The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Category: Machine Shop

Mechanical widgetry

  • Tektronix 492 Spectrum Analyzer Backplane Pin Spacing

    Tek 492 Memory Board
    Tek 492 Memory Board

    My buddy Eks asked me to help fix his new-to-him and guaranteed broken Tek 492 spectrum analyzer, which turned into a tour-de-force effort. One sub-project involved sucking the bits out of an existing “known-good” Tek memory card, which meant building a backplane connector and a circuit that behaved like a 6800 microcontroller… fortunately, it could be a lot slower.

    [Update: It seems searches involving “Tektronix 492” produce this page. You may also be interested in these posts…

    If those aren’t what you’re looking for, note that the correct spelling is “Tektronix“.

    Good luck fixing that gadget: it’s a great instrument when it works!]

    You can tell just by looking that this board was designed back in the day when PCB layout involved flexible adhesive tape traces and little sticky donut pads. Ground plane? We don’t need no stinkin’ ground plane!

    Actually, it’s a four-layer board done with the usual Tek attention to detail. They didn’t need a ground plane because they knew what they were doing. Remember, this is in a spectrum analyzer with an 18-GHz bandwidth and 80 dB dynamic range; a little digital hum and buzz just wouldn’t go unnoticed.

    Tek 492 Backplane Geometry
    Tek 492 Backplane Geometry

    Anyhow, the backplane pins are on a 0.150-inch grid within each block. The center block (pins 13-36) is 0.200 inches from the left block (pins 1-12) and 0.250 from the right block (pins 37-60).

    That means the left and right blocks are neatly aligned on the same 0.150-inch grid, with the middle block offset by 50 mils. You can’t plug the board in backwards unless you really work at it.

    Of course, Eks had some genuine gold-plated Tek pins in his stash: 24 mils square and 32 mils across the diagonal. They have 1/4″ clear above the crimped area that anchors them to the black plastic spacer and are 1/2″ tall overall. They’re not standard header pins, but I suspect you could use some newfangled pins in a pinch.

    Here’s what the reader board finally looked like, hacked traces and all, with the board connector to the rear. The memory board didn’t use all the backplane pins, so I only populated the ones that did something useful. The power-and-ground pins (left side of right pin block) stand separately from the other because I had to solder them to both the top and the bottom of the board: no plated-through holes!

    Tek 492 Memory Board Reader
    Tek 492 Memory Board Reader

    I cannot imagine this being useful to anybody else, but I defined an Eagle part for the connector so I could CNC-drill the board. Drop me a note and I’ll send it to you.

    [Update: this turned into a Circuit Cellar column, so you can fetch a ZIP file from their FTP site that has all manner of useful stuff.]

    Memo to Self: The drill size follows the pin’s diagonal measurement… not the side! Duh.

  • Front Derailleur Cable Breakage: Prevention Thereof

    Front derailleur cable with broken strand
    Front derailleur cable with broken strand

    Although recumbent bikes use ordinary bicycle components, they tend to have somewhat different frame geometries (to put it mildly). Our Tour Easy ‘bents seem to put a particular strain on the front derailleur cables, perhaps because the cable enters from a different angle than the derailleur designers expected. The little finger that’s supposed to guide the cable actually concentrates all the bending force at one spot… precisely where the cable breaks.

    If you look carefully, you’ll see a little brass disk (between the derailleur body and cable) that cradles the cable. I made that for the previous derailleur, but this one has Yet Another Geometry. I know there’s a difference between “high pull” and “low pull” front derailleurs, and perhaps this is the wrong one for this application, but there seems no algorithmic way to sort this stuff out.

    Cable guide pulley
    Cable guide pulley

    The solution is to make Yet Another Cable Guide Pulley, with a groove around the perimeter, an off-center hole, and a notch to clear the finger. It’s not exactly a pulley, but I’m not sure what else to call it. Maybe just a cable guide?

    This was a quick-and-dirty manual lathe project, two days before leaving on a trip: turn down some brass stock, put a groove around the perimeter, part it off, drill a hole, and cut the notch. Not a trace of CNC to be found: all done by guess and by gosh, marked out with Sharpies on the actual part in real time running.

    The general notion is that the cable rides the groove smoothly throughout the derailleur’s entire travel range and, thus, doesn’t bend around the finger. This changes the shift geometry just slightly, but, fortunately, long-wheelbase ‘bents have a sufficiently relaxed chainline that indexed front shifting isn’t much of a problem even with slightly misplaced positions. Besides, that’s why SRAM grip-shifter have all those clicky stops, right?

    (The shifting is actually a bit goobered, with the outer chainring shift a bit too close to the middle. When we get back, I’ll re-do this with somewhat more attention to detail.)

    Pulley in action
    Pulley in action

    Here’s what it looks like in action. I’ve had good success with this sort of thing over the years, so I think this one will work just fine, too. It simply takes one broken cable on each new derailleur to spin up enough enthusiasm for making Yet Another Pulley…

  • Bicycle Water Bottle Cap: Relaxed

    Water bottle cap
    Water bottle cap

    Being cyclists, we were doing the resuable-water-bottle thing long before it became trendy, but now that we use hydration packs, we just tote bottles along when we’re driving or on some other sort of outing. Eventually the bottles wear out / get lost and we page a new one in from the essentially infinite stash in the bottle cupboard.

    This one had a cap that simply couldn’t be pried open with bare hands, no how, no way. I eventually got it open by main force and the threat of high temperatures.

    Turns out there were two problems: the aperture in the pull-up ring is a wee bit small on the sealing nub and the ridge on the screw cap is about two wee bits large for the recess in the ring.

    The former succumbed to an O (letter Oh) drill, which I pulled & pushed through the hole by hand to enlarge the aperture from 0.320 to 0.332. It still seals reasonably well, although it’ll pee a thin stream under more pressure than you should apply to such a bottle, which means I put a slight scratch on the aperture.

    The latter required gently shaving the ridge with a box cutter (gasp). It’s still rather stiff, but entirely workable. That doesn’t affect the seal, because the ring’s skirt is a snug fit against the screw cap.

    Why not just throw the fool thing out? After all, it’s just a freebie water bottle…

    We run on the “Use it up, wear it out, fix it once, wear it out again, then put it on the shelf because maybe you can use the parts for something” principle.

    Now, that’s not the way things are done these days, but it works for us…

  • Bike Helmet Mirror Re-Repair

    Socket with brass reinforcement
    Socket with brass reinforcement

    The front ball joint on the mirror on Mary’s helmet loosened enough that the mirror blew out of position every time we got up to a decent traveling speed. I’ve repaired these mirrors several times before; they’re plastic and tend to fracture / wear out / break at inconvenient moments.

    The first pic shows the mirror (the black surface is reflecting the dark floor joists overhead) with an old blob of epoxy that repaired a break in the outer socket. The socket originally had stylin’ curves joining it to the mirror, which proved to be weak spots that required epoxy fortification.

    This time the socket split axially on the side away from the mirror, which released the pressure on the ball socket that seats into it. I found a chunk of brass tube that fit snugly over the socket, then carved some clearance for the existing epoxy blob. The key feature is that the tube remains a ring, rather than a C-shaped sheet. to maintain pressure around the socket.

    Clamping the reinforcement ring
    Clamping the reinforcement ring

    Here are the various bits, with the reinforcing ring clamped in place. I coated the socket exterior with JB Weld epoxy, slipped the ring in place, and tapped it down with a brass hammer to seat flush with the front face of the socket. That left gaps between the socket opening and the tube that I eased more epoxy into with an awl. A bit more epoxy around the exterior smoothed over that ragged edge.

    The strut at the bottom of the picture ends in a ball joint held by a socket that slips into the mirror socket. The loose brass ring above the mirror is some shim stock that I added some years ago to take up slop between the ball socket and the mirror socket and tighten the ball joint. I suppose that pressure eventually split the outer socket, but so it goes.

    Repaired mirror joint
    Repaired mirror joint

    The clamp squished the outer socket enough to snug it around the ball socket, so when I reassembled the mirror it was fine. To be sure, I dunked the ball in my lifetime supply of Brownell’s Powdered Rosin for a bit more non-slip stickiness.

    I have a box full of defunct bike helmet mirrors, dating back to those old wire-frame square mirrors that clamped onto the original Bell helmets. The newer plastic ones just don’t last; we ride our bikes a lot and even fancy engineering plastic isn’t nearly durable enough. A few bits of metal here and there would dramatically improve the results!

    I’m going to build some durable wire-frame mirrors, but … this will keep us on the road for a while. I suppose I should make a preemptive repair on my helmet mirror while I’m thinking of it…

  • Bike Rim Reflectorization

    Bike wheel with retroreflective tape
    Bike wheel with retroreflective tape

    Here’s a quick-and-easy way to improve the odds of your arriving home safely after dark: add snippets of retroreflective tape to the inside of the rims on your bike.

    Do half the rim in one color and leave the other half untaped (or taped in a contrasting color) so that the rim flashes as the wheel rotates. I originally applied orange tape, of which I have very nearly a lifetime supply, then added white when I got a sheet as part of a surplus deal.

    At 15 mph the 20-inch front wheel blinks at about 4 Hz, which is wonderfully attention-getting. The rear wheel, a more common 700C size, blinks at 3 Hz.

    It helps to measure the space between spokes, then set up a template to cut all the tape pieces the same length. Wipe the big chunks of dirt off the rim, then remove the remaining grunge with alcohol so the tape actually sticks.

    New York State vehicle law considers reflectorized tires as equal to those in-the-spokes reflectors, which is a Good Thing.

    The more you look like a UFO after dark, the less surprised the drivers are and the less hassle you get.

  • Keeping the Screws in Sherline Hold-Down Clamps

    A small improvement: add a snippet of heat stink shrink tubing to the screw in the L-shaped hold-down clamps and the screw won’t go walkabout in your tooling widget case.

    Make it the same length as the distance from the clamp to the surface and it’ll remind you how far to screw on the T-nut when you swap the clamps from tooling plate to milling machine table.

    The Sherline Mill Vise (PN 3551) comes with a set of clamps. They’re also available separately as the 4-Jaw Hold-Down Set (PN 3058).

  • Red Filter for White LED Bike Headlight

    White 5-LED headlight
    White 5-LED headlight

    As I mentioned there, we have white LED bike headlights clamped to the amateur radio antennas on our bikes, facing rearward to eliminate the “But, Officer, I didn’t see him” line from the accident investigation. That works fine during daylight hours, but it’s rather blinding after dark and, in any event, taillights are supposed to be red (after 1 Nov 2009, they may also be amber).

    The easiest way to get that result, without having to tote along Yet Another Light, is to slip a red filter over the white LED lens. This dramatically reduces the light output, because the yellow phosphor used to get white light out of what’s basically a blue LED doesn’t emit much energy in the red end of the spectrum, but it’s plenty good enough to be seen from the requisite 300 feet.

    Amber filters would be a much, much better match to the phosphor and I’ll use them next year when they’re legal.

    For what it’s worth, we’ve discovered that the more we look like UFOs after dark, the more clearance we get. The bikes are extensively reflectorized and lighted, plus we have reflective arm and leg bands. If somebody hits us, it’s because they did it intentionally; that’s usually the story with drunks and punks, alas.

    Red filter components
    Red filter components

    I cut two transparent disks from ordinary electronics packaging material, plus a red disk from the Primary Red filter material mentioned there, stacked them on the headlight, and fired some big heat stink shrink tubing around them. The tubing extended maybe 3 mm past the end of the headlight and shrank into a neat lip that matched the bezel around the lens.

    The tool to have for this sort of job is an Olfa Compass Circle Cutter. It leaves a pin prick in the center of the circle, but if you’re gentle that won’t be a problem in this application.

    The shrunken tubing will be exceedingly difficult to pull off the headlight, so you may want to wrap a layer of tape around the bezel before shrinking. Peel the tape off when you’re done and the tubing will have a few mils more clearance.

    No adhesive on earth will stick to both the polypropylene disks and the heatshrink tubing, but you can try silicone snot if you want. I made the disks just slightly larger than the bezel so that the tubing captures them as it shrinks. These things spend much of their lives in a ziplock baggie, so durability isn’t an issue.

    Red filter installed
    Red filter installed

    In any event, the filter looks like this when it’s installed. Because of the odd way I mounted the headlights, the side lenses aren’t visible (and they’re white, not red), but we have plenty of other light visible from the side.

    For the straight dope on current NYS bicycle laws, go there, click on the “Laws of New York” link, search for “bicycle”, then click on section 1236. It’s New York’s idea of a useful Web interface: get over it.

    The bezels on our lights are beginning to crack, so it’s probably time to start thinking about a killer street-legal day/night amber taillight. High intensity LEDs are dirt cheap these days…