Norchard Line Concentrator
Previously, all signal post telephones (SPTs) had been connected to the signal box using "D" Phones at each end. These phones use a 12 volt battery for signalling and a 3 volt battery for speech and the batteries were a constant source of annoyance. They required changing every so often and of course they were scattered about the railway system making them difficult to service. Then one of the SPTs was knocked off its post by a passing works train and left on the ground. By the time it was reported it had been full of water for some weeks and would no longer work. It was a phone that we could not replace as spares of that type could not be found.
Once it became known that we were attempting to build a concentrator, help came from Chris Hall of the Mid Hants Railway and Mick Bulman of the Telephone Heritage Group. Between them, and with the relay sets spare in our store, we had sufficient equipment to build a system.
At the design stage we were referred to the IRSE recommendations for line concentrators and these can be summarised as :
"The design must include a display of the identity of the telephone (and thus the signal number and by inference, its location) at the signaller's control panel.......The system must ensure that only one conversation can take place to each signaller at any one time, thus preventing the overhearing of operational messages by other drivers. Systems should allow the signaller call back to the driver.
This meant that we were not providing a switchboard but a key and lamp system where it would not be possible to operate more than one speak key at a time.
Mick Bulman came up with a concentrator head (the key and lamp unit) supposedly from a Southern Region source. The keys had to be laid out differently and the whole head had to be completely rewired but it is now a lovely piece of kit for a signal box. It caters for a maximum of nine SPTs and has a simple line tester built in for engineers to use when fault finding on the lines. This was felt desirable as the lines will not pass through the site's MDF and will otherwise not be easy to test.
Operation of the Line Concentrator
1) SPT calls, box answers.
2) SPT calls, no response from Signal Box
3) Signalman calls an SPT, SPT answers.
4) Signalman calls an SPT, No response from SPT.
5) Miscellaneous Operations.
Each line key has three positions. Centralised it connects the line circuit to line to await a call. Downwards (KK) it operates a K relay which switches the line through to the common equipment. Upwards (KL) the line is connected to the line tester. The tester normally sends an earth on both legs to line, therefore any line with the key up is normally earthed both legs and can be left in that state if it should go faulty.
The lamp is normally out. If the SPT calls then it is flashed at a rapid rate. When the call is answered the lamp goes out. Should the call not be answered after a timeout period (2min 30sec) the lamp remains permanently lit. If the signaller calls the SPT, he operates the line key and then momentarily operates a ringing key. This causes ringing to be sent to line and the line lamp flashes slowly in synchronism with the ringing cadence.
Common Equipment and Operator's Circuit
The common equipment has a standard transmission bridge which is connected between any SPT and the signalman's telephone circuit by the operation of the SPT speak key and the operator's key.
The PHA relay circuit is of interest. It was derived from the IRSE text book on Railway Signalling and consists of a relay connected across a bridge circuit. If no speak key is thrown, then no current flows and the PHA relay remains released. If one speak key is thrown, then 450 ohm earths are connected to each side of the PHA relay which is balanced and remains released. If more than one key is thrown then 450 ohms is connected to the right hand side of the PHA relay and 225 ohms or less on the left hand side. This unbalances the circuit and current flows through PHA to operate it. PHA1 disconnects the earth from the SC wire thus preventing any K relay operating in the line circuits. PHA2 also operates the main warning light and the buzzer to tell the operator that he is misoperating the equipment.
A calling L relay in a line circuit, or the PHA relay operating lights the main warning lamp and starts the buzzer. At the same time relay SB starts its slow to operate feature. It takes a second or two to operate. The long/short buzzer key can select whether to use the period after the SB operation to ring the buzzer continuously or the short time before the SB operation to give just a quick call on the buzzer. In either case the main lamp lights continuously. If the sound of the buzzer is annoying, it can be silenced temporarily by operating the buzzer cut off key momentarily. This operates relay BCO which holds to the calling condition. It disconnects the buzzer but leaves the calling condition on to light the main lamp.
Any calling L relay or the operation of the ringing relays RR and RA extends a start earth to the tone, ringing and time pulse relay set.
The ring trip circuit is of interest as it had to be modified from the standard circuit element. The ringing converter has a 2150 ohms output winding on the coil. Whilst this would ring a bell satisfactorily it would not permit sufficient direct current to flow to allow the F relay in the circuit to trip the ring. In the end it was decided to modify the ringing circuit so that DC only flows during the "silent period". Relay RT was added which operates each time ringing occurs. During the silent period RT is released and connects a 270 ohm battery out to line in place of the 2150 ohm earth from the converter. During this period sufficient current flows around a looped line to operate the F relay to trip the ring. Ring trip though cannot occur during the ringing phase.
Tone, Ringing and Time Pulse Equipment
The timing relays are started by an earth on the STA lead. This operates relay ST which in turn causes relays FE and FR to interact and allow relay FE to provide a "flicker earth" pulse. This flicker earth at FE5 flashes any calling lamp rapidly. From here the relay set times out to around 2 mins 30secs.
Relays RA and RB divide the FE1 pulses by two, RC and RD reduce the pulse frequency by a further factor of two, as do relays RE and RF. Over all the pulse frequency is divided by eight. Various timings can be picked off the relay dividers. In particular RGB is operated in a similar cadence to British Ringing and then RUS covers the short gaps in the cadence to give a US type ring. In practice the British ring is not too satisfactory as the rings are too short, so the US ring has been adopted for this installation.
Each time the dividers end a sequence a type 3 uniselector T is operated. Each time T completes a sweep around its bank it operates uniselector TA. When TA moves to outlet 3 relay TP operates. This is after 2 mins 30 sec but the timing can be altered by strapping the earth to a different TA outlet. This releases relay ST and at the same time holds to the start condition. The relay set stops timing. TP3 stops the buzzer from sounding (but the main lamp stays alight). TP7 stops any outgoing ringing. TP5 stops the calling lamp flashing and causes it to glow permanently although at a somewhat reduced brilliance. TP1 connects NU tone to any caller. The situation can be released by the signalman operating a line disconnect key or a speak key.
Both tone generators are similar except for the size of the coupling capacitors. The first two transistors act as a multivibrating pair producing something like a square wave. The third transistor is simply an emitter follower to drive the low impedance of the volume control. The fourth transistor is a further emitter follower with a low impedance output sufficient to produce loud tones in a distant telephone. The resistor network on the left reduces the power feed line to around 18 volts to prevent the transistors being over voltaged and destroyed.
Ringing Converter 9a (modified)
When the concentrator reached the testing stage it was found that the ring converter has a 2150 ohms output winding on the coil. Whilst this would ring a bell satisfactorily it would not permit sufficient direct current to flow to allow a relay in the circuit to trip the ring. In the end it was decided to modify the ringing circuit so that DC only flows during the "silent period", at which time sufficient current flows from a battery which temorarily replaces the ringing converter earth. Ring trip cannot therefore occur during the ringing phase.
Line Test Facility
The line tester normally provides earth both legs on the common wires to the line disconnect keys. Even if more than one line is disconnected, and therefore connected to the tester, they will be firmly earthed and be unable to gain access to each other.
To test a line, only the line disconnect key for that line can be operated. The tester then operates very like any other GPO/BT line tester using a voltmeter connected to the voltmeter terminals T1 and T2. It was difficult to measure loop resistances with local batteries and voltmeter shunts in the usual way, so a "loop" key was added to bring the line out to a separate pair of T3 and T4 terminals. It is proposed to connect a digital ohmeter to these terminals to read loop resistances when required.
Alarm Send and Clock Pulse Receive Equipment
The equipment also receives 30 second battery pulses on the second wire of a pair to Norchard exchange. These pulses operate relay NCP which in turn generate a standard length pulse from relays A,B and SCP to operate clocks in the signal box. The length of the output pulse is dependent on the value of the capacitor across the B relay.