Exchange - Medstead

Medstead

The opening of the line to Medstead saw a small GEC PAX being placed in service in 1983. In very cramped conditions there were 2 cabinets, one for final selectors, line finders, junction relay sets and common equipment, the other cabinet contained group selectors.

Numbers were in the range 51XX but not part of the linked numbering scheme which was added later.

Space was so limited that the equipment required for the linked number working was installed at Ropley, an overhead pole route connected this station to Ropley.

Linked number working arrangements comprised as follows -

Calls to an Alresford extension 22XX or Ropley extension 43XX were routed to one of two common relay sets connected via junctions to Ropley exchange.
The relay set could detect if it had be seized via level 2 or 4, subsequent digits were sent to the distant end.
Call barring was achieved by the examining the second digit dialled, if the call was via level 2 only a second digit of 2 was permitted and via level 4 only a second digit of 3, all other combinations would result in the caller getting unobtainable tone and then force released. Later modified to allow 49 numbers to be dialled for the teleprinter service.

At Ropley the junction terminated on an incoming relay set and the digits were stored on an electronic regenerator; the first incoming digit received was checked. If 3 it was a call for Ropley and the associated incoming selector would be connected and the three stored digits sent to the local selectors.
If the digit was 2 a junction hunter would seize a circuit to Alresford and the three stored digits pulsed out, the incoming selectors were on the second selector shelf there.

Calls to Alton went on direct junctions from level 3, again the incoming selectors were on the second selector shelf.

Part of the changeover from the PAX to the new exchange involved removing the incoming relay set at Ropley, a temporary arrangement took place where a code was needed for Alresford and Ropley calls while this work was undertaken.

from Turntable June 1st 1985

from Turntable January 14th 1984

Three photos taken by Eric Langley

Medstead PAX showing the rear of one cabinet and the external connection point, MDF is too grand a word for this.

Medstead new exchange - showing Rectifier 110, MDF and line tester, a mixture of soldered and IDC blocks, plus a pair of GPO steps.

Standby batteries for the new exchange at Medstead - a modified signalling cabinet.

One major problem with the Strowger system is that a call will connect to the first free piece of equipment, if for any reason this cannot handle the call successfully, for example a selector cannot mechanically step, the call will fail, the caller will try again and if the exchange is not busy there is a good chance they will get the same equipment again with similar results.

A modification that had been made at Alresford and Ropley had removed this problem. After a call a selector or junction relay set would busy itself out of service, when the last item in the group was used the others would be restored to service, safeguards were built into the system to prevent exchange isolation if for some reason only one piece of equipment remained free and it could not be taken into use.

Due the lack of space it was not possible to have these modifications at Medstead and often the exchange became isolated due to a faulty selector.

One alteration made in 1986 was to replace the common equipment relay sets with own design ones to allow for a PABX3 ringer, fitted at the rear of the group selector cabinet, to keep tones standard across the system.

Only known colour picture of the PAX at Medstead, shown is the rear of the final selector/line finder rack, the group selector rack is behind - a rather cramped exchange. Now used as a store cupboard by the station staff.

Medstead developments

The PAX remained in service until 1995 when it was replaced by a new exchange in the new S and T building, this was based on recovered buildings from a BT repeater station at Steeple Landford in Wiltshire. The old PAX equipment went to the Bluebell Railway for further use.

Pictures shows exchange during installation.

The original plan was an exchange based on 2 UAX13 racks, a shortage of bank multiples meant the incoming selector function and second selector switching stage were to be replaced by relay sets.

The picture below shows one of the Digit Absorbing relay sets that was going to be used in place of 2 second selectors, modified to absorb the digit 1 instead of a digit 0. There is an additional CN relay which has been added, this was part of a busying circuit.

This relay set was found in a store in November 2010. There are two circuits in the relay set.

A relay set from a Motor Uni Group Selector was modified, shown below, to take the functions of the incoming selector. Type 4 uni-selector added.

A delay in the completion of the new building prevented the installation of the exchange, by the time the building was ready additional bank multiples were available so a new design was planned.

The new exchange was another hybrid, formed from 3 UAX13 racks, bank multiples from a UAX13, selectors from a PABX3 and UAX14 plus home build relay sets, 25 extensions were provided using line relays from a PABX7 and the 25 point linefinders from UAX CCB equipment.

Extension numbers remained 51XX.

The PABX7 line circuits were mounted in relay set cans, 10 circuits per relay set.

Four relay sets were provided giving a total of 40 circuits, 25 were used for normal extensions and the remaining five circuits in the third relay set were used by an Out Of Area exchange line from Ropley. At the time there was no external line at Medstead, several phones at key locations were connected to an OOA line finder relay set.

As there were only 25 line finder outlets available the fourth relay set was provided for lines with very low outgoing usage, 10 circuits were combined via a uniselector circuit to share one line finder outlet, these extensions were in the numbering range 5141 - 40.

Several modifications had to be made to interface the PABX7 line circuits to the 1st selectors.

One mechanical ringer was provided and later a prototype electonic tone and ringer set was provided, although it worked very well only one was ever made.

The power unit originally installed proved very temperamental with little control over the output voltage, it was later replaced with a PP2040.

Two bothway junctions connected this exchange to Ropley (0 and 1 relay sets) for levels 2 and 4, junction hunters were not used and to Alton via level 3, again two bothway junctions.

This was the ideal size exchange, nice and compact, the other exchanges took up more space but one had to use what equipment was available at the time.

The equipment has been saved for further use on the the railway.

PABX3 final selector.

Left hand rack in picture

Top relay can contained misc relays

Shelf E - final selectors - ex PABX 3

Shelf D - 1st selectors - ex PABX 3

Shelf C - 25 point line finders - ex UAX 13 CCB

Shelf B - PABX 7 line circuit relay sets 10 circuits per relay set

Shelf A - misc relay sets

Right hand relay set

Top shelf - TRT284 call sender

Shelf E - call forwarding unit, electronic ringer and electronic dial tone cards

Shelf D - EPG standby clock

Shelf C - second selectors - ex PABX 3

Shelf B - incoming selectors - ex UAX 13

Shelf A - junction relay sets to Ropley via levels 2 and 4 - ex UAX

Electronic tone generator can be seen in the shelf top right, it is the second unit from the left. Electronic ringer is on the far wall.

Mechanical ringer is at the bottom of the far rack.

Exchange maintenance..

Another view of the original power unit, distribution fuses and MDF which was later replaced with a Box Connection and Krone strips, see below.

The Line tester shown was a type used in TXE4 exchanges and modified for local use.

Originally the four Strowger exchanges had MDFs with solder type blocks, later some blocks with insulation displacement terminations (IDC) were used, the two left hand blocks seen in the Medstead before picture below.
Later the older type MDFs were replaced by box connections with Krone strips, these boxes needed to be located in the position the old frame was plus full service had to be maintained while the work was undertaken.

The pictures below show the replacement work at Medstead in 2003.

Above - before work started, also shown is an Ultimate 106 exchange and a Norstar 7310 phone.

Below - during the change over - temporary jumpers run to keep service working.

Old blocks moved to one side and Box Connection 520 in place, clock unit at top of picture..

Above - after change over completed.

Top management visit Medstead exchange, another wonky picture.

Relay set used to connected recorded announcement during change over of exchanges.

Exchange recovery - the ever helpful Sunday PW gang assisting.

TRS huts at Steeple Langford Wiltshire.

Site after wooden huts removed showing modern TRS building.

Recently found is this picture by Eric Langley showing the construction of the S and T building at Medstead, date unknown.

Mid Hants telephones