Signal Post Telephones
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.
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
the later installations at Ropley and Alton the relay sets have 5 circuits,
giving a total of 10 lines.
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.
changes will not impress the purist.
Original Medstead SPT equipment under test - for more pictures.
|Modern case - Medstead yard ||BR type - Medstead||MHR type - Ropley Down home signal|
SPT concentrator and associated phone, it now has an approriate period handset cord.
to the Telecoms department of the Festiniog Railway
for donating the phone.
and miscellaneous equipment rack at Alresford.
the top shelf are the alarm circuits from the other 3 stations, below is an
electronic Clock and associated
Clock control relays.
B is the SPT equipment, 2 relays sets each with 4 Line Circuits and the Common Equipment
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.
two furthest SPTs had an additional relay set fitted to detect if the line went
the far end a 33k ohm resistor was fitted across the line, this caused a small
current flow that was detected by a
circuit at the local end, if the line went faulty an
alarm was raised. Not very successful as the detector circuits
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
monitored, one of those jobs to be done on a rainy day.
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
happening so by the time the items were discovered they were
old technology it was not easy to replace it so a mixture of old and modern
equipment has now been used.
a the time a direct circuit via the two Norstar
exchanges was installed and tested should it be required to give the diversion
the circuit was in situ it has been made a direct line between the two signal
box Norstar phones.
Ropley and Medstead use vibrator ringers, Alton has an electronic
two occassions Norstar switches have been used to give
temporary SPT service, first at Alresford when the Strowger exchange
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
|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
The top set of relays are the exchange
| || |
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
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.
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.
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
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
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
no longer used. Two lamps indicate if the Alton equipment is in the local or
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
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
This method overcame the problem of SPT handsets being
left off but the LS coils often went faulty leaving the SPT out of service,
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
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
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
be seen a Norstar phone, in no way hidden, they must be made of stronger
stuff in Dorset.
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