Theory of Operation
Common battery systems permit more efficient switchboard use because the Central Office can detect automatically whether an instrument is off the hook. This is indicated by a complete direct current path through the transmitter circuit, and so there is no need to "ring off" as on magneto systems. In addition, transmission quality is higher because an assured talking current is provided all instruments from the Central Office. The price, however, is the need for a better quality line and more complex switching equipment. There are two types of common-battery circuits: sidetone and antisidetone. Sidetone is the sound in the receiver of the speaker’s own voice, which can interfere with communication if the speaker is in a noisy location. Anti-sidetone circuits, introduced later, reduce the sidetone. The basic sidetone circuit (drawn from WE wall phone ["hotel phone"]) is shown in Figure right --->
circuit When the hookswitch is closed, the first contact to make
completes a circuit from L2 through the transmitter and induction coil
primary back to L1
and the Central Office. The low d.c. resistance of the instrument (200
to 300 ohms) operates a relay in the Central Office, flashing a light
on the switchboard. Talking power is provided from the same
source. Receiving circuit The last contact to make on the
hookswitch completes the receiver
circuit through the transmitter, capacitor (condenser in earlier
terminology), and secondary of the induction coil. Voice current from
party induces a current in the secondary which is heard in the
capacitor prevents direct current from passing through the receiver,
which might weaken its permanent magnet. Signalling circuit As with the
magneto system, ringing may be either bridged (from L1 to L2) or
divided (from one line to ground). In the bridged circuit, used on
private lines, the solid ringer connection in the schematic diagrams is
In the divided circuit, used on party lines, employ the wiring shown in the dashed connections. The capacitor performs double duty by also preventingline current from flowing through the ringer, giving the Central Office a false indication of off-hook condition. If ringing is not desired, simply disconnect the two ringer leads and tape.
Figures illustrate different manufacturers’ basic circuits for the candlestick or cradle phone instruments. Note that sidetone circuits require only a three conductor cord from desk instrument to wall box. Note also that all common battery instruments require an induction coil (either in the phone or on the wall). Simply wiring transmitter and receiver in series will destroy the
receiver magnets in time.
Common battery candlestick circuit with separate ringer box
(drawn from 1926 Kellogg desk set)
Common battery candlestick circuit (drawn from early WE desk set)
Masterphone Common Battery Cradle Circuit
Circuit of WE 634A ringer box with 46B induction coil;
for use with the Candlestick or Cradle phones.
This is a sidetone ringer circuit.
Two Western Electric anti-sidetone circuits are described below.
The major differences from the sidetone circuits are:
1. an additional winding in the induction coil, which provides an induced voltage counter to that set up by the local transmitter,
cancelling out part of the sidetone;
2. a different hookswitch arrangement; 3. an additional capacitor; 4. a four-conductor cord.
WE manual anti-sidetone candlestick circuit
WE manual anti-sidetone cradle circuit
Western Electric anti-sidetone induction coils are numbered 146B and 101B. Other manuacturers’ units may be checked to determine whether they contain the additional winding by measuring the resistance between the 1-2 and 3-4 (L1-R and C-GN) windings. If open, it is a sidetone coil; if a low resistance (indicating the additional winding) it is an anti-sidetone coil.
It should be noted that ringer boxes containing an anti-sidetone induction coil can be used only with candlestick and cradle instruments having a four-conductor line cord and two separate parts to the hookswitch. On the other hand, ringer boxes with sidetone induction coils may be used with anti-sidetone-wired candlesticks and cradle instruments by omitting the black cord connection at the ringer box and wiring as a sidetone unit. Otherwise any manufacturer’s ringer boxes and instruments may be intermixed so long as the components conform to the circuits shown.
<- WE 634A ringer box with 146B or 101B antisidetone coil
(use with hand set circuits above )
COMMON BATTERY; DIAL TELEPHONES
Theory of OperationAddition of a dial to a common battery manual instrument permits automatic operation of Central Office equipment. The dial consists of two sets of springs (metal leaf contacts):
1. pulsing springs: these interrupt the complete d.c. path through the instrument a number of times corresponding to the digit dialed (10
times for "0" in the United States and in most other countries);
2. shunting springs: these short out the rest of the instrument while pulsing in order to provide squarer pulses to the dial equipment
and in order to prevent unpleasant clicks in the receiver.
(Western Electric dials open the receiver circuit instead of shunting it.)
WE 553A wall telephone ("hotel phone")
Restoration and TroubleshootingSeveral difficulties commonly occur with aged dials. Patience and ingenuity are essential in repair.
.....1. Bent finger wheel, striking fingerstop or frame: remove and straighten.
.....2. Dust and corrosion: remove with alcohol and pipe cleaners or cotton swab. The governor is particularly sensitive to the slightest ..........amount of dirt. Lightly oil metal parts (except governor).
.....3. Loss of spring tension on finger wheel shaft: this is provided by a coil spring wound around the shaft itself in the center of the dial. The
.......... problem is rectified by adding one or a few turns to the spring. On WE dials this is done from the front, removing the fingerwheel, .......... number plate, and finally the plate which stops the finger wheel shaft from rotating more than one turn. With this latter removed, the .......... shaft can be rotated to provide additional tension. On AE dials tension is increased from the rear by removing the end of the coil .......... spring from the slit in the shaft, twisting, and reinserting. In any case, take care not to release the coil spring and be certain to clear .......... any impediments to free movement before increasing spring tension.
..... 4. Misadjusted contact springs: this may occur from loss of fiber or rubber pushers (which must be replaced or refabricated) or loss of .......... tension (which must be added by bending springs carefully).
Pulsing springs: observe operation while finger wheel is rotated: they should open and close a number of times corresponding to the number pulled, ending closed.
Shunting springs: they should shift just as the dial is rotated, and remain operated for the entire time the pulsing springs pulse. They should return to their normal position just as the finger wheel returns to rest.
Conversion of Manual to Dial
Manual instruments having a dial blank may easily be converted to dial operation by adding a dial, using the appropriate circuit shown on the preceding pages, depending on the hookswitch configuration, whether handset or separate transmitter and receiver, and whether sidetone or anti-sidetone. Instruments without physical provision for a dial may also be converted to dial use, by mounting the dial on an appropriate home-made adapter. Alternatively if one does not wish to alter the original condition of an old telephone, the dial can be mounted separately in a wood or metal case, and connected to the instrument by a four-conductor line cord. "Pinging" of the bell when dialing can be overcome by reversing the bell leads, or by increasing the spring tension on the clapper.