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: Electronics Workbench

Electrical & Electronic gadgets

  • Ooma Telo 2: Speaker FAIL

    The tiny voice inside our Ooma Telo 2 box died, although the VOIP phone service continued to work fine. A bit of searching showed the speaker seems to be the weak link.

    Well, I can fix that.

    Start by prying the recessed top panel off the case:

    Ooma Telo 2 - upper case latches
    Ooma Telo 2 – upper case latches

    Remove the circuit board to expose the tiny speaker, taking care not to rip the tiny wires out of the tiny connector:

    Ooma Telo 2 - OEM speaker to PCB
    Ooma Telo 2 – OEM speaker to PCB

    You can’t measure a dead speaker, but it seems to be an 8 Ω unit.

    The speaker sits in a rubber surround, with a foam rubber cushion against the PCB, tucked into a walled garden stiffening the case:

    Ooma Telo 2 - speaker port
    Ooma Telo 2 – speaker port

    I don’t happen to have a tiny 8 Ω speaker, but I do have a bunch of small speakers (Update: 28 mm OD), so I bulldozed those walls with a flush cutting pliers and a bit of cussing to make room:

    Ooma Telo 2 - modified speaker port
    Ooma Telo 2 – modified speaker port

    Nibble an adapter ring to match the rim of the new speaker, thereby routing the sound out those little holes, and hot-melt glue it in place:

    Ooma Telo 2 - speaker adapter
    Ooma Telo 2 – speaker adapter

    Hot-melt glue the new speaker in place atop the adapter, taking care to fill all the edges / cracks / crevices below it with an impenetrable wall of glop:

    Ooma Telo 2 - replacement speaker installed
    Ooma Telo 2 – replacement speaker installed

    The sealing part turns out to be critical with these little speakers, because a leak from front to back will pretty much cancel all the sound from the cone.

    Cut the wires off the old speaker, affix to the new one, replace the PCB, snap the case lid in place, and it sounds better than new.

    Millions of transistors in those ICs, but Ooma can’t spec a good speaker? Maybe they should have used a bigger speaker to begin with; ya never know.

  • Dell U2711 Monitor: FAIL

    My landscape monitor, a six-year-old Dell U2711, died after a few days of flickering and failure-to-start. As you’d expect with any old electronics, particularly from Dell, it’s the electrolytic caps:

    Dell U2711 Monitor - failed caps
    Dell U2711 Monitor – failed caps

    All of the black-cased caps on the board had bulged cases:

    Dell U2711 Monitor - failed FOAI cap - detail
    Dell U2711 Monitor – failed FOAI cap – detail

    They’re (allegedly) made by FOAI, for whatever that’s worth.

    They’re not really capacitors any more:

    Dell U2711 Monitor - 100 uF 5 ohm cap
    Dell U2711 Monitor – 100 uF 5 ohm cap

    I replaced all of them with cheap eBay caps to no avail. Spot-checking the other (“brown”) caps on the logic board showed they were still good, but the power supply board is firmly glued in place and I can’t get to the HV cap.

    A new monitor arrived two days later and it’s all good again.

  • Transistor Leads vs. Antistatic Foam

    Why you shouldn’t use antistatic foam for long-term storage:

    Anti-static foam - decades of corrosion
    Anti-static foam – decades of corrosion

    The lump emerged from Mad Phil’s parts stash, now residing under a bench at Squidwrench. The 952 date code on the HEP802 JFET suggests he tucked it in around 1980; you’re looking at nigh onto four decades of corrosion.

    Memo to Self: use it or lose it!

  • Hearphone Deterioration

    I bought my Bose Hearphones in late August 2017, so they’re just shy of two years old, and have used them more-or-less daily since then. Although the innards still improve my hearing, the exterior is falling apart:

    Bose Hearphones - cosmetic repairs
    Bose Hearphones – cosmetic repairs

    The conspicuous blue tips come from silicone tape holding the “soft touch” silicone shell together:

    Bose Hearphones - detached band cover
    Bose Hearphones – detached band cover

    The white line seems to be silicone glue holding the hard cover plate to the equally hard base. So far, it’s working, but the two-piece soft cover is peeling away from the very thin adhesive (?) holding it to the hard parts.

    The silicone glue under the flexy cover on the control pod along the right earbud cable hasn’t fared as well:

    Bose Hearphones - failed control cover
    Bose Hearphones – failed control cover

    I blobbed ordinary RTV silicone under the cover, ignoring the caveats about acetic acid corrosion, because I don’t have any platinum-cure silicone on the shelf.

    When the blue tape wears out / falls off, I’ll replace it with black silicone tape going further up the ring to hold the rest of the soft cover in place:

    Bose Hearphones - cosmetic repairs - detail
    Bose Hearphones – cosmetic repairs – detail

    The ear buds have soft silicone strain relief tubes around the cables. The friction holding them in place failed long ago and, because no adhesive will work with silicone, I wrapped enough double-sided tape around the cables to produce a sticky lump jamming them in place:

    Bose Hearphones - ear piece strain relief
    Bose Hearphones – ear piece strain relief

    A bit of the muck sticks out on both ends and I expect to replace the tape every now and again:

    Bose Hearphones - earpiece repairs - detail
    Bose Hearphones – earpiece repairs – detail

    I also expect to replace the non-replaceable lithium battery / cell in about a year, as they’re now barely adequate for a day’s use.

    Fortunately, I can’t see any of this hackery while I’m wearing the things:

    my face I don’t mind it,

    Because I’m behind it —

    ‘Tis the folks in the front that I jar.

    https://www.azquotes.com/quote/1243103

    [Update: Bose apparently had a QC failure on the silicone covering and, much to my surprise, swapped me entirely new Hearphones. The new covering feels slightly different, the USB cable hatch is a distinct piece of plastic, and maybe it’ll survive until the battery gives out. Color me satisfied! ]

  • 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.

  • M20 Camera Operation

    A reader asked how the M20 camera mount on my bike works with respect to the camera’s clock; this description explains a few things missing from the original writeup.

    SJCAM M20 Mount - Tour Easy side view
    SJCAM M20 Mount – Tour Easy side view

    Do you have to set the time & date at start of every ride?

    The internal clock shuts down about ten seconds after you pull the battery. If-and-only-if you swap batteries fast enough, it’ll keep time forever. Screw up once and it snaps back to Epoch Zero.

    “Car mode” automagically begins recording when USB power goes on, but the manual advises:

    TIP: When using your camera as a dashcam, use a car charger cable and remove the internal battery to make sure it does not die out while you travel.

    That’s because the M20 continues to run from its internal battery when USB power drops. After recording an hour of a parking lot or your garage wall, the battery dies and so does the clock.

    Of course, without the internal battery, the clock dies ten seconds after you turn off the car.

    The internal battery has many days of capacity with the camera turned off (whew!), so I conjured the case & PowerCore battery tray to handle our normal rides. The internal battery keeps the clock alive overnight and during the rain we’ve had for the last week, the PowerCore supplies juice during the ride, and I recharge the PowerCore every few weeks.

    The M20 doesn’t draw charging current when I turn it on, but poking the PowerCore’s status button also turns on its outputs, whereupon the M20 decides it should begin charging and, bonus, draw power from the PowerCore during the entire ride. The M20 finishes charging while we ride, but the PowerCore continues supplying power and, when I turn the M20 off, the PowerCore sees no current draw and shuts itself off.

    Only a geek could love a lashup like that, but it works around the M20’s broken clock and removes its battery maintenance hassle.

  • LED Nightlight Base Teardown & Simulation

    I volunteered to take a look inside a small LED nightlight base to see how well it might work as a power supply for other circuitry:

    Nightlight - overview
    Nightlight – overview

    Note: the AC plug is not polarized. Either blade can contact the hot side of the AC line.

    The cadmium-selenide photocell in front turns the white LED on when it sees darkness and off when it sees lightness, with a more-or-less proportional response during dimness. The LED has an obvious 60 Hz flicker, particularly during its partially on phase, so I didn’t expect much inside.

    The component side of the PCB faces toward the blades, which you’re looking along the lengths of:

    Nightlight - PCB component side
    Nightlight – PCB component side

    The solder side faces away from the outlet:

    Nightlight - PCB solder side
    Nightlight – PCB solder side

    Flipping the solder side left-to-right and overlaying the two images produces an X-ray-ish view useful for tracing the circuitry:

    Nightlight - PCB trace overlay
    Nightlight – PCB trace overlay

    Some doodling extracts an LTSpice schematic:

    Nightlight schematic
    Nightlight schematic

    None of the component values seem particularly critical; the diodes and transistor are close approximations to what’s really inside. I think the 100 Ω resistor also serves as a fuse, should anything else go wrong.

    Setting the CdS cell to 1 MΩ = “dark” turns the LED on:

    Nightlight - ON waveforms
    Nightlight – ON waveforms

    Although I don’t trust the numbers very far, the LED current waveform definitely suggests the flicker isn’t all in my head.

    Setting the cell to 10 Ω = “light” turns the LED off, by the simple expedient of clamping the filter capacitor voltage well below the LED’s forward drop:

    Nightlight - OFF waveforms
    Nightlight – OFF waveforms

    When the LED is off, the transistor current is slightly higher than the LED’s on-state current, because saturation voltage:

    Nightlight - OFF - transistor current
    Nightlight – OFF – transistor current

    The current runs right through the 820 nF capacitor, which serves as a more-or-less 3.2 kΩ ballast resistor:

    Nightlight - OFF - 820 nF cap current
    Nightlight – OFF – 820 nF cap current

    It’s a nice film cap and should have a low ESR, but this seems a bit sketchy to me.

    So, basically, the nightlight doesn’t really have a power supply in the usual meaning of the term and isn’t suited for driving anything other than the white LED inside the case. Relocating the LED outside the case is an Extremely Bad Idea™, because the anode is one diode away from what might well be the hot AC line; one little oopsie and you’ve got a lethal shock hazard.