GEC
ST3500 and ST3600

GEC AUTOMATIC TELEPHONE
TABLE TYPE
Catalogue Nos. ST3500 series (Standard) and ST3600 series (Tropical)

Application
A table telephone of modern design, for use with all automatic switching systems.

The corresponding telephone for C.B. manual systems is described in Leaflet STL4.

Construction
The moulded Bakelite case, identical in shape with the latest British Post Office model, combines a robust design with a pleasing simplicity of line.

All components except the dial are mounted on a main assembly plate, which can be withdrawn from the case very easily as a complete unit.

The G.E.C. dial is the British Post Office standard, with a stainless steel finger-plate which retains its high polish indefinitely.

The cradle-switch mechanism has been specially designed to eliminate any likelihood of sticking plungers. Twin contacts give maximum reliability.

The handset is the well-known moulded Bakelite Gecophone pattern, with inset type transmitter and receiver.

The flexible handset cord will give exceedingly long service; the conductors are made from tinsel that withstands 200,000 reciprocations without breaking.

The four-way Bakelite terminal strip, connecting the incoming line wires to the telephone cord, has moulded separators between terminal plates to maintain long leakage paths.

There are separate entrances for the line wires and the cord.

Performance
The transmitter and receiver are British Post Office standards, and the high-efficiency speech circuit gives maximum volume with uniform frequency response and excellent sidetone suppression.

The unfailing efficiency of the G.E.C. dial mechanism is well known and a special spark-quenching circuit protects the dial impulsing contacts during dialling.

If required, a suppression circuit can be included to prevent interference with nearby radio receivers during dialling.

Tropical Model
G.E.C. telephones for service in tropical climates incorporate various special features, which are the result of long experience (see picture above):-

• A shunt is connected across the transmitter to minimise the minute arcing that tends to occur at the granules and electrodes as: the cradle switch operates; the accelerated tendency towards frying often shown by transmitters in the tropics is thereby offset. The effect of any increase in transmitter resistance upon line circuit conditions is also minimised by the shunt. The loss in volume efficiency is very small and is partly cancelled by reduction of sidetone, the nett result being a difference of only 1 db. Should the conditions of use render the protection unnecessary and require the very maximum of volume efficiency, then the shunt may be disconnected by the simple transfer of one wire from one terminal to another.

• Ventilation holes in top and bottom of the case allow air to circulate freely through the case to minimise condensation.

• All apertures are covered with fine wire gauze to prevent the ingress of insects.

• Metal parts are specially finished, and all wiring is specially treated, to resist the effects of high humidity and salt-laden atmospheres.

• Cradle-switch springs have large insulators to ensure long leakage paths.

• Flexible cord conductors are V.I.R. covered and braided overall. Since textile bindings at the ends are not in contact with the conductor, moisture absorption cannot cause leakage. Rubber fittings give mechanical protection at points of entry to the mouldings.

• All coils are specially treated to render them impervious to damp, heat and sudden changes in temperature.
• For very dusty atmospheres, a Bakelite cover can be fitted to the back of the dial to give complete enclosure to the mechanism.

Earlier model shown - identified by the flat switch hook plate

Colours
The standard colour is Black but enquiries are invited for the telephone in Ivory, Jade Green or Chinese Red, with cords finished to match.

Extension Bell
A magneto extension bell in the G.E.C. Cat. No. BE1100 series (Standard) or BE1200 series (Tropical) is recommended for use with this telephone, to provide calling signals at a distant point, indoors or out, or to augment calling signals in very noisy surroundings.

Dimensions
Length - 7.25" (18.4 cm)
Width - 9" (22.9 cm)
Height - 6" (15.2cm )
Weight - 5.5 lbs. (2.5 kgs)

Enquiries and Orders
Please state on all enquiries and orders:

• Finish - whether standard or tropical

• Colour required.

• If batteries are to be supplied and whether they are to be active or inert.

• For component parts, please refer to item by name and quote catalogue number of complete instrument ; for example, "Handset cord for Telephone Cat. No. ST3101".

Taken from the GEC Catalogue Leaflet No. STL.3

The ST prefix model numbers were introduced in 1946.
The diagram numbers are all prefixed with KS and this number is not the telephone part number.

PART NUMBERS
(1955 PART NUMBERS SHOWN IN BRACKETS)

Note: * For use with the 10-line All Relay P.A.X. Cat. No. PX1110 (Standard) and PX1210 (Tropical).

Conversion to the UK Plug and Socket system
 

Circuit Diagram for the ST3608 - Diagram KS9645

To Convert the ST3608 to work on the UK plug and socket system

1. Remove existing line cord.

2. Connect the red wire of the new line cord to L2.

3. Replace the link between E and L1 with a 3.3k resistor.

4. Insulate the green wire of the new line cord.

5. Connect the white wire of the new line cord to L1.

6. Disconnect the black wire from the 2uf capacitor.

7. Connect the blue wire of the new line cord to the black wire and insulate.

8. Connect a Rectifier No. 205 between R and MR.

Circuit Diagram for the ST3506 - Diagram KS8335

Reliance and GPO Telephone No. 332

To Convert the ST3506 to work on the UK plug and socket system

1. Remove existing line cord.

2. Strap terminals 8 and 9 together.

3. Connect the red wire of the new line cord to terminal 9.

4. Replace the link between 1 and 2 with a 3.3k resistor.

5. Connect the green wire of the new line cord to terminal 3.

6. Connect the white wire of the new line cord to terminal 1.

7. Remove the link between terminals 10 and 11.

8. Strap terminals 11 and 12 together.

9. Connect the blue wire of the new line cord to terminal 11.

10. Connect a Rectifier No. 205 between terminals 5 and 6.

Circuit Diagram for the ST3506 - Diagram KS10858
Fitted with a buzzer in place of the bell

Reliance and GPO Telephone No. 332

To Convert the ST3506, with buzzer or bell, to work on the UK plug and socket system

1. Remove existing line cord.

2. Strap terminals 8 and 9 together.

3. Connect the red wire of the new line cord to terminal 9.

4. Replace the link between 1 and 2 with a 3.3k resistor.

5. Connect the green wire of the new line cord to terminal 3.

6. Connect the white wire of the new line cord to terminal 1.

7. Remove the link between terminals 10 and 11.

8. Strap terminals 11 and 12 together.

9. Connect the blue wire of the new line cord to terminal 11.

10. Connect a Rectifier No. 205 between terminals 5 and 6.

Circuit Diagram for the ST3551 - Diagram KS10659

GPO Telephone No. 312

To Convert the ST3551 to work on the UK plug and socket system

1. Remove strap on terminals 10 - 11.
2. Remove strap on terminals 12 - 13.
3. Insert a 3.3K ohm resistor between terminals 11 & 12.
4. Link the terminals terminals 1 - 2.
5. Red of line cord to terminal 8.
6. White of line cord to terminal 1.
7. Blue of line cord to terminal 11.
8. Green of the line cord is spare and can be parked on terminal 7.
9. Rectifier No. 205 (if needed) inserted between terminals 6 & 5.

ADDITIONAL PICTURES

ST3505 Black - standard automatic telephone with Director dial (letters and numbers)

ST 3506 - CB variant made for the Reliance Telephone Co.

ST3506 - Internal view

ST3506 - fitted with a buzzer in place of bells

ST3604 with chassis removed.  This is a later model with the "non-stick" switch hook plungers

ST3608 Ivory
 

ST3608 Black

Pictures below are all ST3608 models

Front view
 

Front view
 

Rear view
 

Manufacturers marking on handset
 

Close up of interior of Terminal Block
 

Circuit diagram pasted to base plate
 

Base plate showing model number
Note the two plates, with mesh covered holes, held in place by the feet
 

Interior view of telephone

Last revised: January 17, 2023