Tube Radio Repair

 

After having fixed many solid state radios, I decided to learn something new (old) and jumped into the world of tube radios. I found my first radio on eBay for €10. The radio lit up, I was told, but had no  sound. Sounded just like what I was after to learn the tricks of tube radio repair and restoration.

But before we jump into that in another blog post, I had some preparation to do. These old radios can have voltages of around 300V D.C. inside and depending on how you put the plug in the wall socket, the chassis can sometimes be hot! Therefore, it is wise to build some protection into the bench. The first step was to source an isolation transformer. This is simply two windings on a transformer that are galvanically separate from one another. There is also no connection to earth.

This means that I can safely touch a hot wire and not get a shock. Obviously, if I were to touch both live and neutral at the same time, I would still get a shock - but hey - I don't plan on doing that. A suitable sized transformer was sourced on eBay.

Now that the power supply was floating, the next issue to solve was how to bring up the voltage in the radio slowly, so that if it did have issues, I could cut the power right away before the magic blue smoke began to escape from everywhere inside the radio. China came to the rescue with a low priced variac.

Now, before I plug anything I get from China into the wall socket, I take it apart and make sure it is electrically safe. In the case of this device, I am glad I did, as the earth connections were made with bolts through the metal case but the case paint had not been removed and thus the connections were not being made correctly. A quick buff with the Dremel and we were good to go. Note that this variac is not an isolation transformer in itself. This unit has a direct connection between neutral on both sides of the transformer. So the neutral wire is still referenced with respect to earth. Therefore the need for a separate isolation transformer. Buyer beware - some ads offer this all in one - and there are devices out there that can do it but you can't be sure until you get it home and test it. 

Finally, a way to reduce or limit the current going into the radio was needed. Often when tube radios are first switched on, they will draw a momentary surge of current and then drop off back to a lower level. This is ok, but if a radio has leaking capacitors, as most do, then they are basically short circuits and they will draw a lot of current, resulting in smoke. A cheap way to implement this is to use a so called dim bulb tester. The hot wire is fed through a light bulb and then onto the socket. So the bulb is in series with the load.

An old tungsten wire bulb is needed - new LED bulbs will not work. The bulb has some neat characteristics which we can make use of.  When it is off, it is almost a dead short with only a few ohms resistance. So the current can flow through it to power the radio. However, once the current draw begins to rise, the wire changes its resistance as it begins to heat up and light. The more it lights up the higher the resistance goes up. The lit lightbulb also warns the tech that high current is flowing and to turn the device off. Similarly, if I were to get a shock the light bulb would like and take most of that current away from me.

Finally, to be able to work on the chassis, it is very helpful to suspend the chassis in a frame. This way the radio can be turned over easily and without damage to the dials, tubes or pulley mechanism. I got this idea from M. Caldeira on his YouTube channel.

I made mine from flat bar aluminum with a profile of 20mm x 3mm. This come in a length of 2450mm which is exactly the right length to make two rectangular frames. One for each side.

Now that everything is prepared, it is time to get working on that first receiver.