Thomas Burns' Analog Intercom System Connects Two Soviet-Era Rotary Phones Together

Built from parts including a dramatically over-spec'd 55V 250A transformer, this two-station intercom system rings a bell.

Former cinematographer turned maker Thomas Burns has turned two Soviet-era rotary telephones into a point-to-point intercom system — using a fully-analog circuit and a vintage 55V transformer capable of putting out an impressive 250A.

"I needed an intercom system for our family's computer repair business. We restore a lot of vintage tech at our shop, so I decided to make this system using two Soviet-made rotary telephones from the 1980s," Burns explains. "Modifying a pair of these phones so that you can speak and listen is relatively simple: it basically only requires around 15V of direct current, plus a resistor. But getting the phones to ring is a bit more complicated, because the ring circuit requires alternating current."

A couple of transformers and a clever circuit turns 1980s Soviet telephones into a point-to-point intercom system. (📹: Workshop Nation)

Traditionally, turning vintage telephones into intercoms involves gutting the hardware and replacing it with modern electronics. With two well-kept phones from the 1980s, though, doing so would have been a disappointment — so Burns turned to classic analog design to put together an intercom circuit which would use the phones' existing ringers, causing one to sound when the handset of the other is picked up.

"I designed this circuit based on some old schematics that have been floating around Russian-speaking electronics circles for decades," Burns explains. "The concept is this: when one of the receivers is lifted, the circuit sends alternating current to the other phone, causing it to ring. When both receivers are lifted, the circuit sends direct current instead, which allows the users to speak and listen."

To get the original ringer to work, though, required at least 53V of power — and with delivery to Burns' office in Tbilisi, Georgia taking rather too long, the maker turned to a local outdoor market and picked up a pair of vintage transformers. The first, capable of a rather excessive 250A, puts out 55V for the ringer; the second puts out 24V to drive the audio circuitry.

With the transformers and control boards housed in a reused steel housing, Burns was able to wire up both phones — without modification. "After a bit of troubleshooting surrounding the phone line connections, we finally got it all up and running," he explains. "The phones ring as intended, and the receivers are transmitting and receiving as we had hoped. The only thing I noticed was that the audio level was a bit low when listening and the sound is a bit crackly, but my friends who grew up in the USSR listened to it and told me this was completely normal for Soviet phones."

Burns' full write-up, including a parts list and schematics for the control circuit, is available on his project page; a video detailing the build is available on the Workshop Nation YouTube channel.

Gareth Halfacree
Freelance journalist, technical author, hacker, tinkerer, erstwhile sysadmin. For hire: freelance@halfacree.co.uk.
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