The Lincolnshire Heritage Exchange

The PABX 7

Early May 2016 I was contacted because a PABX 7 had become available for purchase.  Totally surprised by this as I had given up hope of ever finding one let alone complete one.  The PABX 7 has been on my wish list for a very long time and I can tell you that I had become very excited by this news and there was no hesitation with regard to purchase of this PABX.

One of the key things that had attracted me to the PABX 7 was the switchboard, trim and neat, this is an eye catcher compared to the standard Post Office switchboards of the same time period.  At the age of seventeen I would often be found watching the operator at AEI Semiconductors Lincoln, who used this switchboard,

May 16th 2016:-  Monday morning and a drive over to see the PABX 7 in the West Midlands, I was not disappointed, though the outside of the units had faded with time and had a few scratches, the inside was pristine.  Three and a half hours later I completed the financial transaction, loaded the twelve connecting circuits (two motion selectors) into the car for safe transport, choose the switchboard from the three available and then drove home.  As soon as I got in I cleaned the switchboard inside and out, removing a mass of dust from the extension dial keys, which after cleaning with a damp cloth all worked fine.

You will note from the picture below that I choose a  switchboard with the wooden ends and black extension keys, although the 8 and 9 have been replaced with ivory keys.



The switching equipment consists of 4000 type two-motion selectors and Type 2 Uniselectors. The trunking arrangements, which are shown on Dgm SA 8540, are essentially similar to those of the PABX 1. The equipment is accommodated in two cabinets, known as Equipment's, PABX 7/1 .... and 7/2.... having capacities of 9 + 50 and 11 + 50 respectively these cannot be extended. The Equipment, PABX 7/1 .... which includes the ringing and tone supplies can itself form a complete PABX but generally the PABX 7 will be supplied only to those subscribers requiring both units. Both equipment cabinets have doors back and front for engineering access. Power is supplied by secondary cells float charged by a rectifier from the mains supply, this is of the type described in Q0070. The layout of the equipment is shown on Drgs CD 2289 and CD 2290. Relay- sets of types not shown on the drawings are fitted in any of the positions allocated to miscellaneous circuits shelf jacks and the necessary wiring is provided on site when required. A single desk-mounted cordless switchboard, known as Switchboard SA 8551 (see picture) is provided with a handset as standard, but a lightweight headset (trimphone style) can be supplied as an alternative. The normal capacity of the PABX is reduced by one exchange line and one automatic extension line for each auto-auto inter-PBX line or manual extension provided, and by one exchange line for each manual-manual inter-PBX line provided.



May 23rd 2016:-  Cable for the PABX 7 arrived today, so from the exchange PCP (Primary Cross-connection Point) I pulled a 15m length of 160pr cable through the roof space down into the PABX room.  I then set about terminating the first lot of 160 pairs in the PCP.  Going on to pull a 6mm cable from the Rectifier / Battery feed to the PABX room and terminating the supply in to a fuse box.  The 6mm cable will happily supply 10 amps, into the PABX room.  For interest sack I will mention that I use old Wylex boards with the 5 amp push to reset breakers, they work perfectly with my low voltage supply and are a lot easier than using wired or cartridge fuses, and more responsive.

May 26th 2016:-  Preparation of the site for the PABX 7 has started today, cleaned out a massive amount of junk and tidied up the back end of the PABX room.  Allocated an eight foot square floor area for the two cabinets, which leaves enough room for the doors to be opened back and front. Also decided on the route for the switchboard cable, having already allowed enough pairs to take it to the PCP in the 160 pair from the PABX 7, another 40 pair cable is need from the PCP to the Switchboard.

May 31st 2016:-  Started to pull through a one hundred pair cable from the Primary Cross-connection Point to the office where the PABX 7 switchboard is sited.  The office end has been terminated on to two female RJ21’s for the switchboard and five ten pair Krone blocks for any other use.  I have yet to terminate the other end in the PCP.

June 4th 2016:-  Terminated the 100 pair cable feeding the switchboard in the PCP, also managed to jumper the 75 wires needed for to switchboard to link the office cable with the PABX 7 cable.  So the only wiring left at this point is the jumpers for the exchange lines, jumpers for extensions and the termination of the 160 pair cable in the PABX 7 its self.

June 5th 2016:-  Jumpered some exchange lines through to the PABX 7 and a couple of extensions from the PABX 7 to the office.  Stripped some sheathing off the 160 pair at the PABX end, tied off the groups ready for connecting.

IMG_1241June 14th 2016:-  My mate Shane and I set off for Birmingham, there was heavy rain forecast and we knew we where in for a soaking at some point in the day.  Arriving around noon, we loaded the two heavy cabinets in to the van and then loaded the relay sets at the rear, there must have been around fifty.


Above Left:  The van loaded.   Above Right:  Shane and Roddy moving Cabinet two.


We just got the van loaded and it started to spot  with rain we had been lucky, as within a few minutes it turned from light rain heavy and remained that why until we got home.  Once back in Lincolnshire the rain having stopped we unloaded our cargo and moved it all to the PABX room at the back of the garage block.  It went a lot easier than we thought, mainly due to the PABX 7 still retaining its casters, we were able to run the cabinets across the gravel on boards.

We loaded the common equipment, the exchange lines and the extension relay sets, leaving the others out for the time being.  I wired up the power and we have life.  Now we need to wire up the extensions, switchboard and exchange lines all contained in the 160 pair cable we previously laid to the site.


July 29th 2017:  Time for an update, In the months since my last post, not a lot has happened because work commitments have held me back, that said there has been movement forward.  The extensions and the switchboard were wired up shortly after my last post and since then I have wired up the manual extension relay set in the first cabinet and two inter-PBX circuits in place of exchange line 8 and 9.  The manual extension appears on exchange line key 9 with the extension end terminating on exchange line 9 of the office PMBX.  The inter-PBX lines currently are terminated on Meridian exchange lines and so calls at this time are only incoming, the aim is to install relay sets on another strowger system sometime in the future.  I recorded a video of traffic simulation on the PABX 7, which has proved quite popular.


Six minutes in the life of a PABX 7



















End to Date.

Header Image-003PABX 7






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