Tektronix Circuit Computer: Layout Analysis

Following a linkie I can no longer find led me to retrieve the Tektronix Circuit Computer in my Box o’ Slide Rules:

I’m pretty sure it came from Mad Phil’s collection. One can line up the discolored parts of the decks under their cutout windows to restore it to its previous alignment; most likely it sat at the end of a row of books (remember books?) on his reference shelf.

The reverse side lists the equations it can solve, plus pictorial help for the puzzled:

Some searching reveals the original version had three aluminum disks, shaped and milled and photo-printed, with a honkin’ hex nut holding the cursor in place. The one I have seems like laser-printed card stock between plastic laminating film; they don’t make ’em like that any more, either.

TEK PN 003-023 (the paper edition) runs about thirty bucks (modulo the occasional outlier) on eBay, so we’re not dealing in priceless antiquity here. The manual is readily available as a PDF, with photos in the back.

Some doodling produced key measurements:

All the dimensions are hard inches, of course.

Each log decade spans 18°, with the Inductive Frequency scale at 36° for the square root required to calculate circuit resonance.

Generating the log scales requires handling all possible combinations of:

• Scales increase clockwise
• Scales increase counterclockwise
• Ticks point outward
• Ticks point inward

I used the 1×100 tick on the outer scale of each deck as the 0° reference for the other scales on that deck. The 0° tick appears at the far right of plots & engravings & suchlike.

The L/R Time Constant (tau = τ) pointer on the top deck and the corresponding τL scale on the bottom deck has (what seems like) an arbitrary -150° offset from the 0° reference.

The Inductive Frequency scale has an offset of 2π, the log of which is 0.79818 = 14.37°.

The risetime calculations have a factor of 2.197, offsetting those pointers from their corresponding τ pointer by 0.342 = log(2.197) = 6.15°.

A fair bit of effort produced a GCMC program creating a full-size check plot of the bottom deck on the MPCNC:

By the conservation of perversity, the image is rotated 90° to put the 1 H tick straight up.

The 3018 can’t handle a 7.75 inch = 196 mm disk, but a CD-size (120 mm OD) engraving came out OK on white plastic filled with black crayon:

The millimeter scale over on the right shows the letters stand a bit under 1 mm tall. And, yes, the middle scale should read upside-down.

Properly filling the engraved lines remains an ongoing experiment. More downforce on the diamond or more passes through the G-Code should produce deeper trenches, perhaps with correspondingly higher ridges along the sides. Sanding & polishing the plastic without removing the ink seems tedious.

The Great Dragorn of Kismet observes I have a gift for picking projects at the cutting edge of consumer demand.

More doodles while figuring the GCMC code produced a summary of the scale offsets:

Musings on the parameters of each scale:

How to draw decades of tick marks:

It turned out easier to build vectors of tick mark values and their corresponding lengths, with another list of ticks to be labeled, than to figure out how to automate those values.

More on all this to come …

1. #1 by solaandjin on 2019-11-22 - 09:17

I’ve gotten good marking on laser engraved acrylic by flooding with acrylic paint, letting dry, then knocking it off. It removes relatively easily and gives nice sharp contrast.

• #2 by Ed on 2019-11-22 - 11:01

How much laser power do you need to engrave white(-ish) acrylic?

A slightly charred trench would definitely grab paint better than the slick V-groove from a diamond drag tool.

AFAICT, a 40 W CO2 laser is about the minimum for cutting acrylic. I keep lusting after a Lasersaur, but …

2. #3 by Brent on 2019-11-22 - 10:29

This project is reminiscent of a pilot’s E6B: