Atwater-Kent Model 20>
Approximate Year 1920.It began with a call from my wife's sister and her husband. They had recently purchased a vacation and possible retirement home and had been doing a little antiquing in the area. They had found an Atwater-Kent radio from 1920 and knowing my interest in old radios asked if I wanted it. My wife Sue decided she would buy it for me for Christmas of 2010. The antique store guy said it had 7 tubes when in fact it had, and has 5. He also said it worked and he had heard it working. Upon closer questioning he admitted he had been told it worked. Then he changed his story again and said the person who sold it to him said he had been told it worked. The speaker that came with it wasn't even an Atwater-Kent but an RCA.
When it came into my possession, guess what, it didn't work.
This picture was taken before any attempts were made to get it working.
The set has 2 RF tubes, 1 detector tube and 2 audio amplifier tubes. They all have the number 201-A and look new. There is a logo on top of each one that I was not able to photograph. Some tests showed that the grid leak was open and there was no plate voltage on the first audio tube. All audio coupling is by transformers. The antique man probably thought the two round transformer cans were tubes. I soldered a 3.9 megohm resistor unobtrusively under the chassis across the grid leak holder and a 100 k ohm in the plate of the first audio tube to get some B+ there. I really can't explain why I did that, it is most illogical.
To my surprise I began to hear static and with an antenna connected I was able to tune in a station. I was rather puzzled about how it had worked with an open transformer winding. I would later find that the transformer had a lot of windings and the inter winding capacitance was enough to couple signal from the first audio to the second audio stage. I removed the offending transformer and confirmed that it indeed had an open primary winding. Here is a picture at this point.
I plan to reinstall the open grid leak but I'll have to clean up my bench to find it.
I did a Google search for Atwater-Kent audio transformer and found several sources. Because I didn't want to pay an arm and a leg I opted for a modern interstage transformer. My plan was to hide it in the old transformer can. I was going to melt the tar in the can, pull the old transformer out by its leads and immerse the new one in the melted tar, then let it congeal. Well, you know what they say about the best laid plans of mice and men...
First step, melt the tar, how difficult can that be? I bought a hotplate and a saucepan, found a scrap of wood and drilled a big hole in it, and mounted the transformer in the hole.
Here's some helpful advice for any younger married men reading this article. NEVER! NEVER! NEVER! NEVER! EVER, borrow cooking utensils from your wife's kitchen for a project such as this. That's one of the quickest ways there is to wind up in the doghouse.
You can see the transformer mounted in the hole with two small woodscrews. You can also see the tar. The pan was filled with water which was brought to a boil. After two hours the tar had softened slightly but the transformer wouldn't come out. The melting temperature of the tar was above that of boiling water. Mice 1, man 0.
OK. We need to get hotter than boiling water. That spells cooking oil and increased danger.
Here I am seen probing the tar with a woodscrew to see if it is soft. When It got soft enough to work with it had expanded and a little had run over the board and into the cooking oil. It was never intended that it would be used for cooking after this use anyway. I lifted the contents of the can by the wire and what came out was much bigger than I had anticipated. Then the wires pulled out of the transformer and I watched in dismay as the object sank slowly back into the La Brea tar pit. Mice 2, man nothing. Fortunately it didn't sink quite all the way in. I got a pair of large needle nose pliers, grabbed it, and pulled it out. Now imagine this. Here I am holding a tar covered object which is at a temperature higher than 212 F. I didn't want to put it down on the table. I would be in trouble with the misses if I put it down on the carport floor so I dropped it in the grass. Then I turned my attention to the can. Instead of a container about 85 percent full of tar it was almost empty. I took it out of the hot oil and turned off the hot plate. Mice 3, man nothing.
After the tar covered old transformer cooled I picked it up and scraped off all the grass I could.
Here you see the replacement transformer on the left, the empty can in the middle, and the former guts of the can on the right. Evidently they couldn't make transformers as small in 1920 as they can today. I was curious about how the transformer was made so I took it to the bandsaw. The result is shown below.
As you would expect the transformer is wound with fine copper wire. The core appears to be small rods of iron. It looks as though they didn't fully understand the concept of a magnetic circuit in those days. There appear to be a few iron rods on one side of the transformer but not nearly equal to the cross section area of the central core. The can and chassis probably contributed a little area to the circuit but it still doesn't look like anywhere enough.
Plan B. I'm going to hide the modern transformer inside the can so no one will ever know the difference unless they take out the screws that hold the can In place. I need to put something in there to keep the little transformer from rattling around. Paraffin is out of the question because someone may store the radio in a hot attic long after I have departed for the big electronics workshop in the sky. Foam rubber is out for the same reason. It only has a life of about 25 years before it turns into goo. I have no doubt that the process is accelerated by high temperatures. I think pieces cut from an old towel will be good. While an attic gets hot it's not hot enough to set cloth on fire. The heat inside the cabinet when the radio is operating is minimal. The tubes barely get warm to the touch when operating.
After being immersed in boiling water for 2 hours the paint on the can was a little worse for wear. Another hour in hot cooking oil really did a number on it. I sanded down to bear metal and my artist wife used some of her acrylic paint to put a new coat of black on it. Since the original pattina was gone beyond any chance of recovery I think it's best to make it obvious that the can has been repainted.
Oh yes, there's still that missing grid leak. I have been known to make some mistakes but I wouldn't have been so stupid as to throw it away. Someone familiar with the logo on the tubes might be able to recognize it in this picture.
For those who may be concerned, after the radio was reinstalled in its cabinet and the bench cleaned up, I did find the missing grid-leak.