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:

Tektronix Circuit Computer - front
Tektronix Circuit Computer – front

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:

Tektronix Circuit Computer - rear
Tektronix Circuit Computer – rear

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:

Tektronix Circuit Computer - angle layout
Tektronix Circuit Computer – angle layout

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
  • Text reads from center
  • Text reads from rim

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:

Tektronix Circuit Computer - Bottom Deck - scale check plot
Tektronix Circuit Computer – Bottom Deck – scale check plot

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:

Tek CC bottom - ABS 160g 2400mm-min
Tek CC bottom – ABS 160g 2400mm-min

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:

Tektronix Circuit Computer - scale angle tabulation
Tektronix Circuit Computer – scale angle tabulation

Musings on the parameters of each scale:

Tektronix Circuit Computer - scale parameters
Tektronix Circuit Computer – scale parameters

How to draw decades of tick marks:

Tektronix Circuit Computer - decade tick doodles
Tektronix Circuit Computer – decade tick doodles

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 …

Tour Easy: PTT Switch Replacement

The PTT switch on Mary’s Tour Easy became intermittent:

Tour Easy - failed PTT switch
Tour Easy – failed PTT switch

It’s been sitting there for least five years, as witnessed by the sun-yellowed hot melt glue blob, which is pretty good service from a switch intended for indoor use. The 3D printed button never fell off and, in fact, was difficult to remove, so that worked well.

I took it apart and cleaned the contacts, but to no avail, so her bike now sports a new switch with a similar rounded dome:

Tour Easy - new PTT switch
Tour Easy – new PTT switch

I clipped the wires a bit beyond the terminals and soldered the new switch in place, so it’s the same cable as before.

Now, to see how long this one lasts …

Baofeng UV-5R Squelch Settings

The Baofeng UV-5R radios on our bikes seem absurdly sensitive to intermodulation interference, particularly on rides across the Walkway Over the Hudson, which has a glorious view of the repeaters and paging transmitters atop Illinois Mountain:

Walkway Over The Hudson - Illinois Mountain Antennas
Walkway Over The Hudson – Illinois Mountain Antennas

A better view of the assortment on the right:

Illinois Mountain - North Antennas
Illinois Mountain – North Antennas

And on the left:

Illinois Mountain - South Antennas
Illinois Mountain – South Antennas

Not shown: the Sheriff’s Office transmitter behind us on the left and the Vassar Brothers Hospital / MidHudson pagers on either side at eye level. There’s plenty of RFI boresighted on the Walkway.

Anyhow, none of the Baofeng squelch settings had any effect, which turned out to be a known problem. The default range VHF covered a whopping 6 dB and the UHF wasn’t much better at 18 dB, both at very low RF power levels.

We use the radios in simplex mode, generally within line of sight, so I changed the Service Settings to get really aggressive squelch:

Baofeng UV-5R - Improved Squelch Settings
Baofeng UV-5R – Improved Squelch Settings

I have no way to calibrate the new signal levels, but I’d previously cranked the squelch up to 9 (it doesn’t go any higher) and, left unchanged, the new level makes all the previous interference Go Away™. Another ride over the Walkway with the squelch set to 4 also passed in blissful silence.

If the BF-F9 levels mean anything on a UV-5R, that’s about -100 dBm, 20 dB over the previous -120 dBm at squelch = 9.

The new squelch levels may be too tight for any other use, which doesn’t matter for these radios. As of now, our rides are quiet.

[Update: Setting the squelch to 5 may be necessary for the Walkway, as we both heard a few squawks and bleeps while riding eastbound on a Monday afternoon. ]

Baofeng Big Battery Capacity

I bought a pair of third-party 3800 mA·h batteries for the Baofeng UV-5RE Plus (whatever that means) radios on our bikes. Oddly, the packs carry the same “Model BL-5” identification as 1800 mA·h batteries shipped with the radio:

Baofeng BL-5 Batteries - 1.8 and 3.8 Ah
Baofeng BL-5 Batteries – 1.8 and 3.8 Ah

The obviously mislabeled “Baofeng” battery eliminator also sported a 3800 mA·h label:

Baofeng Battery Eliminator - overview
Baofeng Battery Eliminator – overview

I conjured a “test fixture” from a clamp, copper sheet, and copper tape snippets:

Baofeng battery - test setup
Baofeng battery – test setup

Which produced interesting results:

Baofeng BL-5 3800 mAh packs - Ah - 2019-05
Baofeng BL-5 3800 mAh packs – Ah – 2019-05

The 250 mA load = 15 hour rate seemed reasonable to simulate radios spending most of their time in power-save mode, but the packs still delivered only 2.8 A·h.

The packs also claim an unnaturally precise 28.12 W·h, but they’re still underperformers at 20 W·h:

Baofeng BL-5 3800 mAh packs - 2019-05
Baofeng BL-5 3800 mAh packs – 2019-05

Anyhow, I can run the radios for a week without (worrying about) running out of juice during a ride.

JYE Tech DSO150 Oscilloscope vs. Actual Signals

The DSO150 oscilloscope’s specs give a 200 kHz bandwidth, so a 50 kHz sine wave looks pretty good:

DSO150 - sine wave 50 kHz 10 us-div
DSO150 – sine wave 50 kHz 10 us-div

A 100 kHz sine wave looks chunky, with maybe 25 samples per cycle:

DSO150 - sine wave 100 kHz 10 us-div
DSO150 – sine wave 100 kHz 10 us-div

The DSO150 tops out at 10 µs/div, so you can’t expand the waveform more than you see; 25 samples in 10 µs seems to be 2.5 Msample/s, exceeding the nominal 1 Msample/s spec. I have no explanation.

A 10 kHz square wave shows a blip just before each transition that isn’t on the actual signal:

DSO150 - square wave 10 kHz 20 us-div
DSO150 – square wave 10 kHz 20 us-div

At 50 kHz, there’s not much square left in the wave:

DSO150 - square wave 50 kHz 10 us-div
DSO150 – square wave 50 kHz 10 us-div

And, just for completeness, a 200 kHz square wave completely loses its starch:

DSO150 - square wave 200 kHz 10 us-div
DSO150 – square wave 200 kHz 10 us-div

A 10% (-ish) duty cycle pulse at 25 kHz has frequency components well beyond the scope’s limits, so it’s more of a blip than a pulse:

DSO150 - pulse 25 kHz 10 us-div
DSO150 – pulse 25 kHz 10 us-div

The pulse repetition frequency beats with the scope sampling and sweep speeds to produce weird effects:

DSO150 - pulse 25 kHz 100 us-div
DSO150 – pulse 25 kHz 100 us-div

Tuning the pulse frequency for maximum weirdness:

DSO150 - pulse 15 kHz 200 us-div
DSO150 – pulse 15 kHz 200 us-div

None of this is unique to the DSO150, of course, as all digital scopes (heck, all sampled-data systems) have the same issues. The DSO150’s slow sampling rate just makes them more obvious at lower frequencies.

Key takeaway: use the DSO150 for analog signals in the audio range, up through maybe 50 kHz, and it’ll produce reasonable results.

Using it for digital signals, even at audio frequencies, isn’t appropriate, because the DSO150’s low bandwidth will produce baffling displays.


Baofeng UV-5: Squelch Tail Elimination

Baofeng UV-5 radios can (mostly) eliminate the loud hiss heard at the end of a transmission before the squelch kicks in after the received carrier drops: Menu → 34 STE → ON. A detailed description of the option suggests it’s a 55 Hz subaudible tone sent for 250 milliseconds after the sender releases the PTT and before the transmitter stops sending, with the receiver muting its audio during the tone. Obviously, this requires a Baofend radio at each end of the conversation, which applies to our bikes.

Saying “laaaa” while kerchunking (into a smaller dummy load than the hulk) with STE OFF:

Baofeng - STE OFF - laaaa
Baofeng – STE OFF – laaaa

Compared to the received audio, the squelch tail hiss is really really loud.

Then with STE ON:

Baofeng - STE ON - laaaa
Baofeng – STE ON – laaaa

You can see the STE tone reception start about 250 ms before the audio cuts off, although it’s not at all clear the audio is muted on either end. In any event, there’s no squelch tail worth mentioning, even if there’s an audible tick when the STE tone starts.

Saying nothing with STE ON:

Baofeng - STE ON - silent
Baofeng – STE ON – silent

It’s unlikely the audio output would include the subaudible tone, but you might convince yourself something happens in the 250 ms between the STE blip near midscreen and the final pop (now clipped) as the audio drops.

All in all, a definite improvement!

Baofeng UV-5: Audio Attenuation and Knob Pointer

Perhaps because we’re using better quality earbuds, the Baofeng UV-5 radios on our bikes produce extremely loud audio, even with the volume knob just above its power-on click. Reducing the volume requires a series resistor downstream of the diodes clipping the pops:

Baofeng Headset Audio Attenuation
Baofeng Headset Audio Attenuation

The color codes come from previous work.

Because we have different earbuds and different hearing, my radio has a 140 Ω resistor and Mary’s has a 430 Ω resistor. Getting the right value requires a few iterations of on-road testing, but it’s not particularly critical; the volume knob should end up roughly in the middle of its range.

For now, all the “circuitry” lives among layers of Kapton tape:

Baofeng headset wire plate - detail
Baofeng headset wire plate – detail

Speaking of volume knobs, Baofeng radios have large flat-top cylindrical knobs (unlike Wouxun’s fluted knobs), so I added a pointed snippet of reflective tape to make the position visible:

Baofeng volume knob - reflective pointer
Baofeng volume knob – reflective pointer

The flash lights it up, but there’s enough backlighting behind your (well, my) head to make it easily visible under normal conditions. Once you figure out the proper volume, it’s easy to set the pointer in that direction before every ride.

To the road!