|
|
|
|
|
|
The GPO 300-series was originally developed in cooperation with
Ericsson UK
and is based on the Ericsson N1015, N1016, N1017, N1374 and N1376
models [4], albeit with a British GPO No. 164 handset. This is the same
handset that came with the earlier Telephone No. 162 (the Pyramid).
There were models for use with Local Battery (LB), Central Battery (CB)
and Central Battery Signalling (CBS) systems. Furthermore they could be
configured with or without a dial, with or without a bell, and with up to
three push-buttons just above the dial, for a variety of special functions.
|
|
|
|
During WWII, a speech privacy system was introduced under the name
Frequency Changer.
It was based on the inversion of the audio frequency
spectrum, and is therefore also known as a
voice scrambler.
Telephone sets that were used with the scrambler were quite different
from regular ones, but were based on the chassis of the 300-series,
in particular the 328, 394 and 396.
|
In fact, when there were supply shortages during the war, regular 300-series
phones were often converted for use with the scrambler, either at the GPO
workshop or by technicians in the field.
Original scrambler phones had a 4-digit model number, prefixed with the letters
'SA'. In most cases, they were fitted with green handsets, to discriminate
their lines from regular unprotected ones. They were made of green bakelite,
as with the SA-5063/1
shown in the image on the right, but could also be regular black bakelite ones
that were painted green
as a gap-fill solution.
|
|
|
|
A few examples of phones in the 300-series:
|
332 Basic unit with dial, bell and handset (no buttons) 330 Version for private branch exchange (recall button) 1 312 Version for Shared Service installations (party line) 332 CB Version without dial (and sometimes without drawer) 394 LB 2 or 3-button version for CB systems 396 LB 2 or 3-button version for CBS systems
|
|
A few example of telephone sets that were derived from the 300-series:
|
-
The recall button connects earth (ground) to the telephone loop.
|
The drawing below shows the standard terminal block that is present on
all 300-series GPO phones. It is mounted at the bottom of the metal chassis
and is normally used for connection of the line and the handset. All components
of the telephone, such as the transformer, the bell, the capacitor, the dial
and the hook switch are connected to this terminal block.
Note that on Secraphone voice terminals,
additional wiring is present via the 9K terminal block.
Here is the layout:
|
| |
Connection block T as seen from the bottom of the phone
|
For a standard telephone set, the line is usually connected at 1 (A) and 9 (B),
and the handset is connected at 4 (M), 5 (MR) and 6 (R). There should be
shorting strips between 1 and 2, between 8 and 9 and between 10, 11 and 12,
as indicated by the red lines in the drawing above. If you want to convert
a telephone set for use on 'modern' analogue systems, check out
this website [6].
➤ How to convert a telephone for use on modern networks (off-site)
|
The 394/396 telephone, that is used as the voice terminal of the
Scrambler,
has two or three switches at the top of the bakelite case.
These buttons directly operate the
303A key unit
(also known as a 9K
switch pack) which consists of two 4-pole make-before-break switches (4K)
and one single-pole
make-before-break switch (1K) mounted in between the other two.
For other types of key units, check out
Robert Freshwater's excellent website
[3]. The layout of the 9K:
|
| |
Connection block 9K as seen from the rear of the phone
|
Each of the contacts of this terminal block, with the exception of
contact 28, is wired to one of the contacts of the 9 individual switches.
Contact 28 is not connected (nc).
When looking from the front of the telephone, the three buttons are identified
(from left to right) as A, B and C. Pressing a button, automatically releases
the other buttons. Depending on the configuration, the keys are latching or
momentarily. In our case, only the rightmost button (C) latches when it is
pressed. On some phones, the buttons are also released when the handset is
placed in the cradle (on-hook).
|
| |
The switches inside the 9K switch pack with their terminal numbers
|
Note that the individual switches of the 9K switch pack, are of the so-called
make-before-break type (MBB). This means that when pressing the button, there
is a short moment when all three contacts of the switch are connected together
(shorted). This is the opposite of the more common
break-before-make (BBM) switch.
Although standard switch symbols are used in the diagram above,
make-before-break (MBB) switches are sometimes explained with the
following symbols:
|
| |
Three possible symbols for MBB switches.
|
The individual contacts of each switch are usually named as in the diagram
above. The fixed contact, or mother contact, is known as common (c).
The contact that is connected to the common when the switch is in rest, is
known as the normally closed contact (nc). The contact that is connected to
the common when the button is depressed, is known as normally open (no).
The behaviour of the push-buttons can be 'programmed' with a control plate
that is fitted behind the spring-loaded
latching bracket at the front
of the 303 key assembly. It can be installed in four different orientations
with two positions each, giving a total of
eight combinations as shown here:
For clarity, the notched control plate is shown in red.
The presence of a notch allows the button to be latched, whilst (optionally)
releasing the other buttons. Note that the middle button (when present) can
never be latched. A good suggestion for the scrambler phone is (2).
Configurations:
|
- None of the buttons is locked
- A is locked and can be released by B or C, C is locked and can be released by A and B
- A is locked and can be released by B or C, B and C are not locked
- A and B are not locked, C is locked and can be released by A and B
- None of the buttons is locked (same as 1)
- A is locked and can be released by C only, C is locked and can be released by A only
- A is locked and can be released by C only, B and C are not locked
- C is locked and can be released by A only, A and B are not locked
|
|
Some telephone sets, in particular the SA5063/1 1 that was used as the voice
terminal of the Scrambler Phone
from 1944 onwards, were fitted with a
No. 330B key unit.
Built from the same parts,
it is very similar to the No. 303/A Key Unit,
but lacks the facilities for the middle button.
|
|
Below is a non-exhaustive list of manufacturer codes found on an inside
GPO/BT telephones. The list is not claimed to be complete or correct, but might
be of use when restoring old telephones.
For a more complete overview and additional information, please refer
to this website
[1].
|
|
CB
|
|
Central Battery
No dial fitted
|
|
F
|
|
Figures
Figures (numbers) only on the dial or dial surround
|
|
L
|
|
Letters
Letters and figures on the dial (or dial surround)
|
|
LB
|
|
Local battery
Dial fitted
|
|
R
|
|
Recall
Recall button fitted
|
|
A
|
|
Edison Swan Electric Ltd.
|
|
AA
|
|
Pinching & Walton
|
|
AAG
|
|
Mullards Ltd
London
|
|
AAR
|
|
Associated Automation
|
|
AEG
|
|
GEC Telecommunication
Glenross (Scotland, UK) [not to be confused with the German AEG]
|
|
AEI
|
|
Associated Electrical Industries
Later: Henley Cables.
Henley later took over Siemens Bros and Thorn EMI and was then taken
over itself by GEC Telephones.
|
|
AEK
|
|
GEC-AEI Telecommunications
Kirkaldy (Scotland, UK)
|
|
AK
|
|
Peel Conner
This was the telephone manufacturing branch of GEC UK from 1910 to 1921.
Later markings include GEC and AKE.
|
|
ATL
|
|
Austin Taylor Ltd
Bangor (Whales, UK)
|
|
C
|
|
GEC
Coventry (UK). Currently known as GPT.
|
|
CWL
|
|
Pye Telecommunications
Cambridge (UK)
|
|
DAE
|
|
A.P. Besson Ltd
Hove (Sussex, UK)
|
|
DFM
|
|
Denis Ferranti Meters Ltd
Bangor (Gwynedd, UK)
|
|
E
|
|
British Ericsson
Later: Plessey, later: GPT
|
|
ET
|
|
British Ericsson
Later: Plessey, later: GPT
|
|
EET
|
|
Plessey
Beeston (Nothinghamshire, UK), formerly Ericsson
|
|
EEX
|
|
Plessey
Sunderland (UK), formerly Ericsson
|
|
EMI
|
|
EMI (Valve Division)
Ruislip (Middlesex, UK).
|
|
EMT
|
|
EMI Telephone Division — unconfirmed
Manufacturer of Privacy Set No. 8.
|
|
FB
|
|
Post Office factory
Birmingham (UK)
|
|
FBR
|
|
Post Office factory
Birmingham (UK) Refurbished items
|
|
FED
|
|
LM Ericsson
Sweden
|
|
FH
|
|
GPO Factory
Holloway, London (UK)
|
|
FHA
|
|
GPO Factory
Halloway, London (Assembly)
|
|
FHB
|
|
GPO Factory
Holloway, London (Construction)
|
|
FHR
|
|
GPO Factory
Holloway, London (UK) Refurbished items
|
|
FNR
|
|
GPO Factory
Edinburg (Scotland, UK) Refurbished items
|
|
FRA
|
|
Ferranti Ltd.
|
|
FW
|
|
GPO Factory
Cwmcam (Wales, UK)
|
|
FWG
|
|
STC Consumer Electronics
Cwmcam (Wales, UK)
|
|
FWR
|
|
GPO Factory
Cwmcam (Wales, UK) Refurbished items
|
|
G
|
|
General Electric Company
|
|
GEC
|
|
GEC Telephones, Coventry (UK)
Currently known as GPT
|
|
GEN
|
|
GEC-AEI Telecommunications
Newton Aycliffe, Co. Durham
|
|
GNA
|
|
PYE
Aidre (UK). Later renamed TMC.
|
|
GCL
|
|
EMI
Manufacturer of Privacy Set No. 8.
|
|
GTE
|
|
Dennis Ferranti
Ferranti imported or licensed GTE phones, as part of a joint venture
with GTE USA and GTE Belgium.
|
|
H
|
|
Automatic Tlephone and Electric (ATE)
Later marked as PL (Plessey)
|
|
HAA
|
|
Plessey Strowger Works
Liverpool (UK)
|
|
I
|
|
Ibex Telephones
Harrow (UK)
|
|
IBX
|
|
Ibex Telephones
Harrow (UK)
|
|
KRM
|
|
Krone
Cheltenham (UK)
|
|
PEH
|
|
Pye
Hastings (UK)
|
|
PER
|
|
Plessey
Romford, Essex (UK)
|
|
PL
|
|
Plessey
Later became GEC-Plessey Telephones (GPT).
Many of the war-time 300-series telephones were manufactuered
by Plessey and will therefore have the PL marking.
|
|
PLA
|
|
Plessey
|
|
PLS
|
|
Plessey
Swindon (UK)
|
|
PLX
|
|
Plessey
NewPort, Gwent (UK)
|
|
PX
|
|
Phoenix
Hendon, London (UK). Later PXA.
|
|
PXA
|
|
Phoenix
Hendon, London (UK)
|
|
RAA
|
|
Landys & Gyr
Formerly eronautical & General
|
|
S
|
|
Siemens Brothers
Later AEI, later: GEC, now: GPT.
|
|
SB
|
|
S.G. Brown Ltd.
|
|
SEA
|
|
Siemens-Edison Swan
London N17 (UK). Later part of AEI.
|
|
SEG
|
|
AEI Ltd
Woolwich, London SE18 (UK).
Formerly Siemens Brothers (S).
|
|
SER
|
|
AEI Ltd
West Hartlepool, Co. Durham (UK).
Formerly Siemens Brothers (S).
|
|
SEW
|
|
Siemens Edison Swan
Woolwich, London SE15 (see also SEA).
|
|
SLW
|
|
Plessey
Wigan (UK)
|
|
SPK
|
|
Plessey
Speeke, Liverpool (UK)
|
|
STC
|
|
Standard Telephone Company
New Southgate, London (UK)
|
|
STM
|
|
Standard Telephone Company
Belfast (Northern Ireland)
|
|
TCH
|
|
Standard Telephone Company
Larne (Northern Ireland)
|
|
TE
|
|
Telephone Manufacturing Company (TMC)
Formed 1914. Later manufacturing code 'TEA'.
➤ More
|
|
TEA
|
|
Telephone Manufacturing Company (TMC)
West Dulwich (London SE21, UK). Formed 1914.
➤ More
|
|
TES
|
|
Telephone Manufacturing Company (TMC)
St. Mary Cray (Kent, UK). Opened 1939.
➤ More
|
|
TEW
|
|
Telephone Manufacturing Company (TMC)
Winford (Cheshire, UK).
➤ More
|
|
TGR
|
|
Thorn-Ericsson
Rochester (UK)
|
|
TGW
|
|
Thorn-Ericsson
Scunthorpe (UK)
|
|
TMA
|
|
Telephone Manufacturing Company (TMC)
Airdrie (Lanarkshire, UK).
➤ More
|
|
TMK
|
|
Telephone Manufacturing Company (TMC)
King's Lynn (Norfolk, UK).
➤ More
|
|
TMM
|
|
Telephone Manufacturing Company (TMC)
Malmesbury (Wiltshire, UK).
➤ More
|
|
TPC
|
|
Telephone Manufacturing Company (TMC)
Transmission Division. St. Mary Cray (Kent, UK).
➤ More
|
|
TSD
|
|
Telephone Supplies Ltd.
Aycliff, Co. Durham (UK)
|
|
TTH
|
|
Telephone Manufacturing Company (TMC)
Canterbury (Kent, UK).
➤ More
|
|
TTL
|
|
Telephone Manufacturing Company (TMC)
Livingston, Scotland (UK).
➤ More
|
|
W
|
|
Western Electric
Later: STC.
|
During WWII, most 300-series telephones were manufactured by Plessy and by
TMC (also Pye-TMC), especially the ones that were used for the war effort.
This means that the most likely codes to be found on these devices are
PL, TE, TEA, TES, TMA and GNA.
|
|
Expressions and abbreviations used on this page:
|
|
BT
|
|
British Telecom
Arguably the largest telecom operator of the UK. Previously state-owned
and known as the General Post Office (GPO).
|
|
CB
|
|
Central Battery system
System in which all the energy needed for transmission and signalling
is delivered by the exchange. No local batteries or hand generators are
used at the telephone end.
➤ More [3]
|
|
CBS
|
|
Central Battery Signalling system
Similar to a CB system, except that the mircophone is powered locally
by a battery at the telephone end. Power for signalling is provided by the
exchange as in a CB system. In the UK there were three types of CBS.
➤ More [3]
|
|
GPO
|
|
General Post Office
The state-owned post and telecommunications operator in the UK, before it
was renamed BT and privitised. The GPO was als known as British Post
Office (BPO) and simply as Post Office (PO).
It is currently known as British Telecom (BT).
|
|
LB
|
|
Local battery system
System in which a local battery is used for providing the current for
the speech circuits.
|
|
Magneto
|
|
Hand-cranked electrical generator that provides electricity for signalling
in an (old) telephone system. In some countries known as inductor,
crank ringer, or wake-up unit.
|
|
PBAX
|
|
Private Automatic Branche Exchange  PBX
|
|
PBX
|
|
Private Branche Exchange
Local telephone exchange or switching system, using inside the building
of a private organisation, usually connected to the Public Switched
Telephone Network (PSTN) via Central Office (CO) lines.
|
|
PL
|
|
Plessey
Manufacturing code used on the body of the telephone sets and also
inside, often stamped on the chassis. The manufacturer's code is also
cast inside the bakelite body.
|
|
TE
|
|
TMC (see below)
Manufacturing code used on the body of the telephone sets and also
inside, often stamped on the chassis. The manufacturer's code is also
cast inside the bakelite body.
➤ More
|
|
TMC
|
|
Telephone Manufacturing Company
The original manufacturer of the Secraphone in St. mary Cray (Kent, UK) [6].
➤ More
|
-
Documentation obtained from Robert Freshwater [2].
|
|
|
|
Any links shown in red are currently unavailable.
If you like the information on this website, why not make a donation?
© Crypto Museum. Created: Tuesday 28 January 2014. Last changed: Monday, 15 January 2024 - 16:23 CET.
|
 |
|
|
|
