The following is from AT&T Co.'s Specifications 4068, February, 1921.


Type of set.

The No. 3-A telegraph repeater set as shown in Figures 2 and 3 is the standard of this company for repeating between two telegraph lines operated by the Single Morse system. One of these lines may be merely the local circuit of a duplex repeater as explained later.


The No. 3-A telegraph repeater set is a direct point repeater in which the signals are repeated directly by the armature contact points of the main line relays, without the use of auxiliary transmitters or repeating sounders. This set consists of two relays mounted on a single base. One relay is arranged to have its line windings in series with the west line and its main contacts arranged to open and close the east line, while the other relay is arranged to have its line windings in series with the east line and its main contacts arranged to open and close the west line. Each relay has a double-spool line magnet of 120 ohms resistance and a double-spool locking magnet of 10.5 ohms resistance. The armature of each relay actuates two sets of contact points, one set being in the main line circuit and the second set or auxiliary contacts in the local circuit. The auxiliary contacts of one repeating relay serve to shunt the holding magnet of the other repeating relay and vice versa. The line magnet, holding magnet and retractive spring of each repeating relay act in the same direction on the armature so that the armature is always bearing on the same side of the trunnions. This arrangement prevents lost motion when the bearings become worn. Switches are provided on the repeater set base for opening the local battery circuit when the set is not in use and for the purpose of separating the east and west lines. The retardation coil, resistances and condensers which form a part of the set are mounted on the under side of the base. The various adjustments of the set, which are referred to later, are indicated in Figure 2.

Explanation of Operation.

In Figure .3, the main line contacts of th& east repeating relay are shown to be open, as the key at the distant end of the east line is assumed to be open. This relay is ready to repeat signals from the east line into the west line, which is assumed to be closed at its distant end. The main line contacts of the west relay are held closed by the holding magnet of this relay, the shunt on this magnet having been opened when the east repeating relay opened its auxiliary contacts. When the east line is closed, the armature of the east relay is actuated, closing the main line contacts and the auxiliary contacts of the east relay. The main line contacts cause current to flow through the line magnet of the west relay and the auxiliary contacts shunt out the holding magnets of the west relay. The contacts of the west relay are therefore kept closed when the contacts of the east relay are either open or closed, provided the distant key of the west line remains closed. This sequence of operations al)plies as well to the west relay when signals are being repeated from the west line into the east line.

If an operator in the west line opens a key to “break” while signals are being transmitted from east to west, the armature of the west relay will be released the instant the key in the east line is closed, since no current can flow through the line winding of the west relay while the west key is open, and current will be shunted out of the west holding magnet by the closure of the auxiliary contacts of the, east relay. Consequently, the “break” will be transmitted to the east operator.

The 20-ohm retardation coil in the locking circuit, through its inductive effect when the shunt on the holding magnet is removed, hastens the operation of this magnet. The line contact points are shunted’ by a condenser and resistance in series to reduce the sparking. The 1500-ohm non-inductive resistance reduces sparking at the auxiliary contact points. The retardation coil, condensers and resistances are located in the base of the repeater set.


The No. 3-A telegraph repeater set is used to connect two telegraph circuits together. These telegraph circuits may be Single Morse circuits, half duplex circuits, or a Single Morse circuit and a half duplex circuit.

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