Restoring a Teletype Model 15

That's my site.

I restored five Teletype machines, and they were at major steampunk conventions for almost a decade. Here's our telegraph office.[1]

I know about the guy at the Computer Museum, although I've never heard from him. He built one of my interface boards. I built an interface that drives one of the old teletypes from a USB port. It's entirely powered from the USB port, even though it has to deliver 120V at 60mA.[2] He's also using my "baudotrss" software.[3] He's much more into YouTube than I am. I never dreamed that anyone would want 12 hours of Teletype restoration videos.

These machines were designed to be maintainable, at three levels. First, the big components are hot-swappable. Remove two thumbscrews and the keyboard slides out. A switch closes when you do this to take it out of circuit. Remove three thumbscrews and the typing unit lifts out. Again, a switch closes so this doesn't break the current loop and interfere with other machines on the loop. Replacing the motor takes a screwdriver and about two minutes. That's everything except the cast iron base and some switches and wiring. So a quick swap in the field will get you going if you have the spares. It was routine to swap these big units and take them back to a shop for cleaning and oiling once a year or so.

At the next level of maintenance, there are removable subassemblies. The carriage and the selector assembly, the most complicated subunits, come off easily with tools. So they can be swapped by first-line maintenance if necessary.

Finally, every part can be unscrewed and replaced if necessary. That's why these machines still work. They are totally maintainable. However, they require considerable adjustment after maintenance. Measuring spring tensions and clearances is required, not just parts changing. Fortunately, the very detailed manuals have survived.[4]

The price of this maintainability was the need for too much infrastructure. Maintenance depots. Teletype mechanic schools. The military had two levels of school - one to do installation and maintenance down to sub-assembly replacement, and one for the shop techs who had to be able to totally disassemble and reassemble a machine. All specialized for one or two very specific machines. That's expensive.

[1] https://vimeo.com/124065314 [2] https://github.com/John-Nagle/ttyloopdriver [3] https://github.com/John-Nagle/baudotrss [4] http://www.aetherltd.com/manuals.html

And remember, boys and girls, that the correct sequence is CR LF, and not LF CR. If I see LF CR in your C code I will slap you with a review comment, just out of nostalgia, even though modern printers rarely drip oil. CR can take more than a single character time for the carriage to move to the left and settle, so to avoid print corruption, issue CR first, and then LF so that the carriage also has LF time to settle.

Back many moons ago I used to operate RTTY on ham radio. On the air, since characters can be easily lost, the standard end-of-line sequence was often CR CR LF LF NUL NUL in case a CR or LF got clobbered by a static crash or such, and the NULs are extra settling time. (Usually we also configured "unshift-on-space" because a lost LTRS in Murray code is annoying. We all assumed the other end was running U.S.O.S. so always assumed SPACE took the receiver out of FIGS mode and therefore sent a redundant FIGS after SPACE.)

If you are wondering why UARTs can be configured for 5 data bits and 1.5 stop bits.... Murray code as used in Model 15's is why. Once upon a time I did a Apple II program in 6502 assembly that gave me a "glass TTY", split screen, received text on the top 2/3's of screen, sent on the bottom. Interfaced to a HAL ST-6000 (I loved that box. I still have it collecting dust somewhere). Eventually I ported the glass TTY to a Macintosh I.

Now... where did I leave my cane? And get off my lawn.

CuriousMarc, a volunteer at the Computer History Museum, recently finished a video series restoring two Model 19 teletypes into working order. It's a long series but eventually explains how the devices work at all levels, and shows off some of the more niche features of the ttys. Well worth the watch.

https://www.youtube.com/watch?v=_NuvwndwYSY

Another "rocking chair" story...

I grew up (60's) with a few of these clacking away in my father's HAM shack. He was a member of the Military Affiliate Radio (MARS) program, and used to get things like TTYs for free.

He was really open about letting me mess with things (as long as I didn't take things apart or transmit), so I spent a lot of time playing around with them, feeding punch tapes into them to print things, etc. I knew at the time that punch tapes were a/the recording method for computer programs on mainframes, so I had a lot of "this is a computer" fantasies rolling around in my brain.

We had one of these when I was a kid in the late 1970s. My dad built what was essentially a modem and used it to connect the teletype to an old Motorola single channel radio with a crystal tuned to the amateur radio (Ham radio) teletype repeater in Dallas, Texas. There was a community of Ham radio teletype (RTTY) users in Dallas with similar setups that formed what was essentially a chat room. There was even one Ham that had a PDP-11 minicomputer connected to his system that acted like a chatbot. You could send a message to it to get weather reports and a few other interesting functions. When I would come home from school each day, I would find several feet of paper had come out of the teletype, and I would scan it to see what had been said while I was gone. If I wanted to contribute and the channel was clear, I would flip the transmit switch and start typing. Everything I typed would appear on all of the other rigs that were connected.

We could also connect the modem to my dad's HF radio and chat with RTTY operators all over the world. I would spin the tuning dial until I heard the distinctive RTTY tones. I would adjust the tuner until the pitch sounded correct and then turn on the teletype. Often garbage would come out until I got it tuned in perfectly, and then I might see someone calling to start a conversation, or a conversation already in progress. Sometimes I would catch a teletype art (ASCII art using 5-bit Baudot code) picture in progress. If it looked interesting, I would stay tuned in case they repeated the transmission. This took a while because these machines operated at 60 words per minute (~6 characters/second).

After a while, my dad upgraded to a Model 19, which had a paper-tape punch and reader. This allowed me to create my own teletype art, so I worked several hours creating an R2-D2 that I was very proud of. I sent it to several people and was thrilled one day when I was tuning around and recognized my R2-D2 being sent by someone else.

When my dad later brought home an Apple II, he was able to connect the Apple II to the modem, so we replaced the Model 19 with a Model 28 RO (Read-only), which did not have a keyboard or paper tape, but that was okay because we used the Apple II. Because modern computer printers like an Epson MX-80 were rather expensive then, we used the teletype as our printer, and I even turned in some school papers printed on the teletype in all-caps on yellow paper torn off from a roll. (I'm sure my teachers loved that.) When we eventually did get an Epson printer, it was hard to justify keeping the bulky teletype, so it was sold. We still did some RTTY now and then on the Apple II, but it wasn't quite the same as having the teletype machine left on all day. Of course, most of the rest of the RTTY community was also moving to microcomputers, and the hobby began to rapidly change. Then I went off to college to study computer science and haven't done much with amateur radio since then.