A note arrived from someone who obviously couldn’t pass up an orphaned HP 7475A plotter:
The plotter I received works beautifully, except that the carousel doesn’t rotate. I found a YouTube video showing a 7475a running with the cover off, and there’s a little plastic piece – it looks like a teardrop – that advances the carousel, and is apparently part of the carousel motor assembly. Mine is missing that piece …
The keyword is Geneva drive, a wonderfully simple technique to convert one rotation of the stepper motor into 1/6 turn of the pen carousel, with no need for fancy sensors.
The (unofficial) HP Computer Museum has All The HP 7475A Documents and the Plotter Service Manual shows All The Parts. And, of course, I’ve written a bit about my adventures with an old 7475A.
Back in the day, you could get the entire Pen Carousel Housing Assembly w/ Motor (PN 07475-60175) as a unit and the Carousel Motor Only (PN 3140-0687) as a separate thing, but not the Geneva drive wheel:
The cam’s drive wheel end (in inches, because early 1980s):
- 0.25 thick overall
- 0.10 thick plate under pin end
- 1.09 OD – rounded end
The pin sticking up from the cam:
- 0.154 OD (or fit to slot?)
- 0.16 tall (above base plate)
I have no good (i.e., easy + accurate) way to measure the distance from the motor shaft to the pin, but I doubt it’s critical. As long as the pin doesn’t quite whack the hub end of the slot, it’s all good:
The 0.10 plate + 0.16 pin height don’t quite add up to the 0.25 overall measurement, but that’s certainly measurement error. I’d round the pin length downward and carve the drive from a 1/4 inch sheet.
A 3D printed part would probably work, apart from the accuracy required to fit the D-shaped motor shaft. Perhaps a round hole, reamed to fit the shaft, carefully aligned / positioned, with epoxy filling the D-shaped void, would suffice. A dent in the round hole would give the epoxy something to grab.
I’d be sorely tempted to use an actual metal / plastic rod for the pin, rather than depend on a stack of semi-fused plastic disks. The pin must withstand hitting the end of the “missing” slot during the power-on indexing rotation, because turning the carousel isn’t quite a non-contact sport. Normally, though, it enters the end of the slot without much fuss:
The blocked slot sits at the bottom of that picture, with a small locating pin sticking upward just above the circular feature at the end of the arm: we’re seeing the negative of a plug inserted into the original injection mold.
With a bit of luck, another HP 7475A plotter will fascinate everybody within hearing distance!
[Update: It lives! ]
The Smell of Molten Projects in the Morning 
A Geneva drive is also used in motion picture cameræ and projectors to advance the film, as it gives the needed quick motion with pauses in-between and can easily be synchronized with the shutter. I’ve seen several things built with a stop at one or both extremes so a mechanism can find its home position without a sensor by just advancing far enough that it would be at the stop. The old “QIC” (quarter inch cartridge) drives did this for head positioning, it was fairly noisy and banged away at the stop. It seemed to me to be a little violent and primitive for a precision mechanism.
The rattle clatter snap as the plotter indexes the carousel to Pen 1 for the first time always makes the bystanders twitch. These days, it would surely fail some environmental noise level spec.