The PAX25 is a particularly fascinating piece of electromechanical telephony history. This device is an ancestor to the present-day PBX that is so commonly seen in offices which require inter-office communications via telephone with various dial-out, trunking, intercom, and paging capabilities (all of which this unit also provides).

Introduced in the mid twentieth century, this contraption packs a surprising amount of functionality into a 71cm by 61cm cast iron frame. Sporting a default configuration enabling three concurrent calls (upgradable to four) out of twenty-five extensions, ten-extension conference calling, four tie lines, executive break-in for one prioritized extension, fire-alarm connection, four public exchange lines, fully automatic dial tone, busy tone, and ringing generators, automatic pulse-dial decoding (step-by-step), and even audible indication of blown fuses, and more, this machine does a lot!

All of the timing of operations is provided merely by relays and capacitors. A special trick for creating delays with relays involves addition of copper shorting caps or rings to the relays in order to slow the decay of their magnetic fields and thus provide controlled time-delay. There is one solid-state copper-oxide diode and the rest is entirely electromechanical.

A separate ring generator unit sits outside of the main chassis and provides the vibrating relay and transformers necessary to generate ringing power for the attached extensions.

For extensive details and specifications, check out this article at!

And of course I have a few videos I did with this thing!

An initial demonstration
Another Demo
A Data Connection?!

Some interesting photographs of the unit.

This is how the switch looked when I got it; so shiny and nice even though it was a good fifty years old by that point.

It was pulled from the John Hart electrical generating station back up in Campbell River where I grew up. They’d pulled the unit from the station and given it to the amateur radio club, from which I received it via Jasper

These days, it’s not looking so pretty… So many years of moving, storing things, humidity and nature taking their toll. I simply wasn’t in the state of mine or with the means to give my things a better existence and proper care, but this unit has soldiered through it and it still worked in 2019 when I finally cleaned it out, set it up, and tested it!

I went through every uniselector (those rotating relays) and every regular relay contact, one by one, by hand, to polish and clean all of them. Every relay had all its reeds adjusted for correct tension, closure, and timing… It was a lot of work but well worth it!!