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Build Data: Serial numbers, exchange types and initial allocation of units. |
From 1938 mobile exchanges (MAX 12s) were used to aid the conversions from manual to automatic switching, and also to quickly reinstate service in emergency situations, e.g. a flooded or fire damaged exchange. For growth after the war years, additional mobiles used converted trailers.
In 1948 a new type of body was designed and built at the Yeading and Kidbrooke Central MT workshops to house a MAX 13. The body was mounted on an ex-WD trailer. Twelve box bodies on ex-RAF trailers were also modified at Yeading for use as MAX 12s. By 1949 there were 18 MAX 12s and 3 MAX 13s available for use.
Other mobiles were fitted-out for specific needs, e.g. teleprinter switching. Later mobiles (MNDXs) provided early relief of exchanges as the demand for service increased dramatically during the Subscriber Trunk Dialling era.
Mobiles were always an expedient to cater for any given emergency.
MAX 12s
By 1955 there were 29 MAX 12s available for use. MAX 12s were equipped for 100 lines. Unit dimensions: Height 10 ft. 6in.; Length 17 ft. 6 in.; Width: 7 ft. 6 in.; Approximate weight 6 tons.
MAX 12s
| Region | Serial Nos. |
| Home Counties | 3, 8, 15, 18, 26 |
| Midland | 11, 19, 24, 28 |
| North Eastern | 5, 10, 20, 23, 29 |
| North Western | 4, 22, 25 |
| Scotland | 2, 9, 13, 17 |
| South Western | 1, 6, 12, 16 |
| Wales and Border Counties | 7, 14, 21, 27 |
A number of unequipped vans were adpated to allow the fitting 'Units Auto' equipment as temporary MAXs. These were numbered in the range 101 to 125.
By 1966 there were only 24 MAX 12s remaning.
MAX 13s
By 1955 there were 11 MAX 13s available for use. MAX 13s consisted of two trailers, 'Section A' and 'Section B.' MAX 13s were equipped for 200 lines. Unit dimensions: Height 12 ft. 8 in.; Length 21 ft. 8 in.; Width 7ft. 6 in.; Approximate weight 9 tons.
MAX 13s
| Region | Serial Nos. |
| Home Counties | 1001, 1002 |
| Midland | 1007, 1010 |
| North Eastern | 1008, 1011 |
| North Western | 1005 |
| Scotland | 1003, 1006 |
| South Western | 1009 |
| Wales and Border Counties | 1004 |
By 1966 there were still only 11 MAX 13s in use, but the new MNDX mobiles were carrying the bulk of relief connections.
MNDX Construction
In 1962, 24 Mobile Non-Director Exchanges (MNDXs) and 6 Mobile Tandem Exchanges (MTXs) were on order. Between 1962 and 1971 a reserve of 200 MNDXs was built up.
By 1966 there were 117 MNDXs and 23 MTXs. Line plant waiters stood at 16,142.
In October 1971 worked commenced on building another 200 MNDXs, in the range 200 to 399. Each MNDX was cabled for up to 400 subscribers. A further 40 MTXs were also ordered.
| Telephone Region | Regional Build | Enfield Build | Birmingham Build |
| ETR | 200 - 213 | 214 - 215 | |
| MTR | 221 - 240 | 216 - 220 | |
| NETR | 241 - 255 | 256 - 259 | 250 - 264 |
| NI | 265 | 266 | |
| NWTR | 267 - 276 | 277 - 279 | |
| SCOT | 280 - 308 | 319 - 323 | 309 - 318 |
| SETR | 324 - 344 | 345 - 346 | |
| SWTR | 347 - 366 | 367 - 376 | |
| W & M | 377 - 386 | 387 - 393 | |
| THQ | 394 - 399 | ||
References
PO Telecoms Work Spec TE 8684 (June 1971)
PO Telecoms Work Spec TE 8684 Addendum 1 (October 1971)
EI Teles Auto G 3508 08/07/1955
EI Teles Auto G 3516 05/09/1955
EI Teles Auto G 3519 11/07/1956
EI Teles Auto A 3216 03/10/1968
Engineering Dept. Annual Reports 1936 to 1966
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