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.

Tag: Repairs

If it used to work, it can work again

  • Roof De-Icing Cable Numbers

    Having recently kibitzed on a project using de-icing cables (with some success) to soften PVC pipe for bending, herewith the useful numbers.

    Data printed on the original cable:

    • 100 ft length
    • 120 VAC
    • 800 W

    Derived values:

    • 6.7 A = 800 W / 120 V
    • 8 W/ft = 800 W / 100 ft
    • 1.2 V/ft = 120 V / 100 ft
    • 18 Ω = (120 V)2 / 800 W
    • 180 mΩ/ft = 18 Ω / 100 ft

    The starting point was a 62 ft length of the cable, as I’d long ago converted the end into a heated bed for starting plants early in the spring. That presented a resistance of 11 Ω, drew a current of 11 A, and dissipated 1.3 kW at 21 W/ft. A kilowatt-class dimmer handled the load, but adjoining sections of the cable got hot enough to melt the insulation and terminate the experiment.

    A shorter length of cable might be suitable for a cheap laptop brick power supply. To keep the dissipation under, say, 10 W/ft, we have:

    • 7.5 A = sqrt( 10 W/ft / 180 mΩ/ft )
    • 1.3 V/ft = 7.5 A * 180 mΩ/ft

    The Dell D220P-01 brick on the M2 provides 12 V at 18 A (!) and costs under $20 on eBay:

    • 9 ft = 12 V / 1.3 V/ft
    • 90 W = 12 V * 7.5 A
    • 1.6 Ω = 9 ft * 180 mΩ/ft

    You could run two 9 ft lengths cables in parallel from the same hulking brick. Whether that’s enough to soften a length of PVC pipe from the inside, without having the insulation get all melty, that’s another question…

  • HP ScanJet 3970 Lid Hinge Re-Repair

    As expected, that repair didn’t last very long at all; one hinge fractured along the same line as before. This time, however, we had a visit already in-plan, so I brought along my solvents and clamps.

    Perhaps you wondered how I could have been so remiss as to not brace those thin white flanges. One picture of the unbroken hinge in the “lid down” position is worth a thousand words:

    HP 3970 Scanjet - intact hinge
    HP 3970 Scanjet – intact hinge

    Need more? Here’s another thousand words from the other side:

    HP 3970 Scanjet - intact hinge pivot
    HP 3970 Scanjet – intact hinge pivot

    As the lid opens, the gray tab pivots toward the edge of the lid until it’s nearly parallel, at which point all of the force tries to yank those two flanges apart and then crack the tiny solid part at the pivot pin.

    Eventually, it succeeds. This is a view of the scanner base with the gray tab inserted in its slot, with the broken hinge in the “lid up” position:

    HP 3970 Scanjet - broken hinge pivot
    HP 3970 Scanjet – broken hinge pivot

    Clever design, no?

    I was unable to extract the broken fragment from the gray tab (actually, unwilling to apply more force, as I cracked part of the gray ring around the hinge pin), so this became an in situ repair. Once again, I applied solvent glue and squished the pieces together:

    HP 3970 Scanjet - glued hinge
    HP 3970 Scanjet – glued hinge

    And clamped it while we ate lunch:

    HP 3970 Scanjet - hinge clamping
    HP 3970 Scanjet – hinge clamping

    The brass rod applies the clamping force to the fractured part of the hinge through the pivot point. This isn’t the most stable clamp arrangement you’ve ever seen, but it worked well enough.

    I pushed the scanner back half a foot, so the lid now clunks against the wall just before the hinges reach their limit. Maybe they’ll survive until the next visit…

  • Coaxial Power Plug Tip: Extraction Thereof

    Another coaxial power plug lost its tip inside a lithium ion battery pack used with the APRS + Voice circuitry on our bikes, as I could barely see at the bottom of the socket:

    LiIon Pack - output socket
    LiIon Pack – output socket

    Rather than cutting the pack apart, I buttered up the end of an intact plug with some ABS solvent glue (a hellish homebrew mixture of acetone and MEK), rammed it into the socket, and held it in place for a minute:

    LiIon Pack - undamaged plug insertion
    LiIon Pack – undamaged plug insertion

    The tip emerged on the first try:

    LiIon Pack - rescued plug tip joined
    LiIon Pack – rescued plug tip joined

    Even better, it cracked off the plug without too much effort:

    LiIon Pack - rescued plug tip separated
    LiIon Pack – rescued plug tip separated

    More solvent glue and a few hours of clamping worked fine:

    LiIon Pack - clamped plug tip
    LiIon Pack – clamped plug tip

    That cable is now back in service.

  • BOB Yak Trailer: New New Grenade Pin Straps

    Somehow, I thought those neoprene O-rings that replaced the grenade pin straps on my BOB Yak trailer would last more than one season, but they’ve already rotted out. A bit of rummaging produced a hank of rubber gasket intended to secure window screen in its aluminum frame, so it’s presumably better suited to an outdoor life than O-rings; it comes without provenance, so I have no idea what it’s made of.

    A few snips, a handful of cable ties, and it’s all good again:

    BOB Yak Grenade Pins - new strap
    BOB Yak Grenade Pins – new strap

    But I’m not expecting a decade out of these straps, that’s fer shure…

  • Stapler Latch Spring Replacement

    It’s garden fence stapling season, which means it’s time for the annual stapler annoyance. There’s supposed to be a plastic tab angling under the right side of the latch, pressing against the bottom of the staple channel and forcing the triangular tab on the top out through a small opening. All that’s left is the stub:

    Stapler latch - broken spring tab
    Stapler latch – broken spring tab

    It looks like it ought to be a great 3D printing project, but I came to my senses and snipped off a length of phosphor bronze spring stock, rolled it up, and positioned it inside the opening:

    Stapler latch - metal spring fitting
    Stapler latch – metal spring fitting

    The latch slides into the staple channel until the pin on the side you can see above engages a hole in the channel. The spring looked like this on the way in:

    Stapler latch - before closing
    Stapler latch – before closing

    And now it’s ready for action:

    Stapler latch - in place
    Stapler latch – in place

    Works like a champ and took maybe 15 minutes, tops, to accomplish. Shame it took me a few decades to get around to fixing it, but I finally dropped the clipped-and-filed nail I’d been jamming between the latch and the channel one too many times…

  • Floppy Drive Support in System Rescue CD

    For obscure reasons, I’m kibitzing on a project to rehabilitate an ancient Brother industrial sewing machine. It has a floppy disk drive that stored various custom stitch patterns, but it now crashes / jams / stalls after loading any of the patterns.

    I booted an old PC that had a floppy drive using System Rescue CD, only to discover that /dev/fd0 didn’t exist. A bit of search-fu revealed that the floppy kernel module isn’t automagically loaded: a simple modprobe floppy did the trick, after which mount -o ro /mnt/floppy worked fine (it’s in fstab, even if the kernel module isn’t loaded).

    The floppy was in IBM PC-DOS format, as you might expect in a system with ICs date-coded in the early 90s and an 8085 CPU (not an 8088 or 8086). Applying dd bs=512 if=/dev/fd0 of=/tmp/floppy.bin produced a measly 12 kB file containing the boot sector, many binary zeos, a line or two of pinball panic, and more binary zeros up to the 0x3000 file size, where it ended due to a hard read error.

    So now we know there’s no point in trying to run from the floppy, because there’s nothing to run. According to the instructions, the sewing machine can write to the floppy, so we can examine some of those results to see what the data structures should be.

    A new-to-me off-lease Dell Optiplex 760 that I just picked up (for the M2’s LinuxCNC controller) has a floppy drive, so I can let that old hulk go to the recycler. I don’t see a big duty cycle for the floppy, but ya gotta have stuff…

  • Garden Sprayer: End of Life

    The hose attached to this garden sprayer had failed last season, but the hose fitting had become one with the sprayer. Soaking it with penetrating oil for far longer than seemed necessary didn’t help, so I tried brute force:

    Garden sprayer hose fitting
    Garden sprayer hose fitting

    After convincing myself that wasn’t going to work, I cut the fitting off and tried the old standby of collapsing the threaded shell inward with a small punch:

    Garden sprayer - rolled-in fitting
    Garden sprayer – rolled-in fitting

    That didn’t work, either: the shell really had become one with the sprayer.

    As it turned out, the plastic sprayer body had begun to crack in several high-stress locations and would shortly become Yet Another Project. I cut my losses and tossed the hose and the sprayer.