 2: Do not assume that your house is wired correctly! Older homes or homes with refinished basements (even if the work was done recently) may not follow code. Furthermore, a receptacle that has been replaced may be wired “backwards”.

Your home is powered from a transformer located somewhere outside. The transformer has a center tapped 240 VAC secondary winding. The center tap is usually connected to earth ground at the transformer, and comes into the house as the “neutral” wire. The two end taps of the secondary winding come into the house as the “hot” wire, and each have a 120 VAC potential with respect to the neutral wire. These two hot leads are 180-degrees out of phase and therefore have a 240 VAC potential between them. In residential wiring, the hot leads are usually black, the neutral is white, and the ground is either green or bare.

House Wiring Diagram

The 120V outlets are connected between one of the hot leads and the neutral. The number and type of connections are evenly distributed between the two hot leads, to maintain a power balance between the hot leads. The much larger 240V outlets for ovens and dryers are connected directly across the two hot leads. If the 120V loads on each side of the transformer secondary are balanced, then the neutral lead carries no current and is at ground potential (0V). If the load is unbalanced, (as it always is to some extent) the neutral lead balances the twin loads. This introduces a potential across the neutral wire causing the voltage on the house side of neutral to be above ground by at least a few volts.

Second - The Dangers of A Hot Chassis

Testing - The isolation transformer

Standard Isolation Transformer

Another popular choice, because of the cheap cost and ready availability of these transformers, is using two step-down transformers in a back-to-back arrangement.

Back-To-Back Isolation Transformer

The transformer power rating in VA (Volt-Amps) should be 1.5x – 2x greater than the load (in Watts) you plan to connect to it.

Most Importantly - Safety First

There are several simple ways to reduce hot chassis shock hazard.

1. No exposed metal: Attach all knobs to keep the shafts covered and keep the back of the radio attached to prevent curious hands from contacting the chassis.
2. Make sure the outlets on your workbench are wired properly and verify the polarization of the plugs.
3. Install GFI (Ground Fault Interrupt) outlets on your workbench. In case of a short, they are faster than a fuse!
4. Replace the plug on your radio with a polarized plug.
5. Install an in-line fuse on the radio.
6. Do not ground your chassis unless there is a port for that express purpose.
7. Use an isolation transformer whenever the chassis is removed from the radio.

A while back, a young couple was selling an old tube radio at a neighborhood yard sale. It was a nondescript department store tabletop radio and believe me when I say - that radio was nothing special! It looked as if it had been stored in an attic, then perhaps the basement, and finally the garage - it had seen better days. I stood there staring at that old radio and began to wonder - what if radios could talk? The things they had seen and heard! Oh, what stories they could tell!

The world of antique radios is a wonderful and exciting place. For many people these artistic reminders of the golden age of radio are simple decorations used to accent their antique laden homes. For the younger generations of collectors perhaps, these radios represent what they view as a simpler time. A time without video games, 24-hour cable television, the internet or shopping malls. But, for those that can remember, these nostalgic reminders of a time not so long ago are so much more.

Many remember following the progress of our brave troops in WWII fighting their way across Africa, then Italy and finally Europe. You listened with rapt disbelief when harrowing news reached home of German U-Boats sinking ship after ship right off our very shores and the unforgettable lament of a nation following news of the Pearl Harbor disaster. Stories from the field of our Marines slogging their way from Island to Island slugging it out with a grudging Japanese Imperial Army. These stories meant something to all. Everyone knew someone who was fighting in that war. Many knew someone who was lost or killed in action. Wives lost husbands, mothers and fathers lost sons, a generation of children would grow up - many never knowing their fathers. Radio assured each one - they died heroes, fighting for a just and noble cause. A nation wept and rejoiced as the static laden voices issued forth from the glowing, humming miracle of science called radio proclaimed an end to that epic struggle.

We revel in the ethereal glow of electron tubes, the craftsman-like wooden cabinets and that peculiar, nostalgic sound that only tubes can produce. We spend our time lovingly restoring these old stalwarts from a bygone era. You find yourselves reminiscing as you listen to “Old Time Radio”, with your childhood dreams playing across the expanse of your consciousness. Perhaps you dream of riding with the “The Lone Ranger”, “Red Rider” or “The Cisco Kid” or perhaps of being whisked away on a great adventure with “Little Orphan Annie”. Programming such as “Philo Vance”, “Nero Wolfe”, “Boston Blackie” and “The Saint” stirred your imaginations in a way that television never can. Remember sitting in the dark with only the faint glow of the radio lighting the room? You wanted to sleep with the lights on after listening to “Inner Sanctum”, “Lights Out” or “The Whistler”. Perhaps you imagined yourself transported to “…a million may-be years on a thousand could-be worlds” by “Dimension-X” and “X-Minus-1″.

Some are lucky enough to still own that family heirloom that excited their mind and stimulated their thoughts and dreams for the future. For others, that original family radio is long lost, but a replacement has been acquired and lovingly restored as a reminder of their childhood dreams. These radios are gentle loving reminders of young lives filled with dreams.

Me? I was born at the end of an era. Radio played a big part of my very early years, a time before my parents could afford a television. I missed out on the golden age of radio, but my father instilled a love of all things old, and especially things electrical. In 1962 the one-eyed monster entered our home, and in 1964 I helped my dad build our first color television - a heathkit. Soon after that, my father and I built my first shortwave radio, also a heathkit. Later, my dad and I built a hi-fi stereo system for our family. I grew up on tubes, tube radios and tube television. When I went in the Army, much of the equipment still operated with tubes. I love restoring and repairing old things - especially radios.

Oh- that radio from the local yard sale? It sits on my project shelf awaiting it’s chance for resurrection.