700 type telephones were introduced in 1959 and the basic circuitry has more or less remained the same.

A simplified 700 type circuit may be drawn as follows:-

The line has an impedance represented by ZL. Resistors R1 & R2 with the two capacitors comprise a balance impedance and the diagram can be further simplified as shown in the following drawing:-

Although ZB is shown as a resistance it is in fact an impedance whose value depends on the frequency of the AC signal.

The transmitter is energised from the exchange battery and when spoken into produces varying DC current. If only the alternating part of this current is considered, then the transmitter can be treated as an alternating voltage generator, which at any one instance could cause a current to flow in the direction shown by the arrows.

When transmitting therefore winding 2 has the greater ampere turns product and will control the flux in the core and the direction of the induced emf will be as shown.

The current due to the transmitter will flow through the balanced impedance ZB and a potential difference will be developed across this. Under conditions of perfect balance the value will be the same as the emf induced in winding 3. The emf in winding 3 would cause a current to flow in the receiver but as the voltage across the balance is exactly the same, but opposite in direction, the resulting pd across the receiver is zero and therefore no current will flow in the receiver.

Output from the transmitter is shared between the balance and the line, and therefore no current floes in winding 3 or the receiver.

Representing the voltage across the balance and the receiver as of equal voltages the effect could be shown as in the left hand figure below:-

The receiver and winding 3 could then be neglected since no current flows in this circuit and the transmitting circuit could be considered as follows in the right hand diagram:-

The voltages across winding 1 and 2 are in series aiding and are applied to the line with the balance impedance in series. The induction coil therefore acts as an auto transformer with 402 turns on the primary and 402 + 670 turns on the secondary.
The impedance matches the line for maximum power transfer.

Under receive conditions the transmitter behaves as a resistor of low value. The current path shown in the next diagram is via winding 1 and the transmitter. Winding 1 has the greater ampere turns product in this case and controls the flux in the core and hence the induced emf's.

The emf's induced in windings 2 and 3 cause a current to flow in the receiver. The potential difference developed across the receiver is the same as the emf induced in winding 3. The resulting potential difference across the balance ZB will be zero and no current will flow in the balance.
The input power from the line is shared between the receiver and the transmitter. Receive current can be shown as in the diagram below where ZB has been neglected since no current flows in that circuit.

The induction coil acts as an autotransformer with a ratio of approximately 1.1.

The foregoing explanations assume ideal conditions that would not be achieved in real life. In any event complete suppression of side tone is undesirable since the telephone would appear to be dead to the user. Lines in a practical telephone system also vary considerably and the values chosen for the component parts give a good compromise under average conditions.

Original field trials of the 700 type telephone showed that it was too sensitive on short lines, whilst working well on long lines.
An automatic regulator was introduced to reduce the sensitivity on short lines, whilst allowing full efficiency on long lines.
The regulator consists of a network of rectifiers and a resistor bulb (diodes and thermistor).

The purpose of the regulator is to vary the amount of ac speech current flowing through the microphone and receiver. This is achieved by shunting the two components with a rectifier. If the rectifier is forward biased it will permit current to pass through it freely. However, if the rectifier is reverse biased then current will not flow. Different degrees of forward bias will cause different values of speech current to flow through it.

The resistor bulb or thermistor (which is a non linear resistor having a positive temperature coefficient of resistance) is effectively in parallel with the rectifier. As varying amounts of dc line current flow through the resistor bulb this applies a varying amount of forward bias to the rectifier.

The more the rectifier is forward biased, the more current will flow through it. As the rectifier shunts the microphone this causes lower amounts of current to through it. The converse is also true.

Whilst dialling is in progress the dial off normal springs short circuit the microphone, to prevent damage due to line surges and the receiver, to prevent audible clicks and to reduce circuit inductance.

The dial springs are prevented from arcing by a spark quench circuit, shown in the diagrams below. R1 is the sum of the resistance's 150ohms, 7ohms and 11ohms. R2 is the sum of the resistance's 23ohms, RU and the bell. The spark quench circuit of R1 (approx. 18ohms) and the 1.8 microfarad capacitor absorb the high pulsing currents, effectively eliminating sparking across the dial impulse contacts, prolonging their life.

To prevent bell tinkle when dialling a bell shunt circuit is used and this comprises of the two resistors R1 and R2 (total resistance approx. 80ohms) in parallel with the bell (1000ohms) effectively forming a short circuit. Thus the bell will not operate.


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Last revised: December 20, 2010