Powering Up Your Radio Safely With a Dim-bulb Tester

The moment you bring home a "new old" radio or TV, the temptation is to plug it in and try it out. That's always a bad idea.

For all you know, the set may have a short circuit in the power cord, defective electrolytic capacitors in the power supply, or (very often) failed paper tubular capacitors. Turning it on prematurely may damage expensive parts or even start a fire.

Many experienced restorers replace all of the electrolytic capacitors in a set before attempting to start it up. If your radio is valuable or it has great personal value, that's the prudent course. Read Replacing Capacitors in Old Radios and TVs for details.

There are other basic steps, such as inspecting for ruined parts, testing tubes, and cleaning controls, which should also be performed before power-up. These are detailed in the article, First Steps In Restoration. If you haven't already done those steps, please do them now.

What Is a Dim-bulb Tester?

A dim-bulb tester lets you try out a radio or TV under safe conditions and see whether it has problems in its power supply. You can build one in an evening.

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. If you don't know anyone like that, try contacting an area collector club; you might find someone willing to help. The ARC website has a list of clubs in the USA and throughout the world.

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 thus draw more power. For example, my Hallicrafters SX-88 shortwave radio has 20 tubes and draws 138 watts. A vintage TV may draw even more. To use a dim-bulb tester with these, you'll need to use higher wattage bulbs. (If the bulb's wattage is too low, it will light brightly even if your radio has no problems, and your radio won't play at all.) Generally speaking, your radio should play normally without lighting up a bulb that is 1.5 to 2 times the radio's stated wattage.


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.

This photo shows my homebrew variac, which is housed in a clear Lucite case and includes a voltmeter and ammeter:

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 my simple dim-bulb tester and it 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.

©1995-2015 Philip I. Nelson, all rights reserved