Recently, I noticed an irresistible offer on Craigslist: a majestic radio of 3C70 AM / Short Wave for only $ 50 US. This model dates from the 1930s, when such radios came to magnificent wooden cabinets. The specimen I fell on was still in possession of the original owner, who listened to her with her family when she was a little girl. The wooden and speaker fabric was well preserved, probably very similar to what they attacked Pearl Harbor. I grabbed it.
At the very least, I knew I needed to replace a bunch of capacitors. But after examining the underside of the chassis, I realized that I would do much more, because a large part of the original wire insulation was disintegrated. So started a trip that finally led me to build my own version of critical restoration technology: a Bulb Dimb tester.
My trip started with online research which revealed a circuit diagram for my radio, as well as many tips from vintage electronics experts. The main piece of wisdom was “Be careful”. Even when it is new, electronics from the era of the empty tube could be dangerous. Being the prudent type, I wanted to take all appropriate security measures.
In particular, when you work with the electronics of the tube era, you need to resist the urge to connect it to see if it works. Several decades and electrolytic capacitors are almost guaranteed to be bad. And many others could also be bad. Instead, do the repairs and upgrades you determine are necessary first. Even then, don’t just connect your relic and return the power switch. Better to start it slowly to look for signs of problems.
How does a Dimbulb tester work?
But how do you slowly activate old equipment? This concept is foreign to me, having grown up in the era of the transistor. And it was then that I learned the Dim-Bulb testers. They take advantage of the fact that the resistance of an ordinary incandescent bulb increases considerably as the filament warms up. The tester is between your device and the wall candle. The bulb is wired in series with the electric line and acts as a current limiter: even if a component or a wire in your device fails and causes a short, the current circulating in the device will not exceed the current which would normally flow through the bulb. You can control the maximum current using bulbs of different wattages.
Legend: because the Dimbulb tester relies on an incandescent bulb [top middle]A certain retro look is guaranteed. I looked at this aesthetic using vintage analog meters [top left and right]and have a custom metal front panel by a panel manufacturer [bottom].Provost James
Of course, you can tinker with such a tester together using just an output box, a lamp base and a switch. But I decided to do everything on the safety front and build a more complete bulb DIM tester, something that looks like a design that I saw online that includes a variable transformer with panel meters to monitor voltage and current. And for fun, I decided to give my tester a vintage look.
I chased on ebay for vintage bits (or those who could pass as vintage). Although the effort to make my tester is old increasing the cost and slows down the construction, I was starting to like the idea of restoring the old electronics like a new hobby, so I thought: why not?
The end result was a unit which included two counters of triplett analog panels which, better that I can understand, date from a short time after the Second World War. It also includes three indicative lights which must come from the 1950s. They adorned a panel before I made by ordering a personalized aluminum panel and cutting the openings using holes saws.
The Dimbulb tester allows me to go up the voltage applied to the old equipment. The bulb resistance prevents damaged current flows to the equipment while looking for any signs of disorder.Provost James
Choosing the appropriate speaker for my Ersatz test instrument was one of the biggest challenges. Large enclosures tend to cost dearly, and I also had trouble finding something that would not have seemed out of words in the television repair workshops of my youth. The solution was to buy a damaged vintage test instrument (a signal generator equipped with a tube), remove the chassis and use its painted steel speaker. I bought it for less than I paid for a new speaker. I also bought a small collection of incandescent bulbs from different wings. The assembly of my tester was simple.
But I hadn’t finished. In my surveys on how the repair of vintage electronics safely, I learned to use an isolation transformer to help protect against shocks. I played with the idea of building one in the enclosure of my Dimb tester, but I decided that it was more practical to buy an autonomous unit. I got one for a good price, but it took a little work to repair and modify it so that it really isolated the entry of the exit. (Curiously, commercial units generally do not offer complete isolation – You must modify them for that.) I can simply connect my device to my Bulb Dimb tester, plug the tester into the isolation transformer, then connect the transformer into the wall.
With my tester finished ready to use, I carefully examined the wiring and components of my majestic radio and ordered what I think I will have to repair it. I have just received the box of MOUSER components, so repair and direct tests start shortly. I must add that by working on my Dim-Bulb tester, I could not resist making another $ 50 antiquity purchase: a Zenith AM / FM table radio from the late 1950s. The person to which I bought it said that it works, but I now know that there is a good way and a bad way to check this statement. So I have a lot to keep busy in my new hobby – as well as the equipment I need to continue it safely.
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