How to Interface a Soviet Багта-50 1955 Rotary-Dial Telephone with the GSM Network

Багта-50

On the way back from one of several trips to Russia I purchased a telephone at a flea market in Estonia. As you can see from these pictures, I also picked up some aluminum statues of Vladimir Ilych Lenin.

The telephone was a Багта-50 (that is, Bagta-50), built in the Soviet Union in 1955. According to what was written in crayon or wax pencil on a tightly folded piece of paper inside the phone, it had received its most recent maintenance in 1968.

I used this on a POTS (Plain Old Telephone Service) landline connection for a number of years, but in 2007 I dropped the landline and went all-GSM. That was useful in some ways, but meant that the Багта-50 was no longer usable due to shortcomings in the GSM system design. Until I finally got around to interfacing it to GSM....

Soviet Багта-50 rotary telephone purchased at a flea market in Tallinn, Estonia, now adapted to GSM service.  At rear, an aluminum statue of Vladimir Lenin.

Багта-50 Exterior

Lenin looks over the Soviet Багта-50 telephone.
Soviet Багта-50 telephone from the side, on-hook.
Soviet Багта-50 telephone, off-hook.  Who's calling?
Soviet Багта-50 telephone between stereo Vladimir Lenin statues.
Soviet Багта-50 telephone, close-up view of the rotary dial.  It has the Russian dial code: 1=A, 2=B, 3=V, 4=G, 5=D, 6=E, 7=Zh, 8=I, 9=K, 0=L.
Soviet Багта-50 telephone, view from above.
Soviet Багта-50 telephone, square overhead view.

Багта-50 Handset

Soviet Багта-50 telephone handset, rear view.
Soviet Багта-50 telephone handset, three-quarter view

This shows the earpiece (at left) and mouthpiece (at right) within the handset.

The original handset cord was ruined, the insulation and the outer jacket were rotting away. I replaced it with heavy 3-conductor cable suitable for 20-amp power service.

Too large, clumsy and unwieldy for service as a handset cord — exactly what was needed for authenticity!

Soviet Багта-50 telephone handset, earpiece detail.
Soviet Багта-50 telephone handset, earpiece and mouthpiece exposed.
Soviet Багта-50 telephone handset, mouthpiece detail.

Багта-50 Interior

Here you see the circuit diagram found on the inside of the bottom plate. Click here or on the image for a larger version.

Soviet Багта-50 telephone interior, ringer and terminal block.

The images here and below show the interior of the phone, inside the base plate.

You can see both the terminal strip and the ringer with its two brown wrapped solenoids. The tightly rolled maintenance ticket was slipped underneath the ringer coils.

The useful terminals are labeled on the strip as:

МГ М     Л2 Л1
Т          

The labeling on the schematic only partially agrees. Disagreement between schematics and reality are familiar from my previous experiences with Soviet hardware.

МГ  mouthpiece +
М  earpiece +
Т  mouthpiece and earpiece -
Л2, Л1  to red/green POTS lines
Soviet Багта-50 telephone, interior terminal block.

Interfacing the Багта-50 to GSM

There are at least two obvious and major drawbacks to the GSM system:

  1. Incompatibility with rotary-dial signaling
  2. Difficulty of connecting to sophisticated hardware like the Багта-50

The first seems insurmountable without adding inappropriate complexity to the system. But both drawbacks can be worked around by interfacing the 1950s Soviet technology to an existing GSM device.

This was made more difficult because of the bizarre and proprietary connector used in the Nokia hardware design.

My phone is a Nokia 6086, type RM-260, GSM quad-band. For many years most Nokia handsets used the "Pop-Port" connector, described on the useful pinouts.ru site. The Pop-Port contacts are:

Nokia 6086 GSM phone Pop-Port connector, close-up on phone.  Small coaxial power connector at left, Pop-Port connector at right with data, control, and audio lines.
Nokia pop-port pinout.

While in New York on a job, I went to Chinatown and bought a cheap Nokia-compatible earpiece from a sidewalk vendor along Canal Street. Audio almost as bad as the Багта-50 with none of the esthetic advantages. But it provided a Pop-Port connector for just US$ 5....

Tiny wires! And worse yet, they were the heinous Litz wires.

The cable has been split into the two jacketed components, and each has two enamel-insulated Litz wire bundles. A red/green pair for the earpiece and a gold/green pair for the microphone.

Nokia 6086 GSM phone and a cheap GSM headset purchased on Canal Street in Chinatown in Manhattan, New York.
Tiny Litz wires with two Soviet 1-ruble coins and a Moscow Metro token.

It can be very difficult to make good Litz wire connections. Here's what I did:

  1. Carefully wrapped the wires through and onto the terminals on a 1/8" / 3.2mm stereo plug
  2. Heated the plug contacts with a soldering iron
  3. Melted solder onto the terminals, immersing the Litz wire
  4. Kept the solder molten for several seconds to remove the enamel from the Litz wire and float it away
Tiny Litz wires with two Soviet 1-ruble coins and a Moscow Metro token, cropped and full size.
Soldering the Litz wires using a small vise.
Connector for Nokia interface, interior view showing its internal connections.

I had already determined that the connections on this Pop-Port connector were as shown below:

Nokia pop-port pinout.

I drilled a 1/4" / 6.4mm hole in the phone body next to the existing hole for the handset cord. I then mounted a stereo jack there. The jack is barely visible to the left of the handset cable in the first picture below.

Connector for Nokia interface, exterior view.
Connector for Nokia GSM interface, exterior view with new Pop-Port interface in place.

The short cable to the Nokia connector can just stay permanently attached to the Багта-50.

I tried leaving the handset connected to the impedance matching network in the telephone, and then connecting those terminals to the jack leading to the Nokia unit.

Impedance Багта-50
(correct)
Nokia
(incorrect)
Microphone 800Ω 1200Ω
Earphone 450Ω 35Ω

However, and I see this as yet another shortcoming of the GSM system design, the Nokia GSM unit could not handle the impedances presented by the Багта-50's impedance matching network. Here are the measured DC resistances for the Багта-50 and a Nokia earbud unit. You can see that the Nokia unit is pretty far out of spec.

So, I disconnected the handset lines from the internal terminal strip. Yes, that means that all I am really using is the handpiece itself. But it does keep the impedances close enough to keep the Nokia handset reasonably happy.

I initially had serious trouble with feedback caused by physical resonance of the Багта-50 handset coupled with the high gain of the Nokia external interface. Yet another drawback of the Nokia design! I solved that by packing some cotton gauze pads around both the earpiece and mouthpiece elements.

Initial reports indicate the audio quality is rather poor:

Therefore the performance is exactly as I had hoped!

Here it is in operation! In Soviet Union, phone calls you!

The Soviet/GSM phone in operation!  It makes a call!  In Soviet Union, phone calls you!
Nokia VoIP / GSM network.

There is another benefit that I had not considered when I started the project — I can carry the entire unit somewhere and use it as a just-barely-portable phone. Despite appearances, it needs no wired connection.

Using T-Mobile's "Hotspot @Home" service, the Багта-50 can also be involved in an 802.11 wireless LAN and VoIP (Voice Over IP) networking.

The 1970 Leningrad Telephone Directory

The library in West Lafayette, Indiana had a 1970 Leningrad telephone directory at a recent book sale. Quite a bargain at just US$ 1.50! I have scanned the cover and some of the pages. Contact me if you need something looked up.

Front cover of 1970 Leningrad telephone directory.

The front cover. From the Telecommunications Ministry of the USSR (Ministerstvo Svyazi SSSR)

Emergency numbers inside front cover of 1970 Leningrad telephone directory.

Inside the front cover, Emergency contacts: Fire (01), Militsiya or Police (02), Ambulance (03), LenGaz emergency services (04). Non-emergency: Movie schedules (05), Telegraph office (066), International telephone station (07), The Talking Clock (08), and the Information Center (09).

National medical emergency numbers inside front cover of 1970 Leningrad telephone directory.

Inside facing page, national medical emergency centers.

Cover page of 1970 Leningrad telephone directory.

Cover page.

Cover page and groovy logo inside 1970 Leningrad telephone directory.

Publishing details and a groovy logo.

Directory of information in 1970 Leningrad telephone directory.

All types of information are available!

Main PTT office, just off Nevsky Prospekt, in 1970 Leningrad telephone directory.

The main PTT office in downtown, just off Nevsky Prospekt.

International calling instructions in 1970 Leningrad telephone directory.

International calling details, where by "international" they mean "outside of Leningradskaya Oblast'".

Videotelephone calling instructions in 1970 Leningrad telephone directory.

Videotelephone details. Really! According to this, videotelephone connections were available between Moscow, Kiev, Minsk, Tashkent, Tbilisi, L'vov, Kazan, Saratov, Smolensk, Kharkov, Dnepropetrovsk, Vilnius, and Kaunas.

Telegraphy instructions in 1970 Leningrad telephone directory.

If videotelephoning is too futuristic, perhaps some telegraphy?

Radio (radiotranslayatsionnoy) network details in 1970 Leningrad telephone directory.

The "radio" system, or radiotranslayatsionnoy networks, used to distribute audio programming via carrier current. Your "radio" was pretty much just a small speaker plugged into the city speaker wires.

Non-residential listings in 1970 Leningrad telephone directory.

And here begin the listings. This book has non-residential listings only.

Start of hospital listings in 1970 Leningrad telephone directory.  Hospital of the Scientific Academy of the USSR through October Gynecological Hospital.

The start of the hospital (bol'nitsy) listings. Why hospitals? I did some work in a hospital there, although the one where I worked was built in the early 1970s and isn't listed here.
Hospital of the Scientific Academy of the USSR
through
"October" Gynecological Hospital

Hospital listings in 1970 Leningrad telephone directory.  Smol'ninskovo Gynecological Hospital through Hospital of the Red Triangle Association.

Smol'ninskovo Gynecological Hospital
through
Hospital of the "Red Triangle" Association

Hospital listings in 1970 Leningrad telephone directory.  Number 1 Hospital of the October Association through Chronic Disease Hospital in the name of Karl Marx.

Number 1 Hospital of the October Association
through
Chronic Disease Hospital in the name of Karl Marx,
and then starting the Children's Hospitals.

Hospital listings in 1970 Leningrad telephone directory.  Children's hospitals.

The rest of the Children's Hospitals.

Other Soviet / GSM Projects and Багта / Bagta Information

Brano Meres added Bluetooth to a 1960s Soviet army telephone handpiece, although his site just has a single picture and no details of what he did or how.

Here are about the only other instances of Багта/Bagta telephone apparatuses that Google knows about:

Since that page says that "BAGTA" is an acronym for Bakelīta Automātiskais Galda Telefona Aparāts, and the page is hosted at http://forum.myriga.info/, and I bought mine in Estonia, I suspect that the BAGTA-50 is a Latvian phone.

Someone else merged a British rotary telephone with a DECT phone.

A number of people have asked me where they could purchase a Багта-50. Well, maybe two or three people. Anyway, outside of trying to find one at the same flea market outside Tallinn, Estonia, where I bought mine, I have no idea. However, at right you see a very a Багта-50-like telephone sitting on a display table outside an antique shop in Centro Storico, the historic old city, at the core of Napoli (Naples), Italy.

Other useful GSM information include these lists of international GSM providers and maps of their coverage: gsmworldmaps.com gsmworld.com

Other abuses of technology on my site