The Interstate Crystal Set

By "Proton"
Published in "The Sunday Mail", Brisbane, Australia, 10 April 1932.

The improved interstate crystal set is, without doubt, the most popular crystal set. It has been published and republished in these columns, yet the demands for circuit details became incessant. In order to meet this demand for this popular set I am describing it again this week.

The set was originally designed to meet the "B" class station reception problem in so for as a crystal set was concerned, and the majority of readers have been quite successful in separating all four local station without interference.

Apart from the local success, the receiver has more than lived up to its name by tuning in without amplification a large number of inter-state stations. The popularity of the receiver has been astounding, and many letters have been received from all states of the Commonwealth and New Zealand requiring details, and one Adelaide reader has built 45 of these sets for different radio enthusiasts in South Australia.

Circuit diagram for an interstate crystal set
The Interstate crystal set circuit diagram

The following components are required to build this receiver:

1 One bakelite panel, 12 by 7 by 3/16 inch
2 Two Advance .0005 mfd condensers
3 One glass enclosed detector
4 Two plain 3 inch diameter dials
5 One A.G.N 6 point inductance switch
6 Five nickel plated terminals
7 One .00015 fixed condenser
8 One 2 inch and one 3.5 inch length of 3 inch diameter bakelite former
9 1/4 lb 22 dsc wire
10 One B battery wander plug
11 One single English valve pin socket
12 One packet of covered hook-up wire
13 Sundry screws and solder

How to construct

1. First make the tuner. Take the 2in length of 3in bakelite former and wind on 30 turns, commencing at the extreme edge of one end. This will leave about half an inch of former at one end, and in this mount the battery wander plug.

2. Now take the 3 1/2 inch length of former, wind on 42 turns of the dsc wire, commencing at the extreme edge, which will leave about three-quarters at the opposite end to the windings. With the coils wound they may be mounted in tuner form on the baseboard in a position similar to that shown in the sketch.

3. Mount the fixed 42-turns coil on a pair of wooden blocks the same height as the pin socket as shown in the sketch. Next mount the moving coil by first screwing the pin socket into the baseboard in such a position that when it has the coil mounted on it the coil will just swing across the mouth of the stationary coil.

4. It will have been gathered by now that the wander plug mounted on the coil is plugged into the pin socket, the two serving as a bearing enabling the two coils to be coupled either tightly or loosely to each other. With the coils mounted proceed to mount up to the rest of the components as shown in the sketch. It will prove a good plan to mount the 6-point switch upside down as it makes the tapping easier.

Wiring-up is simple

The wiring-up will prove very simple if the sketch is carefully followed. I would suggest that the constructor makes the tapping of the fixed coil the first job. This is tapped at the beginning of the coil (1), the fourth turn, ninth turn, thirteenth turn, eighteenth turn, and the twenty-first turn.

It will be noted that there are two aerial terminals, one connecting direct to the coil and the other connected direct through a .00015 fixed condenser. This makes the set adaptable to both short and long aerials and enables the amateurs to be tuned in on wave lengths below 200 meters. The aerial terminal with the condenser is for a long aerial.

The operation of the set is simplicity itself. Two controls are tuned until maximum volume is received then adjust the switch to the best point. If the reader cannot tune in the four Brisbane stations with his present set he should try this one, it will do the job.