The Mystery Crystal Set

The construction details of the "Mystery" crystal set were first published in Our Wireless Circle section of The Sunday Mail newspaper in Brisbane on July 3rd 1932. It proved to be very popular with the readers and a second construction article was published on July 17th 1932 along with readers letters and questions, and another on April 16th 1933. Almost every Sunday during this period there were letters and comments regarding the Mystery Crystal Set in the Sunday Mail.

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) - about 6 weeks wages for a radio service man.

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

The first 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.

Ray Creighton

The Mystery Crystal Set

By "Proton"
Published in "The Sunday Mail", Brisbane, Australia 3 July 1932

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 Improved Interstate Crystal Set or the DX Crystal Set, 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.

Mystery Crystal Set circuit diagram
The Mystery crystal set schematic

The components necessary to build this freak crystal set are:

1 One piece of bakelite panel, 10 by 7 by 3/16 inch
2 One wooden baseboard, 9 by 7 by 1/2 inch
3 One .0005 mfd variable condenser
4 One 3 inch plain dial
5 One glass-enclosed crystal detector
6 Five terminals, NP type
7 One piece of 3 inch coil former, 3 inches long
8 1/4 lb of 24 SWG (( 23 AWG ) D.S.C (Double Silk Covered) coil wire
9 2 oz of 30 SWG ( 29 AWG ) D.S.C (Double Silk Covered) coil wire
10 One coil of hook-up wire, solder and wood screws
11 One switch arm, two contact studs and one .001 mfd fixed condenser

Construction Of Set

The first constructional step is the winding of the coil. As this is a little unusual, I will endeavour to make it as simple as possible. Wind 12 turns of 24 gauge D.S.C wire on one end of the former, and, without breaking the wire, stop winding and punch two holes in the former, and thread the end of the 30 gauge wire through these holes to make it secure. Then continue the winding with both the 24 and 30 gauge wire so that, for the next 25 turns, the coil is so wound as to have a turn of the 30 gauge wire between each turn of 24 gauge wire. When 36 turns of 24 gauge wire, and 25 of the 30 gauge wire have been wound on, stop winding, and, without breaking the 24 gauge wire, break the 30 gauge wire, and secure it by punching two holes in the former and threading it through these. Now continue winding the 24 gauge wire for another 13 turns, and then securely fasten by punching a further two holes in the former, and the coil is complete.

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 µF 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.