CIRCUIT DESCRIPTION OF PREPAYMENT CALL OFFICES


Circuit description - Auto working
Circuit description - CB working

P.O. ENGINEERING DEPT.
ENGINEERING INSTRUCTIONS
TELEPHONES
CALL OFFICES
A 4001
Issue 5, 9.2.59

PREPAYMENT COIN-BOX INSTALLATIONS (AUTOMATIC AREAS)
Public Call-Offices Plan No. 5A and Subscribers’ D.E.L. Circuits
Circuit Description

1. General
This Instruction describes the circuit operation of prepayment coin-box installations in automatic areas.  In U.A.X. Nos. 5 and 6 areas, slight differences apply and these are described in par. 11. Par. 12 describes the special conditions that apply when Relay Control of Operator Re-call is used.

2. Outgoing call
A call to a subscriber in the unit-fee area is made by lifting the telephone handset, waiting for dialling tone, inserting four pennies into the coin-box and dialling the number required. When the called subscriber answers, button ‘A’ must be pressed to allow the caller to speak. The circuit action described is with reference to Dgm. N 2408, Panel 1.

3. Lifting of handset
When the handset is lifted, a loop is extended to the exchange via the A line, RB 14 and 13, 35-ohm winding of induction coil, gravity-switch springs 2 and 1, telephone transmitter, D4 and 5, gravity-switch springs 4 and 3, CB6 and 7 to the B line. As the dial pulsing springs are short-circuited via D6 and 7 and BA1O and 9 in parallel with CB3 and 2, dialling at this stage is not possible.

4. Insertion of coins
The insertion of the first penny operates the coin-bar and its associated CB springs.
CB1 and 2 short-circuit the telephone transmitter to prevent the caller being able to speak for the present.
CB2 and 3 remove one of the short-circuiting paths from D4 and 5.
CB4 and 5 connect the 120-ohm resistor across the telephone receiver. This degrades the receiver to prevent it being used fraudulently as a transmitter without pressing button ‘A’.
CB6 and 7 remove the short-circuit from the coin transmitter but this has no significance on local calls.
After the insertion of the second, third and fourth pennies, the balance arm operates.
BA9 and 10 remove the second short-circuit path from D4 and 5, so making it possible to dial.

5. Dialling conditions
During dialling Dl and 2 (in conjunction with D4 and 3) place the 30-ohm non-inductive winding of the induction coil in series with the 2 ~F capacitor across D4 and 5 to form a spark quench. The 30-ohm winding also shunts the bell to prevent bell tinkling during dialling. The induction coil, receiver and telephone transmitter are short-circuited by D3 and 4 during pulsing. The dialling loop to line is given via A line, RB14 and 13, D3, 4 and 5, gravity switch springs 4 and 3, and the coin transmitter to B line.

6. Called subscriber answers
On hearing the called subscriber answer, the caller presses button ‘A’. This action restores the CB spring-set and deposits the money into the cash box.
CB1 and 2 remove the short-circuit from the telephone transmitter
CB4 and 5 remove the shunt from the receiver
CB6 and 7 replace a short-circuit across the coin transmitter.

Thus transmission conditions equivalent to a Telephone No. 232 are set up.

7. Ineffective calls
If busy or N.U. tone is heard by the caller or no reply is received, button ‘B’ is pressed. This action returns the money to the caller, restores the GB spring-set and operates the RB springs which are mechanically linked to an escapement mechanism which delays the restoration of the RB springs for a period of approximately 7 seconds.

  • RB 13 and 14 break the loop to the exchange and clear the line
  • RB1 1 and 12 are spare
  • CB 1 to 7 restore the circuit so that when the RB springs release another call may be made.

8. Incoming calls
On incoming calls, ringing current is received on the B line and passes via CB7 and 6, capacitor and bell, RB13 and 14 to the A line. When the handset is lifted transmission conditions are set up.

9. Assistance calls
The Dial, Automatic, No. 13 normally used in prepayment coin-box installations is so arranged that when the digit ‘0’ or ‘9’ is dialled, D6 and 7 open during the period the dial is returning to normal. This permits ‘0’ or ‘999’ calls to be made without inserting any money. After the caller has dialled ‘0’ the operator asks him to insert coins for the fee. The first coin inserted, whatever its denomination, operates the coin-bar and CB springs.
CB6 and 7 remove the short-circuit from the coin transmitter so that the coins may signal their respective tones to the operator. A penny strikes the wire gong once. A sixpence strikes the bell gong once and a shilling twice. Thus the operator checks the insertion of the necessary money.

CBI and 2 short-circuit the telephone transmitter to prevent the caller from being able to speak for the present.
CB4 and 5 connect the 120-ohm resistor across the telephone receiver. As for local calls, this degrades the efficiency of the receiver and prevents its fraudulent use a~ a transmitter at this stage. A full short circuit cannot be used for this purpose as it may be necessary for the operator to give instructions to the caller.
The caller presses button ‘A’ either when he hears the called party on the line or when the operator asks him. Transmission conditions are then set up in the same way as for the local call already described.
In areas where ‘100’ is dialled for assistance calls a Dial, Automatic, No. 20 must be used. This dial is so arranged that D6 and 7 are normally open, but they close when any digit from 2 to 8 inclusive is dialled.

10. Early type bell-sets
The circuit shown on Panels 1-3 of Dgm. N 2408 uses a Bell-set No. 33, Mark 3. Mark 2 bell-sets are exactly similar electrically but have a modified mounting plate. Mark 1 bellsets are slightly different electrically and the circuit is shown on Panels 4 - 5 of Dgm. N 2408 (yellow).
In the Mark 3 bell-set, dialling on local calls is not possible unless both GB and BA spring-sets operate whereas in the Mark 1 bell-set dialling may take place if only the BA spring-set is operated. The latest arrangement prevents a type of fraud using mutilated coins (see D 3001). The Mark 1 bell-set also differs in that it has an earlier type of induction coil.

11. U.A.X. Nos. 5 and 6 areas
The circuit arrangements used in these areas are shown on Dgm.N 2407 and are similar to those in other automatic areas except in respect of ‘0’ calls.

In these areas it is necessary to dial ‘01’ for assistance calls. When ‘0’ is dialled:- D6 and 7 disconnect the short-circuit from the 75-ohm relay S which operates to the exchange battery and earth.

S1 removes the short-circuit from D4 and 5 and, since the relay holds during pulsing, allows the ten pulses to be sent to line. S2 prevents D6 and 7 from short-circuiting relay S when the dial returns to normal and so permits a second digit to be dialled. Subsequent operations are similar to those described for other automatic areas. Relay S finally releases when the exchange loop is broken at the end of the call.

12. Relay Control of ‘Operator Re-call’
When Bell-sets No. 45 and 46 are used to combat fraud, (see D 3001), the circuit is as shown in Dgm. N 2421 or N 2422.

In each case, when the handset is raised relay CR operates in series with the loop and CR1 and CR2 short-circuit the gravity switch springs 1 and 2 and 3 and 4 respectively. This has no effect on the normal working of the circuit.

When however the gravity switch springs 1 and 2 and 3 and 4 restore at the end of the call or when re-calling the operator they have no immediate effect but springs 6 and 7 short-circuit relay CR which releases after a delay period of approximately 250ms.

The circuit CR1 and 2 break the loop and restore the circuit to normal.


P.O. ENGINEERING DEPT.
ENGINEERING INSTRUCTIONS
TELEPHONES
CALL OFFICES
A 4002
Issue 3, 20.12.66

PREPAYMENT CALL-OFFICE INSTALLATIONS
(C.B. MANUAL AREAS)
Public Call-offices Plan No. 5A and Subscribers’ D.E.L. Circuits
Circuit Description

1. General
This Instruction describes the circuit operation of prepayment call-office installations in C.B. manual areas. The circuit arrangement is shown in Dgm. N 2406, and the type of coin-collecting box used is Box, Coin-collecting, No. 14..., Complete.

2. Incoming calls
Special operation is not required to receive incoming calls; the equipment functions as on a direct exchange line.

3. Outgoing calls
The exchange equipment of call-office lines is arranged to prevent loop-calling, and to make the calling signal dependent upon the insertion of the necessary coins. The modification consists of removing the earth from the A-wire or Li at the cut-off relay (see Dgm. N 1186, Figs. 3 and 4).

4. Lifting of handset and insertion of coins
When the telephone handset is removed from its cradle and four pennies are inserted, an earth is extended to the exchange via L2, the 350-ohm (or 1000-ohm) resistor, coin-bar springs 4 and 5 (spring assembly 1), balance-arm springs 8 and 9 (spring assembly 2), 500-ohm winding of Coil, Bridging, No. 7C, gravity switch springs 1 and 2, 17-ohm winding of the induction coil, and springs 13 and 14 of the refund spring-set (spring assembly 3). Springs 6 and 7 of the coin-bar spring-set remove the short-circuit from the 150-ohm polarized relay and the coin-box transmitter in series, and springs 1 and 2 place a 200-ohm non-inductive shunt across the receiver.

5. Operator answers
The operator answers and the caller states the number required. Prior to the depression of button A, a balanced earth is applied to both lines via the 500-ohm coils of the Coil, Bridging, No. 7G.

6. Operation of the polarized relay
The exchange call-office cord-circuits are arranged to reverse the tip and ring of the answering cord when the calling cord is inserted into a jack (see Dgm. N 1186, Fig. 1). This reversal causes the operation of the polarized relay, the contacts of which short-circuit the telephone transmitter. The caller is therefore unable to speak to the called subscriber until button A is depressed.

7. Called subscriber answers
On hearing the called subscriber answer, the caller depresses button A. This action causes the money to be deposited in the
cash box, and also restores the coin-bar and balance arm spring-sets to normal. The earth is removed from the circuit by coin-bar springs 4 and 5 and balance arm springs 8 and 9. Coin-bar springs 1 and 2 remove the 200-ohm shunt from the receiver, and springs 6 and 7 place a short-circuit across the polarized relay and the coin-box transmitter. The relay releases and removes the short-circuit from the telephone transmitter.

8. Called subscriber busy
If the required number is busy or unobtainable, the caller either receives busy tone or is advised to press button B. The caller then replaces the telephone handset and depresses button B, which causes the money to be returned via the refund chute and also operates the refund springset for a period of approximately seven seconds. Springs 13 and 14 of the refund spring-set disconnect L2, and springs 11 and 12 place an earth on Li via the coil of the 2000-ohm relay. This relay operates to battery from the cord-circuit, and locks to an earth via its own contact. The other contact of the relay disconnects L1. A clear is given to the exchange, and the call-office circuit is disconnected until the operator withdraws the plug from the jack. The 2000-ohm relay then releases and restores the circuit to normal.

9. Calls outside the unit-fee area but controlled by local operator
The caller inserts the local fee to obtain the operator, states the number required and is then told to insert the extra fee. The operator checks the fee, coin-by-coin, by means of the coin-box transmitter which picks up the distinctive sounds of a penny striking a wire-gong once, a sixpence striking a bell-gong once or a shilling striking the same bellgong twice. The calling plug is now used to obtain the wanted subscriber, and the call proceeds, as indicated in pars. 6, 7 and 8.

10. Calls controlled by trunk operator
Having inserted the local fee to obtain the operator, the caller is told to press button B. After the seven seconds disconnexion period, a connexion is set-up between the controlling exchange and the caller, via the B-position at the local exchange. The wanted subscriber is obtained before the caller is requested to insert the total fee, which the trunk operator checks, as described in par. 9. The caller is then told to press button A, and when this has been done, the operator connects the two subscribers. The polarized relay does not operate in this case.

11. Emergency calls
Public call-offices which are accessible from the public highway are fitted with emergency press-buttons, the operation of which enables a caller to call the exchange without inserting money in the coin-box. The operation of the emergency calling button places an earth on L2 via a 50-ohm resistor. The application of this low-resistance earth operates a relay which is in series with the normal line relay (see Dgm. N 1186, Fig. 4). This relay connects an interrupted earth to the calling-lamp circuit, causing the lamp to flash and thus indicating that the call must receive prompt
attention.

 
 
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Last revised: September 19, 2010

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