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.

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 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.

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.

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 was to fit a PABX3 ringer 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 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. Numbers remained 51XX.

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, later replaced with a PP2040.

Junctions connected this exchange to Ropley (0 and 1 relay sets) for levels 2 and 4 and Alton via level 3.

As there were only 25 extension line circuits a 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.

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

Shelf D - 1st selectors

Shelf C - 25 point line finders

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

Shelf B - incoming selectors

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

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. 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 of 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.

Before - also shown is an Ultimate 106 exchange.

During - temporary jumpers run to keep service working.

Old blocks moved to one side and box connection in place, clock unit at top of picture.

After - new Box Connection.

Exchange recovery - the ever helpful Sunday PW gang assisting

TRS huts at Steeple Langford Wiltshire

Site after wooden huts removed showing modern building

Mid Hants telephones