Threaded wires record phone numbers

If you thought programming your 90s VCR was difficult, wait until you see this Russian telephone dialer that [Mike] took over on the mikeselectricalstuff YouTube channel (video under the pause). [Mike] a few years ago i had this soviet era machine from the 1980s and finally got into it to learn what makes it work. The automatic dialer plugs into the telephone line, much like an old-fashioned answering machine. It provides the user with 40 predefined phone numbers, organized into two banks of 20, and a speakerphone to monitor the connection process. It uses pulse dialing – no dial tone. What is surprising is the way you program the numbers. Since it was built in the Soviet Union in the 1980s, he didn’t expect a microcontroller. But he didn’t expect a transformer core “string” memory either.

The phone is normally located on a platform on the left side of the machine. The elevation of the platform exposes a bank of toroidal cores, arranged in seven rows of four. Each line corresponds to a compound number and the four cores are used to encode a single number. At the top and bottom of the programming board are two 40-pin connectors, each pin corresponding to one of the preset phone numbers. A bunch of jumper wires would have been provided, and you program each number by threading a long wire through the appropriate cores, connecting it to the top and bottom connectors much like a modern solderless breadboard. It is also interesting to see the components and the technique of construction of this printed circuit board. For example, diodes have the strip at the end of the anode, not the cathode as we are used to today. The transistor boxes are mounted upside down like dead spiders.

Digitized drawings (click to view full size)

Mike tries to decipher the diagrams and delves enough to understand how it works. This design is an interesting solution to the problem of building an automatic dialer at this time. If you want to know more about main memory, here is an article we wrote on decrypting an Apollo string memory module, perhaps one of the most well-known examples of this technology. We’ve also covered a few projects using chord memory techniques but on a small scale, here and here.

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