Friday, June 27, 2014

Atwater Kent Model 20 "Big Box" - 1924

Ok, you got me. This isn't technically an amateur radio blog post but I would argue that radio in the early 1920's was the pursuit of the amateur, the enthusiast and the tinkerer. The amount of crossover between commercial broadcast radio and amateur radio in those days was much greater than it is now. Anyway, that's my excuse ... but why the Atwater Kent Model 20?

The model 20 or AK20
Well, to make along story short, I found an example of this Atwater Kent model sitting in a flea market stall in Hot Spings Arkansas. It looked like it had seen better days but was fully complete including the five '01A' vacuum tubes. The owner wanted $200 for the radio and explained the better part was the included horn type loud speaker which was worth $150. The horn speaker looked to be in fairly bad shape and lacking any way to test it I cheekily asked if he would take $50 for the radio ... "Sure", he said and my wallet was instantly $50 lighter.

Now I have a few vintage radios at home, perhaps a few too many depending on who you ask, and I consider them all to be interesting examples in their own right. More or less money, time and research was employed to create these radios and a great variety of circuits and designs was the result.

First impressions told me this was different, I had never owned anything like the Atwater Kent radio before. It was certainly older than my other radios but it was also constructed unlike any other radio I owned. Even under layers of grime, dust and mud dauber's nests this was obviously an article of quality. The finish on the wood cabinet and the metal front panel had seen some heat and were very badly checked in addition to the 90 or so years that had taken its toll. On the lip of the hinged top was a protected area which still showed how the finish looked originally, the dark mahogany and subtle gloss hinting that it had once looked very smart indeed.

The model 20 compact or AK20C
Once I had Internet access again I started to research the model 20 and try and learn something about it. It was at this stage, after some initial confusion, that I noticed that my model 20 was the older and perhaps slightly rarer "Big Box" version. There isn't much between them but some important differences crop up as far as restoration goes, for example:

Most references I found online suggested that the front panel of the model 20 was painted with a crinkle finish however this was only true of the later versions.

In an Atwater Kent advertising booklet, published at the time, they write, "The front of the cabinet is of metal with a deep brown mat surface which brings out the sparkling sheen of the lighter brown Bakelite dials, knobs, etc., and the nickel-plated trimmings."

I'll need to clean my Bakelite parts and possibly wax them to bring up the original "sparkling sheen" of the tuning, aerial tap and filament rheostat knobs.

A suggestion from the Antique Radio Forums that Rust-Oleum Earth Brown paint followed by black KIWI boot polish applied using 0000 steel wool sounds like it would come pretty close to producing the right finish and will probably be the path I choose when I refinish the front panel.

This AK20, from The Backwood Realm, appears
to have the original finish and looks similar to mine.
For the woodwork we're fortunate to find the description below and some more research indicates that nitro-cellulose lacquer was used at the time and is still available now.

"The cabinet of the Model 20 Receiver, pictured above, is of solid mahogany, stained a dark brown, then shellacked, triply lacquered, and rubbed to a dull, glossy, long-wearing finish."

Overall it seems very understated compared to modern electronics, even down to "The name plate, of dull bronze, is both distinctive and unobtrusive."

I hardly need to say, to most folks at least, that Ebay is a wonderful, and terrible, place to shop for antique radio parts. Wonderful because at any one time there are parts available for just about any radio you might have and terrible because you'll be jostling with tens, or hundred, or thousands of other collectors and re-sellers ... many of which have some very deep pockets indeed!

Unused Atwater Kent Radio Log card.
After vowing I would never pay Ebay prices I lasted approximately 3 minutes and then started looking on Ebay for some of the things I knew I was missing.

Before too long I had found an all-important unused "Atwater Kent Radio Log" card and envelope. I intend to scan these items in with a high resolution flat bed scanner and make copies to use and share.

Also I was able to get a set of three tuning knobs since one of the originals on my radio was shattered. Although someone had thoughtfully placed the remains inside the cabinet it was never going to piece back together well enough to look "right"

I had found the Atwater Kent Radio Instruction Book Vol. 2 online and printed out a copy but couldn't resist the temptation to pick up an original as well as another envelope along with some instruction sheets for various models of AK loud speakers.

I already own a modern ARBE-III battery eliminator so this really left only one thing to find ... a loud speaker.

Even the parts you don't see, are expensive.
I should have known that finding "the right loud speaker" for the model 20 would not be a simple task. While it will work with many different speakers including later cone style Atwater Kent units and those manufactured by other companies, the "right" speaker ... the type shown in all the period advertising literature ... is the Atwater Kent type M, L, H or R radio speaker.

All but one of those models allegedly used pot metal in their construction which eventually succumbs to the dreaded "pot metal disease" making it crack and expand. I have dealt with pot metal disease before and have no intention of doing battle with it again ... this left one model, the model M, as the one I was looking for.

Ok, so there was one model M listed on Ebay and it looked to be in good condition as well as "working" according to the seller. In fact the more I looked at it, the better it looked ... it seemed to be in very nearly new condition. After I finished drooling over the photographs I decided that I would limit myself to $100 not including the $50 shipping involved. The shipping was more than average and I hoped this would keep bids low and reduce the amount of interest. This seemed to work for a while ... the bids topped out at around $80 and I thought I might be on a winner, however this was short lived and the bids jumped to over $100 and kept on going. "Ah well", I thought, "such is life". I knew there would be an vintage and antique radio auction coming up in a month and I could probably find something there at a reasonable price.

The Atwater Kent Type M Loud Speaker.
At this point I took a shower, which is something I would recommend to anyone suffering some disappointment ... or a bath. Worst case you end up cleaner if still disappointed which is surely a small improvement.

At the same time my wife, who perpetually despairs of ever buying the right gift for a husband who is notoriously difficult to buy for, noticed my dejection and moved over to my computer which I had foolishly left logged into Ebay (I should know better, really).

A furious round of bidding started between my wife and a number of others who knew little of her determination to win this auction. I won't mention the final amount, I would like to say because, "One doesn't discuss money" but mainly because it still makes me a little woozy to think about.

If you were bidding on that same speaker then I apologize, but you never really stood a chance.

After further research I've learnt a bit more about the internals of the AK20 and uncovered the meaning behind its three tuning dials and various other controls. In order to provide more amplification, not to mention more selectivity, the AK20 uses three largely identical tuned circuits. Each tuned circuit is controlled by a separate knob on the front panel and all three must be "in agreement" before you will hear anything from the loudspeaker. Next comes the detector circuit which converts the amplitude modulated radio frequencies into audio frequencies and finally two further stages of audio amplification.

The other controls on the front panel are the aerial tuning switch which selects different taps on the first set of coils and the filament rheostats. The filament rheostats, in addition to correcting the voltage supplied by the batteries to the vacuum tube filaments, also act as the volume control by lowering the filament voltage which in turn lowers the volume from its maximum to a comfortable listening level.

Next steps will involve re-finishing the front panel and cabinet, checking tubes along with the few capacitors and resistors, then a careful power-up with high impedance headphones ... the AK loud speaker can wait until I am absolutely sure that it is working as intended!

I hope to update the blog as I progress, as well as uploading pictures and more detailed data such as circuit diagrams and service information HERE.