The Secret Tunnels of South Heighton

The GPO VF Equipment Room. Before & After 2000.

These pictures show the GPO VF Equipment room as found in 1993, and after clean-up by the Friends of HMS Forward.

When I visited the tunnel in 1993 I could find no logical reason for these pits, yet clearly there had to be. It was not until I visited the tunnel a year later with two former GPO linesmen of the day, that they provided a plausible explanation. This had been the GPO VF (Voice Frequency) Modem Equipment room where, in short, 'teleprinter speak' was converted to 'telephone speak', and vice-versa. In modern parlance, think of a computer connected via a modem to a telephone line; but back in the 1940s before transistors were invented, contemporary 'modulator/demodulators' used valves and sensitive relays, were the size of a family suitcase, and each consumed tens of watts of power. The smallest telephone equipment rack on which four of these modems could be mounted was 8 ft 6.5 ins (2M 60) tall, and would not fit into a 8 ft (2M 43) tunnel. The arched roof could not be raised, so the floor had to be lowered; hence the pits. Discovering how many, where and how, meant draining 700 gallons of accumulated condensation, removing hundredweights of hardcore, and searching for evidence.

The upshot was that the near pit accommodated nine racks of modems, and the far pit was home to a Main Distribution Frame (MDF) and some miscellaneous racks (MAR). The MDF provided a flexibility point where external cables, and internal cables together with every item of telephone/teleprinter equipment was terminated. It was then a simple matter of connecting whatever to whatever with a short length of expendable 'jumper wire' according to the needs of the moment.

One most noticeable point is the reduction in humidity due to the reduction of decaying rubbish strewn all over the floors which, as can be seen, are dust-dry now because of thermo-assisted natural air flow from the western entrance.


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All illustrations and text on this site are 1941-2017 Geoffrey Ellis, or The Friends of HMS Forward, or Nick Catford.