Rob's web

Simple code-practice oscillator

Home - Techniek - Electronica - Radiotechniek - Radio amateur bladen - QST - Simple code-practice oscillator

Loudspeaker output with one transistor.

To practice code you need a key and some sort of tone generator that simulates the sound of a radiotelegraph signal. The tone oscillator described here is inexpensive and portable - you don't need an external source of power. By adding a relay, you can also use the gadget for monitoring your keying.

Pic 1
Tiny but potent! This photograph shows the completed unit ready for code practice. The small speaker puts out enough volume for group practice.

One of the requirements for obtaining an amateur license is the ability to send and receive International Morse code. In order to learn to recognize the sound of the different characters, a code-practice oscillator is a required piece of apparatus for the beginner. The oscillator described in this article is capable of producing an audio tone similar in sound to the code signals one hears when listening to the ham bands with a communications receiver.

The volume from the loudspeaker used in the unit is enough to be heard across the average- sized room, making the oscillator suitable for code-practice groups. The oscillator also can be used as a keying monitor by adding a double-pole, single-throw keying relay. Many operators find it difficult to send legible code without monitoring, and the Novice, particularly, needs some method for listening to his own sending when he goes on the air.

The circuit

As the reader can see from Fig. 1A, the circuit of the code-practice oscillator is quite simple. It consists of a CK722 transistor, a capacitor, a resistor, an output transformer and speaker, and a dry-cell battery. The 9-volt battery used in the oscillator circuit is assembled by connecting six 1 %2-volt penlite cells in series. The oscillator is keyed by opening and closing the connection between the transformer center tap and the battery.

Fig 1
Fig. 1. (A) Circuit diagram of the code-practice oscillator.
(B) Circuit for using the code-practice oscillator as a keying monitor.

BT19 volt battery; six 1½ volt penlite cells in series (see text).
C110 nF disk ceramic capacitor.
K1Keying relay, double pole, single-throw, 6 volt a.c. coil (Advance GHA/2C/6VA or equivalent).
LS1Loudspeaker, 2½ inches, permanent-magnet replacement type. 3.2 ohm voice coil (Allied Radio 81 D066, Lafayette Radio SK-39, Argonne AR-95).
Q1Transistor, CK722.
R1See text.
T1Output transformer, 12,000 ohm primary to 3.2 ohm voice coil (Thordarson 22S48)

Fig. 1B shows the connections for adding the keying relay. A relay having a 6-volt a.c. coil is used. One pair of contacts is used to key the transmitter and the other pair to operate the oscillator. The 6 volts a.c. can be obtained from the transmitter. A commercial rig usually has an auxiliary power socket on the rear, and the power for the relay can be taken from this point; check your instruction manual to see if your rig has such a power take-off. If it doesn't, you can get the six volts either from the heater pins on one of the 6.3-volt tubes in the rig, or directly from the 6.3-volt winding on the power transformer.

Incidentally, the maximum voltage across the key with the contacts open will be only 6.3 volts, so you can't get a dangerous shock if you accidentally touch the key terminals.

Construction details

The oscillator shown in the photographs is built in a 3 × 4 × 5 inch aluminum box, all components being mounted on one side of the box. Be careful not to mount any of the parts too close to the edge or you won't be able to fit the completed unit into the box.

Pic 2
The speaker is mounted directly above the output transformer T1. At the right of T1 is the two-terminal strip (Millen 37302) for the key connections. The remaining components are mounted on the four-terminal tie point.

A four-terminal tie point is used for mounting C1, R1, Q1, and for connecting the leads from T1. Special care must be taken when soldering the CK722 leads as too much heat can ruin the transistor. When you are ready to mount the CK722 use a pair of long-nose pliers to hold the lead being soldered, grasping the lead close to the transistor body. The pliers will absorb most of the heat before it can reach the transistor.

The CK722 has three leads. The lead closest to the red dot on the body should be connected to one end - either one - of the primary winding of T1 (the other end goes to C1 and R1 as shown in Fig. 1). The center transistor lead should be connected to the junction of C1 and R1. The remaining lead goes to the positive terminal of the battery.

Note how the batteries are taped together to form a single pack. In order to connect the batteries in series you must know which are the positive and negative terminals on a single cell. The tip of the cell is the positive, or plus connection, and the metal shell is negative or minus. Connect the cells in series by soldering a short length of wire between the tip of one cell and the case of an adjoining one, as shown in Fig. 2. Then fold the assembly as shown in the photograph and wrap with tape.

Fig 2
Fig. 2. This drawing shows how to connect the six 1½ volt penlite cells in series to obtain 9 volts for the oscillator. The photograph of the inside of the unit shows how the batteries are taped together to form a single pack.

After the wiring is finished, connect your key leads to the two terminals and try the oscillator. If you don't like the pitch of the audio tone you can lower it by changing R1, to 47 kΩ, or can raise it by using 100 kΩ at R1.

Lewis G. McCoy, W1ICP.