The Dim-bulb Radio Tester

The moment you bring home a "new old" radio, the first temptation is to plug it in and try it out. That's always a bad idea. For all you know, the set may have short circuits, a defective power supply, dried-out electrolytic capacitors, or (very often) failed paper tubular capacitors. Plugging it directly into the wall may damage the radio or even start a fire.

Many experienced restorers replace all of the old paper and electrolytic capacitors in a radio before attempting to start it up. If your radio is valuable or has great personal value, that's the prudent course. Read Replacing Capacitors in Old Radios for details. If you don't want to "recap" your radio before powering it up, read on . . . .

A dim-bulb tester lets you try out a radio under safe conditions and see whether it has problems in its power supply. You can build one in about as much time as it took to write this page.

Building a Dim-bulb Tester

Here's my dim-bulb tester: a few leftover electrical parts mounted on a piece of scrap wood. It includes an on/off switch, although the switch is optional.

The sketch shows how I wired the tester.

As the diagram shows, the dim-bulb tester puts a light bulb between your radio and the AC power in the wall. This way, if your radio has a short circuit, it will just light up the bulb instead of causing damage.

The resistance in the light bulb limits the power that can reach your radio. By starting with a low-wattage bulb, and gradually building up to higher wattages, you can slowly increase the power that the radio receives.

Safety Note: the dim-bulb tester involves high voltage current. If you aren't experienced with household wiring, or this diagram looks confusing, get assistance from someone more experienced.

Using the Tester

To use the tester, start with a 15-watt bulb. If it lights brightly, stop! You have a short circuit in the power supply, and should not plug the radio into the wall before investigating and repairing the fault.

If the 15-watt bulb doesn't light, move on to larger wattages: 40, 60, and 100 in turn. Generally speaking, if the larger wattages don't glow, it's safe to plug your radio into the wall outlet.

Passing the dim-bulb test doesn't mean your radio works perfectly, of course. There are many other faults that can't be detected by this device. But it does allow you a safe startup.

An easy variation would be to use a three-way light socket. Then you could install something like a 30-70-100 watt bulb and simply turn the light's switch to increase the wattage.

A dim-bulb tester can be used to diagnose some basic power supply problems. Here is a handy procedure which I found in an old radio service book. It is used to check the transformer and input filter capacitor in a transformer-type power supply. (This will not work with an "AC/DC" type power supply, which lacks a power transformer.)

1. Remove all tubes from the radio. Make a note of where each tube belongs, so you can later replace it in the correct socket.
2. Place a 25-watt or 40-watt bulb in the dim-bulb tester and plug the radio into the tester.
3. A good transformer will cause the lamp to glow dimly after a few moments. If the lamp glows brightly, you have a short circuit; the transformer should then be disconnected and checked.
4. If the transformer is OK, put in the rectifier tube, put a 100-watt bulb in the dim-bulb tester, and try again. If the rectifier tube lights up and the lamp glows brightly, you have a short-circuit in the filter capacitor of the power supply.

When you replace the tubes in the radio, be sure to put them back into the right sockets! Although this method is quick and convenient, there are other ways to check the transformer and filter capacitors, of course.

Note Your Radio's Wattage

If your radio is a typical five- or six-tube set, it probably uses about 30 to 35 watts of power. The radio's wattage is often stated on a label on the back or inside. For such a set, using a bulb up to about 100 watts in your dim-bulb tester is appropriate.

More complicated radios have more tubes and will draw more power. For example, my Hallicrafters SX-88 shortwave radio has 20 tubes and draws 138 watts. A vintage TV may have even more. To use a dim-bulb tester with these, you'll need to use somewhat higher wattage bulbs.

Alternatives

My workshop also includes a device called a variac, which lets you start up a radio at low voltage and gradually increase it. This can allow you to detect problems before something terrible happens. However, it's best to use a variac that has a meter and to understand how much power your radio should draw in the first place. If you don't know what you're doing, and simply use an un-metered variac to slowly increase the supply voltage, you might still damage your radio.

Several years after writing this article, I ran across a commercial tester built by the Christy Electronics company in Chicago. It serves the same purpose as this simple unit but includes additional parts, such an ammeter, making it more versatile. See the Christy Electronic Tester article for more information.

This radio construction project, including all descriptions, diagrams, photos, and the underlying electronic design, is published here for the noncommercial use of radio hobbyists. You may print and reproduce these project instructions for your personal use. Commercial use of this material is strictly forbidden.