Recently I noticed an irresistible offer on Craigslist: a Majestic 3C70 AM/shortwave radio for just US $50. This model dates from the 1930s, when such radios came in gorgeous wooden cabinets. The specimen I stumbled on was still in the possession of the original owner, who used to listen to it with her family when she was a little girl. The wood and speaker fabric were nicely preserved, probably looking much as they when Japan attacked Pearl Harbor. I snatched it up.

I knew at the very least I’d need to replace a bunch of capacitors. But after scrutinizing the underside of the chassis, I realized I’d be doing a lot more, as much of the original wire insulation had disintegrated. Thus began a journey that eventually led me to build my own version of a critical piece of restoration technology: a dim-bulb tester.

My journey started with online searching that turned up a circuit diagram for my radio, along with plenty of advice from vintage-electronics restoration experts. The chief piece of wisdom was “Be careful.” Even when new, electronics of the vacuum-tube era could be dangerous. Being the cautious type, I wanted to take all appropriate safety measures.

In particular, when working with tube-era electronics, you should resist the urge to just plug it in to see if it works. Decades-old paper and electrolytic capacitors are almost guaranteed to be bad. And much else could be amiss as well. Instead, make the repairs and upgrades you determine are needed first. Even then, don’t just plug in your relic and flip the power switch. Better to start it up gently to look for signs of trouble.

How Does a Dim-Bulb Tester Work?

But how do you turn on old equipment gently? That concept was foreign to me, having grown up in the transistor era. And this is when I learned about dim-bulb testers. They take advantage of the fact that the resistance of an ordinary incandescent light bulb increases markedly as the filament heats up. The tester sits between your device and the wall plug. The bulb is wired in series to the power line and acts as a current limiter: Even if a component or wire in your device fails and causes a short, the current flowing into the device won’t exceed the current that would normally flow through the bulb. You can control the maximum current by using bulbs of different wattages.

Caption: Because the dim-bulb tester relies on an incandescent bulb [top middle], a certain retro look is guaranteed. I leaned into this aesthetic by using vintage analog meters [top left and right], and having a metal front panel custom-made by a sign maker [bottom].James Provost

Sure, you can cobble together such a tester using just an outlet box, a lamp base, and a switch. But I decided to go all out on the safety front and build a more fully featured dim-bulb tester, something akin to a design that I saw online that includes a variable transformer along with panel meters to monitor voltage and current. And for fun, I decided to give my tester a vintage look.

I hunted on eBay for vintage bits and pieces (or ones that could pass as vintage). While the effort to make my tester look old increased the cost and slowed construction, I was beginning to like the idea of restoring old electronics as a new hobby, so I figured: Why not?

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The end result was a unit that included two Triplett analog panel meters that, best I can figure out, date from shortly after the Second World War. It also includes three indicator lights that must be from the 1950s. They adorn a front panel that I fabricated by ordering a custom aluminum sign and cutting the openings using hole saws.

The dim-bulb tester allows me to ramp up the voltage applied to old equipment. The resistance of the bulb prevents damaging current flows to the equipment while looking for any signs of troublet.James Provost

Choosing the proper enclosure for my ersatz test instrument was one of the bigger challenges. Large enclosures tend to be expensive, and I also struggled to find something that wouldn’t have looked out of place in the TV repair shops of my youth. The solution was to purchase a damaged vintage test instrument (a tube-equipped signal generator), pull the chassis out, and use its painted steel enclosure. I bought it for less than I would’ve paid for a new enclosure. I also bought a small collection of incandescent light bulbs of different wattages. Assembling my tester was straightforward.

I wasn’t quite done, though. In my investigations into how repair vintage electronics safely, I learned about using an isolation transformer to help protect against shocks. I toyed with the idea of building one into my dim-bulb tester’s enclosure, but I decided it was more practical to purchase a stand-alone unit. I got a used one for a good price, but it took some work to fix and modify it so that it truly isolated the input from the output. (Oddly enough, commercial units don’t typically offer full isolation—you have to mod them for this.) I figure that I can just plug my device into my dim-bulb tester, plug the tester into the isolation transformer, then plug the transformer into the wall.

With my completed tester ready to go, I carefully examined the wiring and components of my Majestic radio and ordered what I think I’ll need to fix it. I’ve just received the box of components from Mouser, so repair and live testing will begin shortly. I should add that while working on my dim-bulb tester, I couldn’t resist making another $50 antique-radio purchase: a Zenith AM/FM tabletop radio from the late 1950s. The person I bought it from said that it works, but I now know there’s a right way and a wrong way to verify that assertion. So I’ve got plenty to keep me busy in my newfound hobby—along with the gear I need to pursue it safely.