Parts to construct a crystal set were relatively cheap and construction fairly simple so they were very popular at this time for people living close to radio stations. Your local radio dealer could construct the mystery crystal set for approx. 25 to 30 shillings ($2.50 to $3.00). At this time a cheap 4 valve radio (Radiola Junior) cost 24 Pounds 10 shillings ($49.00) approx. 6 weeks wages for a radio service man.

In 1932 there were only 4 radio stations in Brisbane, 4QG, 4BC, 4BK, 4BH and radio broadcasting in Brisbane was only seven years old, at the time there were a total of 17 radio stations in Australia

This circuit is unusual due to the earth connection being on the secondary side of the coil and only the aerial being connected to the tuned primary of the coil.

Note: I used a Brach fixed crystal detector ( It is the orange fuse like device at the back of the crystal set ) instead of a cats whisker, square bus wire for the connections, and I made the base and panel from Jarrah a fine Western Australian timber.

Note: When constructing my Mystery Crystal Set I found I got best selectivity when my aerial was connected to the capacitor moving plates and bottom of the coil. aerial connection, this appears to be the opposite to the connections shown above. B.A. = Broad Aerial, S.A. = Sharp Aerial.

The Mystery crystal receiver is so called because I do not know just why it should be so good, and after trying it out for about a fortnight I am more amazed at the results than before. It is without a doubt the best crystal set that I have heard. Some of the Sunday Mail crystal receivers have attained Australia-wide fame, and it is quite a common occurrence to receive requests from readers in every State of the Commonwealth, asking for details of the Interstate or DX Crystal sets, but, this crystal set, to my mind, eclipses them all.

When you look at the diagram you will note that it is quite a different arrangement from that which you normally see in crystal circuits, but nevertheless it is a simple crystal receiver that will cost only a few shillings, and sufficiently selective to separate all the local stations without any overlap, and bring them in with enough volume to make the reception enjoyable. As compared with the Improved Interstate, this set is definitely superior. It tunes in the B stations with greater ease and with more volume, while 4QG's volume will surprise you as it did me.

The coil, like the whole circuit, is a most unusual arrangement, consisting of two coils wound together, turn to turn on the one former. The aerial coil, which is tuned has two aerial points without any earth connection. The detection and output circuit is untuned, and has the receiver's earth connection, a very unusual arrangement.

THE COMPONENTS
The components necessary to build this freak crystal set are:

One piece of bakelite panel, 10 by 7 by 3/16 inch
One wooden baseboard, 9 by 7 by 1/2 inch
One .0005 mfd variable condenser
One 3 inch plain dial
One glass-enclosed crystal detector
Five terminals, NP type
One piece of 3 inch coil former, 3 inches long
1/4 lb of 24 SWG (( 23 AWG ) D.S.C (Double Silk Covered) coil wire
2 oz of 30 SWG ( 29 AWG ) D.S.C (Double Silk Covered) coil wire
One coil of hook-up wire, solder and wood screws
One switch arm, two contact studs and one .001 mfd fixed condenser

The .005 mfd condenser is mounted on the centre of the panel, and the three inch dial is then fitted. The crystal detector is then mounted over the condenser, and the switch arm and the two studs is mounted under the condenser dial. The serial terminals are mounted on the left hand side of the panel along with the earth terminal, while the two phone terminals should be mounted on the right hand side of the panel. The coil is mounted on the baseboard directly behind the .0005 mfd variable condenser.

The wiring up of the receiver is a very easy matter, as will be seen from the diagram, but to avoid any misunderstanding it should be noted that the 50 turn coil of 24 gauge wire is the tuned aerial coil, while the 25 turn 30 gauge wire coil is untuned and connects to the crystal detector and phone circuit.

OPERATION OF THE RECEIVER The operation of this receiver is just as simple as the construction. The first point to note is that the switch and the two studs vary the selectivity, for when the switch arm is in contact with the stud S in the diagram, the set is very selective, and will tune in all four locals without any interference, but when the switch arm is in contact with stud B the receiver is much broader in its tuning and interference may occur, but the volume is greatly increased. The .0005 mfd condenser tunes in the various stations in the normal way. A point worthy of mention is the phone condenser. This condenser is usually omitted, but in this set it will be found to increase the volume quite considerably.

The Mystery crystal set is really a definite solution to the inexpensive crystal receiver selectivity problem and all readers who build up the set have an excellent receiver possessing excellent punch and selectivity.

Now for a few hints. Firstly, if you desire to get the best results, build the set up in the manner instructed. Something else might work just as well, and on the other hand it might not, and everyone of the few components in the set have a definite purpose, even the most despised phone condenser improving the results. The receiver works better with a cats whisker type of detector than a permanent type. In some cases and indoor aerial is giving satisfaction, but this type is not always satisfactory for reception of all four local stations.

PRAISE FOR THE SET
(Letter from a Sunday Mail Reader July 17th 1932)
The merit of this receiver will be more readily appreciated after reading the letter from J E of Ashgrove:

I have successfully constructed your Mystery Crystal Set. As you already know, I have tried out a good few these last seven years. I used, on account of having a lot in hand, No 23 D.S.C wire for the 50 turn coil and 24 D.S.C for the 25 turn coil, using a .0015 mfd condenser between phones, having 120ft overall aerial. I met with amazing results. I bought in 4BG with wonderful punch. It was audible on the speaker (scientific type) two rooms away, on stud B, but when I switched on to stud A 4QG's volume decreased a bit, but I pulled in 4BK,4BC,4BH, and K4LW on the speaker. It was so strong that they could be heard 30ft from set.

If you have any questions regarding the Mystery Crystal Set Email me at ray@clarion.org.au, I hope you enjoy this article, I enjoyed making the Mystery crystal set and it came 3rd out of 19 crystal sets for performance, a lot of thought went into the construction to make it as close to a 1932 set as possible, I must have done a good job as it won the best reproduction section. Ray Creighton Brisbane Australia