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

  • Read-Only MicroSDHC Card

    I iterated this sequence three times before I caught on:

    • ssh into Raspberry Pi
    • Edit /etc/rc.local, save changes
    • Reboot, observe the changes had no effect
    • cat /etc/rc.local shows no changes

    Then I:

    • Edited / saved
    • Listed the file to verify the changes
    • Rebooted, observe no effect from changes
    • Listed the file again: the changes were gone

    Huh.

    Defunct 8 GB MicroSDHC card
    Defunct 8 GB MicroSDHC card

    It turns out the card went read-only without warning, so I was displaying the contents of the file cache buffers after the edit, not the data stored on the card. Rebooting started with empty caches, read the previous file contents, and behaved accordingly.

    The F3 utilities now live in the Ubuntu repository and no longer require compiling from source. The result:

    sudo f3probe --time-ops /dev/sdb
F3 probe 6.0
Copyright (C) 2010 Digirati Internet LTDA.
This is free software; see the source for copying conditions.

WARNING: Probing normally takes from a few seconds to 15 minutes, but
         it can take longer. Please be patient.

Probe finished, recovering blocks... Done

Bad news: The device `/dev/sdb' is damaged

Device geometry:
	         *Usable* size: 0.00 Byte (0 blocks)
	        Announced size: 7.35 GB (15415296 blocks)
	                Module: 8.00 GB (2^33 Bytes)
	Approximate cache size: 0.00 Byte (0 blocks), need-reset=no
	   Physical block size: 512.00 Byte (2^9 Bytes)

Probe time: 164.4ms
 Operation: total time / count = avg time
      Read: 107.1ms / 4098 = 26us
     Write: 56.6ms / 2049 = 27us
     Reset: 0us / 0 = 0us

    That card has been kicking around for a while and started out as a no-name generic in some random gadget. Of course, those fancy Sony MicroSD cards weren’t shining examples of durability, either.

    I’m mildly astonished the streaming player worked perfectly with what amounts to a read-only filesystem, but that’s what caching is all about: there was no need to write the data to “disk”.

  • Fordham FG-801 Function Generator Recalibration

    While I had the case open, I checked the FG-801’s calibration:

    Fordham FG-801 Fn Gen - circuit board
    Fordham FG-801 Fn Gen – circuit board

    Look at all those parts!

    The raw filtered DC power supplies run a bit high and the output voltages & frequencies were off by a little, but not too much after all these years.

    Page 11 of the instruction manual gives the setup and calibration adjustments (clicky for more dots):

    Fordham FG-801 Manual - Page 11
    Fordham FG-801 Manual – Page 11

    Page 12 gives some values that should be true:

    Fordham FG-801 Manual - Page 12
    Fordham FG-801 Manual – Page 12

    For whatever reason, the manual isn’t available on The InterWebs, so here it is for your amusement:

    Fordham FG-801 Sweep Function Generator – Instruction Manual.pdf

  • Fordham FG-801 Function Generator Power Switch

    The power switch in my trusty Fordham FG-801 Function Generator failed with an accumulation of oxidation / crud on the contacts. That’s fix-able, but the switch contained not one, but two powerful springs, and puked its guts all over the floor around the Squidwrench Operating Table. Even with (a preponderance of) the parts in hand, I couldn’t figure out how to reassemble the thing; the only way out was to replace the switch.

    The OEM switch had a 0.360+ inch diameter pushbutton that fit into a ⅜ inch hole and, alas, my remaining stock of line-voltage switches had toggle levers and used ¼ inch holes. So I converted a bit of aluminum rod into a suitable bushing:

    Fordham FG-801 Fn Gen - new switch hardware
    Fordham FG-801 Fn Gen – new switch hardware

    The lock washer in the middle started with a much wider tab that I filed down into a tooth for the dent from a #2 center drill. Protip: center drills don’t walk off like twist drills, even when you hand-hold the front panel at the drill press with all the electronics dangling below.

    The bushing dimension doodle:

    Fordham FG-801 Function Generator - Replacement Switch Bushing
    Fordham FG-801 Function Generator – Replacement Switch Bushing

    The internal wiring routes the 120 VAC line conductor to the switch, then to the fuse, then to the transformer. I don’t know whether it’s better to have an unfused switch or an unswitched fuse (surely there’s a UL spec for that), but I didn’t change anything. The new switch, being slightly smaller and mounting directly on the panel, required a new wire (the blue one) from the fuse:

    Fordham FG-801 Fn Gen - power switch - installed
    Fordham FG-801 Fn Gen – power switch – installed

    The OEM switch mounted on two round brass standoffs and, wonder to tell, the new switch fit between them!

    From the front, the new switch looks like it grew there:

    Fordham FG-801 Fn Gen - switch in action
    Fordham FG-801 Fn Gen – switch in action

    The PCB mounts to the top of the case with one screw and four hexagonal brass standoffs. The standoffs have 6-32 tapped holes on one end and a 6-32 stud on the other; one of those stud had broken off. A 6-32 stainless steel screw secured in a clearance hole with a dab of epoxy solved that problem:

    Fordham FG-801 Fn Gen - standoff stud
    Fordham FG-801 Fn Gen – standoff stud

    I stood it vertically and tweaked the screw to be perpendicular while the epoxy cured.

    Memo to Self: The next time around, put a nut on the stud to make sure the answer comes out right. I didn’t do this time to avoid epoxying the nut to the standoff.

    Done!

  • Maxell CR2032 Lithium Cell: Early Failure

    The Hobo datalogger buried in the dirt under the patio kvetched about a low battery, which produced this surprising result:

    Maxell CR2032 cell - early failure
    Maxell CR2032 cell – early failure

    Cells from the same lot have been doing just fine in the other dataloggers, so I hope this is a one-off weak cell and not the harbinger of another run of dead cells.

  • Snowthrower Shear Bolts

    The snowthrower (I’ve always called it a snowblower, but that’s just me) ate a ski pole (*), handle-tether-end first, and the right-side shear bolt worked perfectly when the right-hand auger slammed to a stop. A bit of drift punch rapping extracted the sorry lump at the bottom:

    Sheared MTD Snowthrower Bolts
    Sheared MTD Snowthrower Bolts

    The missing nut and bolt head may eventually surface, but I’m not losing any sleep over them.

    I popped a replacement shear bolt from the heap (thank you, Aitch!) and thought the nut went on rather stiffly. The nuts have a crimp in the middle to make them vibration-proof, but this one seemed stiffer than usual and, lo and behold, the bolt snapped just before I thought the nut had gotten far enough.

    The nut on the second replacement shear bolt required much less torque, didn’t (let me) snap the bolt, and I finished the mission. That’s the third or fourth shear bolt I’ve used since getting the thrower in 2007, so there’s a package of six in transit.

    Part Number 710-0809A, 5/16-18 x 1.5 inch.

    (*) One of Mary’s gardening cronies works for a sporting goods store, has access to an unlimited supply of slightly bent ski poles, and shares the bounty for use as garden stakes.

  • Mica Compression Capacitor: Unsolderable Pins

    The mica compression capacitors have a finish on the pins that turned out to be completely un-solderable:

    Mica compression capacitor - solder vs pin
    Mica compression capacitor – solder vs pin

    Some casual searching suggests this is a problem with sulfur contamination of the tin-lead solder layer. I can’t vouch for any of that, as the flat areas forming the capacitor seem to be silver-plated, but …

    After some flailing around, I completely disassembled the capacitor, applied 800 grit sandpaper to remove all of the solder / flux / corrosion / tarnish / surface plating from the pins, dabbed on some RMA flux, then applied a thin layer of solder to both sides. Fortunately, the capacitor could be disassembled; they don’t make ’em like that any more.

    The solder layers must be thin, because the slots in the ceramic base must pass two or three pins apiece: four or six solder layers add too much thickness. Solder-wick is my friend!

    For reference, the 700 pF side looks like this:

    Mica compression capacitor - 700 pF disassembled
    Mica compression capacitor – 700 pF disassembled

    The steel washer does not have a mica washer underneath (as does the washer on the 400 pF right side). The two grayish steel plates go on the top.

  • Improvised Repairs Done Wrong

    Mary’s relatives encountered this repair in a rental flat during Thanksgiving week:

    Door handle - hex head bolt
    Door handle – hex head bolt

    Don’t have a hex bolt with the right thread? No problem: just use a sheet-metal screw, perhaps with a self-drilling point:

    Door handle - metal screw
    Door handle – metal screw

    Those hex heads let you apply more torque with less risk of stabbing yourself in the palm, which strikes me as an all-around Good Thing. I prefer socket-head cap screws, myself, but I’ll admit they’re an acquired taste.

    I’d like to think I wouldn’t do that …