Improvement of the “Mystery”

Crystal Radio

by Ken Harthun

The “Mystery Crystal Set”, designed by a fellow who called himself “Proton”, was first published in the Sunday Mail newspaper in Brisbane, Australia in July, 1932. It was quite popular at the time and has again gained favor with many of us due to the articles posted at http://www.clarion.org.au/crystalset/mystery.html by Ray Creighton. Demand by readers of the Sunday Mail for improvements to the set resulted in the version known as the “Mystery Plus Crystal Set” that was published in April, 1933.

Although the “mystery” aspect has been explained quite well in postings to the Crystal Set Radio Club by Ben Tongue (messages 2172 & 2173 and relevant schematics), it is still an intriguing hook-up. There is no doubt that this set is a good performer; however, having built both versions of the medium wave Mystery set, and considering my experiments with a short wave version (http://www.qsl.net/kc4iwt/xtal/SWMystery), I was convinced that an improvement could be made. Furthermore, there was just something about the mystery set that nagged me, but I couldn’t seem to put my finger on it. So I went back to source -- the original articles -- and looked them over very carefully to see if I had missed anything.

In his first article, Proton says, “The receiver works better with a cats whisker type of detector than a permanent type”. I have to agree emphatically with his observation. Selectivity of the original Mystery design was very poor with a 1N34 diode, no matter which antenna connection point I used on the primary coil. I heard at least three stations at once, albeit different ones, no matter where I tuned. Switching to my pyrite detector was a vast improvement, but still not as good as I wanted. My Perikon detector was much better and I found I could “tune out” some of the interference and still get good volume on the station I was listening to.

OK, I got some improvement there. Granted, I lost a bit of sensitivity in the process, but greater selectivity is my goal, so the sacrifice is worth it. Next, I tried adding the extra coil ala “The Mystery Plus”. Everything stayed about the same and I noticed little, if any, improvement in selectivity despite the claim of “Very Selective and Sensitive” that was published in the article.

An improvement did result, however, when I wound the 15-turn coil about ¼” below the main coil. Selectivity improved immensely! This makes sense if you consider that winding the extra coil over top of the main windings results in very tight coupling. Moving the extra coil off of the main windings and spacing it somewhat results in a looser coupling and therefore increases the selectivity. There is room for experimentation here, i.e., what is the “optimum” spacing?

Let’s take a look at another performance factor. Unlike listeners in 1932, I was using a set of sensitive crystal headphones. In the early days, the 2000-ohm magnetic headphones were the standard. Some of these headsets lacked sensitivity, as we all know. Switching to my magnetic HS-16A’s (less sensitive) improved things. I lost some volume but I could still hear all the stations at an intelligible level with almost no interference.

Whether or not I have discovered anything major here is open to debate, but my observations are certainly interesting. My conclusions based on these observations are food for thought and I will leave it to you to decide if I have succeeded in creating the “Deluxe Mystery Crystal Radio”:

1. Wind the extra coil at the bottom of the main windings, spaced ¼” (or more).

2. Antenna to top of extra coil, ground to bottom.

3. Use a real crystal detector, galena, pyrite, Perikon, etc., NOT a diode

4. Sensitive headphones are not required.

The Mystery Set, even with these “improvements”, is not a DX set nor was it designed to be. It is, however, the best simple set I’ve tried for casual listening on local signals. But I’m not finished playing with this set. I may yet “improve” it further before I’m satisfied that I’ve done everything possible to get the best performance while staying faithful to the original designs.

I welcome any and all comments, suggestions and help.