For this project I used two Soviet-era telephones: a Czech-made "Tesla" model and a Latvian-made TA-68. Both of these were manufactured in the 1980s and were two of the most widely-used phones in the USSR.
I initially wanted to use two identical TA-68 phones for this project, but one of the phones wouldn't work properly with our circuit. The phone itself seemed to be functional, but after a bit of investigating I noticed that the phone's resistance (impedance?) was much, much higher than that of the other phone. I measured this with my multimeter by taking a resistance reading between the two wires in the phone's cable. The problem phone measured approximately 1 MΩ, whereas the other unit measured only 1 kΩ.
I tested all the phones that I had at the workshop until I found two with similar resistance. This is something that that needs to be checked before you start this build!
Powering the Circuit
I designed this circuit based on some old schematics that have been floating around Russian-speaking electronics circles for decades. 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.
The circuit uses two transformers (or one transformer with two secondary windings) to reduce 220V mains voltage down to the levels needed for the project. For the speaking/listening part of the circuit we enough AC voltage that we can rectify into around 20-40V DC. The ringer, however, runs on alternating current. But how do we know how much AC voltage we need? Most rotary phone ringers in the US require around 50V to operate. I live outside the US and I don't have very good information about the telephone system in the USSR, so I used a variable transformer that I restored in a previous project to find the exact voltage needed to activate the phone's ringer. I did this by attaching the Variac's outputs directly to the ringer's coil inside the phone. Starting at around 60V (with the phone ringing), I slowly dialed down the voltage level until the ringer stopped. This told us that the minimum voltage needed to operate the phone's ringer was around 53V.
The most difficult part of this build was finding the right transformers. I live in Tbilisi, Georgia, and ordering parts from places like Digikey or Mouser typically takes two or three weeks for delivery. So I decided to shop locally and spent many a weekend wandering through the labyrinth of the Eliava outdoor market. Eventually, after much searching, I managed to find two transformers that would work for this build.
One transformer puts out about 55V (from 220V), which is acceptable for our ringer circuit. The second transformer I found puts out 24V, which is perfect for the speaking/listening part of the circuit. It's worth mentioning here that if you decide to use only one transformer, it must have two secondary windings, not one winding with two outputs. A center-tapped transformer with two different outputs will not work for this project.
If I could do it again, I would have taken the time to order two smaller transformers, as this would have significantly reduced the size and weight of the build. But as we so often do with electronics projects, I made do with what I had available.
One more note about the AC supply for this project: I had read that the AC voltage used to power the ringer on phones like this is usually at a lower frequency (20Hz?) than the frequency of mains voltage (50Hz where I live). Changing the frequency of alternating current sounded like a complicated job, so I chose to ignore this and push ahead. In the end, the ringer worked fine at 50Hz.
What I learned about converting AC to DC voltage
I used a full bridge rectifier to convert the AC to DC for the speaking/listening part of the circuit. But after putting it...Read more »