The Strowger 11 Digit Residence Set

The Strowger 11 Digit Wall Telephone is quite interesting and is one that many collectors would like to own. Despite their frequent appearance on eBay, they sell at reasonably high prices and with the Australian dollar were it is, they are quite expensive here.

Most that appear on eBay are the 1905 model that is CB and originally had a “ring” button on the front. Automatic ringing was not provided on these early exchanges so once the number had been dialled, the caller had to press the Ring button the make the called telephone ring. Many are missing the button as they were removed when refurbished as automatic ringing was installed.

Here is a typical example that appeared on eBay recently. Notice the hole on the lower part of the finger-stop bracket – this is where the ring button was. The dial has eleven finger holes with the eleventh hole labelled “Long Distance”. Even so, the dial can only generate ten pulses; the eleventh hole generates the same number of pulses as the zero hole. The extra hole was provided because the designers of the time thought that the subscribers would not cope with having a single hole that would be used to dial ‘0’ and call the long distance operator. If the number “1045” were to be called, the subscriber would be worried that the long distance operator would answer when the ‘0’ was dialled.

If you have looked inside any that have appeared on eBay you might have noticed that many of them have a line transformer fitted. Why is that?

These telephones were used in large numbers in the USA and Canada and when they were finally replaced they flooded the surplus market. As they operate on the so called “three wire system”, they are not compatible with later exchanges so they could not be reused on more modern installations.

An enterprising purveyor of surplus equipment decided that these old phones could be used as intercoms. All that was needed was a way to make the called telephone’s bell ding. A set of instructions was prepared that used an added button (or the dial) to interrupt DC current in a line transformer to generate a high voltage pulse to ding the bell – problem solved.

This example appears to be using the dial to generate the ringing pulses. The green-white and red wires between the dial and the case are part of the intercom kit.

Here is part of the instructions issued by the Radio Supply Co Ltd describing how to convert a Strowger 11 Digit Wall telephone into a multi station intercom.

Here is the pre-modification circuit.