The Series 400 Telephone.

400AT with 20/4 terminal block (APO version).

The series 400 telephone was released in Australia in 1957, as a replacement for the series 300 which had been standard issue since the late 1930's. It was originally designed in Britain as a series 700 (not to be confused with the later all plastic 706).
For an extensive background to the design and development of the 400, I recommend a very well written article on another website:

The following article expands on this from an Australian perspective, and describes how to use your series 400 in the modern day. It was actually the purchase of a central battery series 400 telephone that really started my telephone collecting back in 1982. At the time, bakelite 300 and 400 series phones were being sold by Tempe Disposals in Sydney. I bought several central battery types for $2 each, and a few automatic series 300's for $3 each. Those prices would be unheard of today!

Versions of the series 400.
Two basic types existed; the BPO (British Post Office), and the locally designed APO (Australian Post Office). The phones were available as table or wall versions. All were available for automatic, central battery, or magneto exchanges. The magneto version will be described in another article, since it is quite different to the automatic and central battery types. Colours were black or ivory. Connection to the exchange line was either by a 20/4 terminal block, a four pin bakelite plug, or the later six pin 603 plug. Early models used cloth covered cords, which were later replaced by PVC. This included a coiled handset cord.

A Automatic
CB Central Battery
M Magneto
T Table
W Wall
P Portable
K Key
D Duplex

For example, a 400 CBW is a series 400 central battery wall mounted phone. A 400 AT is an automatic table/desk phone.
Australian made phones have STC or AWA stamped on the base to indicate manufacturer. If this is not visible, the capacitor and/or bell motor will be stamped.
The year of manufacture is two digit; e.g. 60 is 1960. For example, "AWA 60 P.M.G. S.1/402" means the phone was made by AWA in 1960, is the property of the PMG, and is a black table model for central battery exchanges.
Ericsson made phones have an "E" prefix for the year; e.g. E59. Components inside the phone; e.g. the transmitter and receiver also have a two digit year code.

The following table shows the serial numbers for the different phones. Some of this information was obtained from the 1965 Technician's Handbook. It appears that ivory models in automatic and CB were no longer being issued, since they are not listed. The ivory AT in the list is from my own collection. One could guess the ivory CB is S1/405. Default colour is black unless indicated otherwise.
Serial Description
1/401 400 AT (APO)
1/402 400 CBT (APO)
1/403 400 MT 
1/404 400 AT (Ivory BPO Ericsson)
1/405* 400 CBT (Ivory BPO) *Not confirmed.
1/408 400 MT (Ivory BPO Ericsson)
1/409 400 MP (Ivory BPO Ericsson) 
1/410 400 AD
1/411 400 AT (Black BPO Ericsson)
1/411A 400 AT (Black BPO)
1/413 400 AW
1/414 400 CBW
1/415 400 MW
1/416 400 MK
1/417 400 CBK
1/418 400 AK

Telephones were reconditioned and reissued, so do not be surprised if the date codes on the parts are not all the same. Sometimes the circuit diagram inside the case is for a different phone, or a new circuit diagram has been pasted over an original - this is an immediate indication of a non original case. Later issued and reconditioned phones had a 603 plug fitted. In this case, the two digit year code stamped into the plug grommet (and later, on the plug itself) will also give further clues as to the phone's history.

A Look at Some Different Types.

400 CBT Central Battery Table (APO). Fitted with a 603 plug.

400 CBW Central Battery Wall (APO). Exchange line connects directly to internal terminal block. Note the cloth covered handset cord.

400 AW Automatic Wall (APO).

400 APK. Automatic Portable Key (APO). Note the handle which designates the "P". The key switch has been removed, but its former location is obvious.

Portable phones were fitted with a handle made from chrome plated brass rod. Since they were intended to be moved to different locations in the premises, they were fitted with a plug and socket connection. Where phones were to remain in a fixed location, they were connected with a 20/4 terminal block. Originally, a four pin bakelite plug and socket were used for portable phones. However, once the 603 and 610 plug and socket became available in 1963, as part of the development of the series 800 Colorfone, this became the preferred method of connection for all installations.
A key switch was available to prevent outgoing calls. A typical example of where this might be used would be in a holiday rental house. Back in the day, I would take my own telephone with alligator clips attached, to connect directly to the exchange line, should it be necessary to make a call. Alternatively, it might be used at a place of work, where only certain staff were permitted to make calls.

Ivory 400 AT (made by Ericsson).  This is a BPO version - evident from the outside by the drawer dummy at the front.

The standard issue was black, but ivory was also available. Along with portable and or/parallel services, and key switches, choosing an ivory phone would increase the rental charges. For this reason, most surviving phones are black.

APO and BPO versions.
The initial design of the 400 series phone was done by the British Post Office. Since it used the case from the series 300, it too had a chassis mounted inside the casing. Above this was the dial and gravity switch. Below was a baseplate, with a small drawer for keeping a list of phone numbers. This drawer was never issued with the phones in Australia, since the PMG thought they were too fragile. As a result, locally issued BPO models had a bakelite dummy to fill the drawer opening. Interestingly, it appears the series 400 never took off in the UK, since its development was just before the plastic 706 type.
Unique to the APO design is the wall mounted version. The casing for this was designed in Australia, and it uses the same baseplate chassis as the table models.

Chassis is mounted to the inside of the case, in the BPO version. Gravity switch and dial are behind the chassis.

Note the bakelite drawer dummy.

Since Australian phones were never to have the drawer, the design was altered in that a thicker baseplate was used, which became the chassis. The same construction had already been used in the APO series 300 phones. All components, except for the dial, were screwed directly to the baseplate. It is actually a neater design, and makes for improved serviceability, since the dial and gravity switch are immediately accessible.

Inside the APO version.

Design of the Series 400.
The series 400 was designed to a replacement for the existing series 300, in that transmission and receiving efficiency were improved, along with long line performance. Despite the case being a legacy from the series 300, electrically the phones are quite different with regards to the speech circuit. The most obvious external difference between the series 300 and 400 is a new handset design. It is of hollow bakelite, with a curved contour, so as to place the transmitter closer to the user's mouth, without requiring a horn. A four wire connection to the handset is now used, instead of the three wire handset of the series 300 phones. A rubber grommet was installed where the line and handset cords entered the case.
While the No.13 transmitter was retained, it was now connected directly to the wiring, instead of by spring contacts, and a more sensitive rocking armature receiver (type 4T) was introduced. The latter was used until the end of series 800 production in the 1980's. The Anti Side Tone Induction Coil circuit is where most of the changes are. The notes below, from the PMG training publication, "Telephony II" go into further detail:

4T receiver, No.13 transmitter, and 20/4 terminal block. This particular phone has had the later receiver overload protector added (the black square object on the back of the receiver). It limits the receiver voltage using two inverse parallel silicon diodes.


Circuits for both BPO and APO versions. These show the use of 603 and 610 plugs and sockets.

Electrically, the BPO and APO versions are identical. Because of the physically different terminal blocks used, the terminal designations do, however, differ between the two.

Wiring the Series 400 Phone.

Where the loop resistance of the exchange line is low, the transmitter may be damaged by excess current. In this situation, a 330R resistor is connected in series with the line.

Basic connection of one phone to the exchange line.

Portable installation using four pin plugs and sockets. The astute reader will notice the mistake with the telephone to plug wiring - the red and white wire labelling should be swapped.

Parallel connection of two or more phones.

Using Your Series 400 Phone in the Modern Day.
If you have a central battery version, it can be connected to the telephone output of the modem or ATA (Analog Telephone Adaptor), and be used to take incoming calls. If you have an automatic version, you'll need a Dialgizmo to generate the DTMF tones from the loop disconnect pulses.

Internal Connections & Strain Cords.

Strain cords must be secured. Note alternative attachment point - the hole just above the capacitor.

The line and handset cords have attached to them short lengths of shoe lace. These are for securing the cords to prevent any physical strain on the wires. The two strain cords can be tied together and wrapped around the top of the gravity switch bracket as shown above, or individually tied through the holes on the sides of the bracket. If the strain cords are ignored, physical strain and movement will eventually cause a failed connection at the terminal lugs. Internally, the wires are cotton and tinsel, and are relatively fragile.

Handset and line cord connections for APO versions. Telephones originally fitted with a 603 plug have a different line cord colour code. This phone is a CB version. Automatic phones have the dial connected to the furthest row of terminals.

For BPO versions, line and handset connections are as shown for single use working. This phone has a four conductor cloth covered line cord. The blue wire is not used and is connected to terminal 7 (blank).

The handset cord conductors will always be connected as shown. There may be some difference in line cord connections, depending on the wiring plan, but for a single telephone using the older wiring colours as used with a terminal block, they are connected as shown above. The circuit diagram shows a link between terminals 1 and 2, but I have yet to see this in any telephone I have examined. Instead, the link is made at the terminal block.
For the BPO version, the strain cords can be secured under the chassis mounting screws, as shown above, although official procedure is to tie them to the two pillars at the back of the terminal block, adjacent to the cord entry.

Connection via Terminal Block.

Single telephone connected to exchange line. Terminals are 1 to 4 from left to right.

For telephones connected via a 20/4 terminal block, the connections for a single telephone are as above. Since there is no link between terminals 1 and 2 inside the phone, the link is connected at the terminal block between the white and green wires. If this link is not connected, the bell will not ring. Note that the strain cord is secured under the screw of terminal 4.
If you're connecting the phone to a modem, etc., via a line cord with an RJ11 plug, this will use the U.S. colour code. In this instance, connect the line cord red to terminal 1 and 2 of the terminal block, and line cord green to terminal 3. Since there is nothing polarised inside the phone, either way round will in actual fact work.

Connection via Four Pin Plug & Socket.

Previously used with series 300 portable phones, these four pin plugs and sockets were phased out in the mid 1960's.

These plugs and sockets are the least likely seen method of connection today. They are based on standard bakelite electrical fittings, having the same pin dimensions, but of course a different configuration.

Plug and socket connections for single phone use. The socket shown is connected to a modern line cord using U.S. colours. The phone is linked internally between terminals 1 and 2.

The line cords use the same white, green, red (and blue, where four conductor), colour code as per phones connected to a terminal block. Again, the terminals 1 and 2 must be linked inside the phone, or at the socket, when the phone is used as a single unit.

Connection via 603 Plug and 610 Socket.
Since the 400 series was still being issued and installed after the introduction of the 800 series, some phones are fitted with a 603 plug. The plug and socket wiring is therefore the same as the 800. The line cord wiring colour code was also changed in 1962, so that all phones with a 603 plug use the new colour code. An exception to this is sometimes found in the present day where someone has 'converted' a phone from terminal block connection to a plug. Rather than destroy an original line cord doing this, since the lugs have to be cut off, a better option is to connect the 603 (or RJ11) plug to the terminal block via a short cord. A bodge sometimes seen today, particularly on phones 'restored' for sale, is that the original line cord has been replaced with a modern flat type with RJ11 plug. No attempt is made to recreate the original strain cord and terminal lugs.
Function Old  New
Exchange Line 1 (+) White White
Exchange Line 2 (-) Red Blue
Bell Green Red
Miscellaneous Blue Black
Line cord colour coding.

Note the new colour code for the line cord.

A common complaint when using vintage phones fitted with 603 plugs, with modern devices, is that they don't ring. This has been covered in the article on the series 800 phone. The problem is that most extension cords and RJ11 adaptors only make connections to pins 2 and 6 of the 610 socket. For modern electronic phones this is satisfactory, but for older phones with three wire line cords, the bell circuit becomes inoperative. In this instance it is necessary to link terminals 2 and 3 of the 610 socket, or the equivalent terminals inside the phone (terminals 1 and 2 for series 400).

Exchange line connected to one telephone. Note the link between terminals 2 and 3 in the socket.

This socket is connected to a U.S. colour coded line cord. For the 400 series phone to ring, it was necessary to install a link between terminals 2 and 3.

Parallel Operation.
Where more than one series 400 phone is connected across the line, they should be connected for three wire working as shown in Plan No.2 shown previously. Failure to do so will cause bell tinkling during dialling and gravity switch operation. Additionally, the line is loaded with extra capacitance which can cause dialling impulse distortion. The subject is covered in more detail in this article.