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Signal Post Telephones


For reasons forgotten in the mists of time SPTs became a responsibility of the Telecoms department, there being no

railway SPT equipment available new equipment was made using Post Office 3000 type relays.


The original relay sets for the phones were built in multiples of 4 circuits and these are fitted at Alresford and Medstead,

each having 8 lines installed.


In the later installations at Ropley and Alton the relay sets have 5 circuits, giving a total of 10 lines.


Alton and Alresford exchanges have ex Strowger exchange racks fitted for the SPT relay sets, lack of space at 

Ropley and Medstead exchanges meant the equipment is fitted in the signal boxes, here we have used cut down

UAX13 racks, thanks to the railway Building Department for doing the metal work.



Shown above is the latest variant of a modified phone using the components from a late model 700 type phone.

It is a simpler design and makes any maintenance easier, fitted in Medstead signal box for local SPT circuits.


The changes will not impress the purist.



 

Original Medstead SPT equipment under test - for more pictures.


Modern case - Medstead yard BR type - MedsteadMHR type - Ropley Down home signal
 
 
 




Alresford SPT concentrator and associated phone, it now has an approriate period handset cord.

Thanks to the Telecoms department of the Festiniog Railway for donating the phone.




SPT and miscellaneous equipment rack at Alresford.


On the top shelf are the alarm circuits from the other 3 stations, below is an electronic Clock and associated Clock control relays.


Shelf B is the SPT equipment, 2 relays sets each with 4 Line Circuits and the Common Equipment relay set.


On the shelf below is the Electronic Ring Tone generator and on the right just in shot is the equipment that generates the

30 second clock pulse, there are main and standby generators.


Alresford's two furthest SPTs had an additional relay set fitted to detect if the line went faulty.

At the far end a 33k ohm resistor was fitted across the line, this caused a small current flow that was detected by a

transistor circuit at the local end, if the line went faulty an alarm was raised. Not very successful as the detector circuits

often went faulty, usually after a thunder storm. These extra relay sets were later removed as they were a fault liability.


A plan was made to use opto-isolators instead, this had the advantage of a reduction in space required and all circuits

could be monitored, one of those jobs to be done on a rainy day. 


Alton has a requirement that when the box is closed the SPTs are diverted to Medstead signal box. The equipment that

had been stored for this job was removed from secure storage and dumped 6 miles away in the open, nobody had told us

this was happening so by the time the items were discovered they were ruined.


Being old technology it was not easy to replace it so a mixture of old and modern equipment has now been used.


For a the time a direct circuit via the two Norstar exchanges was installed and tested should it be required to give the diversion service.

As the circuit was in situ it has been made a direct line between the two signal box Norstar phones.


Alresford, Ropley and Medstead use vibrator ringers, Alton has an electronic one.


On two occassions Norstar switches have been used to give temporary SPT service, first at Alresford when the Strowger exchange

was recovered and a new SPT rack was being installed. Second at Ropley, the relay sets from the old installation were required to

be used during testing of the new system. 


Alton - showing the start of installation
of the SPT rack also shown is the PP2040.


On the right the SPT rack at Alton with all 

the relays sets in position, a quick count

shows there are around 130 relays,

3 uni-selectors plus some more relays

at Medstead.



The top set of relays are the exchange

alarm system.

  


Relay set at Alton to switch SPTs to Medstead, 10 circuits, relays have been used to switch lines to reduce

the wiring to the selection keys in signal box.


Under auto working, which is most of the time, the relays are released and calls go to Medstead. Only when

the box is opened are the relays operated.


During the 2009 Spring steam gala the system was placed in service for testing, a phone acting as a SPT

was installed in Alton signal box for these tests. The idea was to see if there were any unforseen problems.


Apart from a noisy cable pair, which there was not sufficient time to sort out, there were no problems experienced,

thanks to the Traffic Department staff for giving the system intensive use, over 100 calls were made in 3 days.




What lurks under the panel in Alton signal box, termination of the SPT Key and Lamp unit, shown are Krone connection 

strips, phone circuit board, buzzer and a converter to change 50v to 5v for the lamps.




The relay sets that are required to extend the SPT circuits from Alton to Medstead when Alton signal box is closed, 2 relay sets either end.


First and fourth sets for Alton and second and third sets for Medstead signal box.




Medstead - new equipment.

 

The top shelf has 2 relays sets each with 4 Line Circuits and the Common Equipment relay set, the bottom shelf

has the Ringer, with 2 spaces reserved for Alton circuits.




Concentrator at Medstead signal box where the circuits terminate when Alton box is closed.

 

At the bottom of the wooden box are the 10 keys for the Alton SPTs and the associated handset on the side, the Green key is the

CALL key and the Grey key on the far right allowed remote swiching of the Alton equipment from local to remote mode and vice vers

 but is no longer used. Two lamps indicate if the Alton equipment is in the local or remote mode.

Modern equipment is being used as someone totally unconnected with the Telecoms department decided to throw out the older style

keys that were to be used, very thoughtful of them.

 

Before building the parts for the Alton circuits a poll was conducted amongst the signalmen and the few who bothered to reply were

happy with modern items.

 

The top row of keys are for the Medstead SPTs.

 

The top right hand key being in a down position shows the concentrator is switched off, any one attempting to call from a SPT

will get a continuous tone instead of ring tone.




Medstead  - new relay sets for Alton connection fitted - March 2009.

 

Initially it was planned to have a bothway SPT circuit between Medstead and Alton but it got complicated with the limited number

of relays there was space for so the final scheme had an outgoing and an incoming circuit.

 

The method of operation is straighforward, to call from Medstead to Alton the required SPT key is operated, this puts a marking

condition on a uni-selector bank. The non locking CALL key is operated and the uni-selector steps at 10 pulses per second (10 pps )

until the marked outlet is reached, a corresponding number of pulses are sent to the incoming relay set at Alton which acts in the

same manner as a Final Selector, it tests the line and if free connect ring tone to the caller and ring current to the SPT. If a call is

made and the SPTs are in the local mode Number Unobtainable tone is returned.

 

An Alton SPT calling Medstead when in remote mode causes a Line Finder to step at 10 pulses per second to find the calling line

and Ring tone is connected, a corresponding number of pulses are sent to Medstead to indicate which SPT is making the call.

If the Medstead concentrator is switched off the line to Alton is reversed , this is detected at the distant end and Number Unobtainable

tone is returned if someone attempts to use a SPT.

 

For a while a Panasonic 308 switch was used at Ropley for the SPTs but was not popular being replaced with a relay based system shown below. 

 



Ropley (old box)- the SPT relay sets were located under the desk.

 

From the left - 4 Line Circuits, Ringer and Common Equipment.

 

Each line circuit had a LS relay coil, this had a 50 + 50 ohm coil differentially connected in the line, on lifting the SPT handset

the LR relay would operate but not the LS relay, the line lamp would come on, there would be no audible indication. 

A caller would press a CALL button on the phone, this connected an earth condition to one leg unbalancing the line allowing

the LS relay to operate which operated LZ in the Common Equipment relay set, this caused the buzzer to sound and the lamp

associated with the SPT to flash.

This method overcame the problem of SPT handsets being left off but the LS coils often went faulty leaving the SPT out of service,

the LS relay was removed from the circuit.    




Ropley (new box)- replacement SPT equipment, the top shelf has 2 relays sets each with 5 Line Circuits

and the Common Equipment relay set, the bottom shelf has the Ringer on the left plus a spare Ringer.

On the right are the electronic Ring Tone generator and Krone connection box.

 

The equipment is now enclosed in the cupboard shown below.






It will be noted on top of the cupboard is a Norstar phone, this is for emergency use for incoming calls from off site callers.

When fitted it caused certain people to go into a state of apoplexy. It seems modern phones are a no no but modern

13 amp electrical fittings are alright.

 

After this item was added there was a view of Swanage signal box on the local TV news, in a very prominent position could

be seen a Norstar phone, in no way hidden, they must be made of stronger stuff in Dorset.


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