Electric lights are everywhere. So much so, we tend to take them for granted. Electric lights have come a long way from the their early start in 1802. Light bulbs, and now Light Emitting Diodes (LED’s) are in almost every appliance on the market, including coffee makers, and radios.
Truthfully, I am unable to determine the first radio to incorporate an electric light within the chassis. I strongly suspect that the inclusion of an electric light was based upon an observation that vacuum tubes (or thermionic valves, as they were once called) have a tendency to glow when power is applied to them. I have read it both ways as to which inspired which, bulb or tube. I really don’t know. What I do know is that at least in 1934, possibly even before then, radios were incorporating electric lights as a part of the overall design.
What purpose does a light bulb serve in a radio? On a purely functional level, they serve no real function. Light bulbs don’t do a thing to make a radio work right. In fact, I have never observed a radio to not work, even if the light bulb is dead. It would seem that the light merely is an aesthetic device, meant to light up the tuning dial or band spread meter, though it is helpful if you must operate in an otherwise dark room, or closet.
So, what happens when a light bulb dies? Truthfully, that is when things get tricky. In our modern age, incandescent bulbs are becoming increasingly difficult to find. If you happen to own a vintage/antique rig, and it requires light bulbs for the tuning dial, you might find yourself at a severe disadvantage, and unable to find a suitable direct replacement. This is a situation that I am certain to find myself in eventually, as I happen to own a 1934 Patterson PR-10 Allwave Receiver.
I admit, some hardware and electronic stores do carry some type of specialty light bulbs, but they are few, far between, and often quite expensive. However, if you are willing to pay the price for it, and it is there, it is most certainly worth it to buy something that is almost guaranteed to work. However, if it doesn’t exist, or your budget is quite tight, you might be up the proverbial creek. Or are you?
At the beginning of this article, I mentioned LED’s. Now I am well aware that these modern devices are not the greatest choice of replacement for the older style screw in incandescent bulbs that our older rigs came with, but they can be made to work. However, a few basics need to be understood about how an LED works.
To start with, the voltage of rating of an LED is about 2.6 VDC, and it must be forward biased. This means that, unlike an incandescent bulb, there is a definite direction of current flow. This also means that you can simply solder in an LED and expect it to just work. The simple truth of the matter is that your antique radio may be providing far more voltage, or amperage, than a typical LED can handle.
That leads to next concern, voltage and amperage. An incandescent light bulb can talk more current than an LED can. Thus, it would be wise to take a volt/ohm meter and test the connectors of where the light bulb goes to determine how much voltage and resistance are present at the point in which the light bulb connects. Some bulbs feed directly off the 117 VAC power supply, and make an LED completely unsuitable for use as a replacement for an incandescent bulb. Even if the voltage is direct current, you still need to discern which point leads to ground and which is live, thus a schematic diagram of the radio in question might prove helpful.
This leads to the next question, if direct current is on the circuit for the bulb, how do we connect an LED? I would hope that you haven’t thrown away that burned out bulb yet, and I would also hope you wrote down everything you can about it, in hopes of finding a proper bulb. That being said, break the bulb.
Yes, break it. You will need to base of the bulb to solder in a resistor to reduce current to the LED and the LED itself. The resistor will have to be soldered inline with the input side of the LED, which should be well marked. The LED output should be soldered to the part of the base that leads to ground. As early model radios varied in terms of bulb voltage from brand to brand, and model to model, I can not give proper specifications regarding how much resistance will be needed to provide enough voltage for the LED without providing too much. Sorry, even I have limitations. After that, you can simply screw the bulb base in and push the resistor off to the side to keep it from obstructing the light from the LED.
I will be honest, this is NOT ideal. However, as is the main principle of Mechanical Ham, it is functional. It does what it needs to, and without getting overly complicated. However, I can not promise that this will work with every radio made. I have encountered quite a few radios, including old general coverage receivers, in which it is impractical, if not impossible to replace the original light bulbs. Such radio are not useless, but they are more difficult to tune if not in an otherwise well lit room.
Yes, light bulbs are virtually everywhere, and that includes in our radios. However, if that light should burn out, it doesn’t mean our radio is gone. Either we can find a way to replace the bulb, incorporate an alternative to the burned out light bulb, or do without it.
Honestly, even knowing what I’d have to do to put an LED in to replace a light bulb that is no longer manufactured, I think I’d rather just go without the light. The way I see it, if the radio works properly, and I’m in an already well lit room, there is no need to replace something that doesn’t really provide anything extra.
If you really need that light, do what you can, if possible. If you can do without it, think twice before doing anything. You may save yourself some time and headache, and you might even find that proper bulb at a ham flea market. No sense damaging a classic radio unnecessarily.