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

  • Ampeg B-12-XY Cap Autopsy

    Before trashing (*) all those caps from the Ampeg, I marched them past a capacitance meter that gives the dissipation factor D. As D = tan δ = ESR / ¦X¦, we know ESR =  D*¦X¦ at the meter’s 1 kHz test frequency. We don’t know the magnitude of the total reactance X (the meter doesn’t tell us that) and in this case we can’t assume the ESR will be small with respect to the capacitive reactance Xc = 1/2πfC.

    Ampeg capacitors
    Ampeg capacitors

    The smaller green 0.022 µF Cornell-Dubilier caps all came in with D=0.05, so they’re marginal.

    The larger green 0.15 µF Cornell-Dubilier caps had D=0.00 and the black 0.1 µF was D=0.01. Those are OK.

    The small black caps had D=0.14. Yikes! The larger one and the yellow cap had D= 0.01 or 0.02.

    The blue Ducati (!) electrolytics ranged from 0.06 to 0.48. That was without reforming, as the last time Phil turned it on, the finals about melted down: I wasn’t going to risk that again just to find out if you can reform all the electrolytic caps without the tubes in place.

    So, yeah, some of the coupling caps were exceedingly bad. If you’d like to rub the values & data against the schematic to find out which one(s) were killing the finals, go ahead.

    All of the measured capacitance values were within spitting distance of their nominal values.

    [Update: Eks points out that I really should measure the leakage at operating voltage, so as to find the current that would drive the grids off their normal bias points. That’s a project for another day… ]

    (*) They’re in the e-waste recycling box, of course.

    The raw data, not that anybody cares:

  • Monthly Aphorism: On Complexity

    • When faced with a problem you don’t understand, do any part of it you do understand, then look at it again.

    Heinlein, of course: The Moon is a Harsh Mistress

    Yes, this algorithm can stall you at a local maximum, but that’s better than remaining stuck in the starting gate while you’re thinking too much.

    In less high-falutin’ terms: Don’t just sit there, do something!

  • Bicycle Water Pack Leak Repair

    So the hydration pack I’ve been using for a few years started piddling all over the floor, whereupon some debugging revealed a pinhole leak where the large thermally sealed flange meets the bag side. Nothing, but nothing adheres to the polyethylene (or some such) bag material, but a blob of acrylic caulk (armored with a layer of electrical tape, not shown) may suffice for a while.

    Hydration pack leak repair blob
    Hydration pack leak repair blob

    I did the same thing to the other side as a prophylactic measure…

  • Exposed Stepper Motor Windings

    Got a stepper motor from halfway around the planet from the usual eBay source, intended for a direct-drive extruder (at some point). This one has integral wire leads, which is fine with me, but the opening in the rear endcap reveals a bit more of the innards than one usually sees:

    ACT 17HS5425 stepper - exposed winding
    ACT 17HS5425 stepper – exposed winding

    Yup, that’s one winding peeking out. Although the wire insulation should take care of anything conductive, I’d expect the same casual attention to detail in the winding terminals.

    I’d worry more if this were being used in a metal-cutting operation, but a snippet of heatshrink tubing and a blob of hot-melt glue seem in order.

    For what it’s worth, the motor is an ACT 17HS5425:

    • 1.8°/step
    • 48 mm case length
    • 3.1 V
    • 2.5 A
    • 1.25 Ω
    • 1.8 mH
    • 48 oz·in holding torque
    • 2.8 oz·in detent torque
    • 68 oz·in rotor torque

    No torque curves and nothing more in the way of a datasheet.

  • Tour Easy: Squeaky Pedal

    Of late my Tour Easy has developed a squeak at the pedal-go-round rate. It has Performance Bike Campus pedals, with SPD cleats on one side and a rat-trap surface on the other, and only the SPD side squeaked.

    Turns out that the two little mounting screws holding the cleat dingus worked their way loose.

    SPD pedal screws
    SPD pedal screws

    I should probably ease some lube under that plate, just to be sure, but the simple fix worked fine…

    (And, yeah, I should clean it, just once, to see what it’s like, right?)

  • Ampeg B-12-XY: Recapping

    Just for completeness, here’s the original underside:

    Ampeg B-12-XY - Underside - old caps
    Ampeg B-12-XY – Underside – old caps

    And with the new caps, many from Eks’ stash and a few from mine:

    Ampeg B-12-XY - Underside - new caps
    Ampeg B-12-XY – Underside – new caps

    With all those in place, the firebottles lit up properly, the power tube plates remained dark, and it sounded great. The edge-lit engraved acrylic panel in the middle is a wonderful custom mod!

    Ampeg B-12-XY Firebottles
    Ampeg B-12-XY Firebottles

    It’s in mint condition, with the original footswitch and a remote Echo speaker box with a pair of drivers:

    Ampeg B-12-XY - ready to rock
    Ampeg B-12-XY – ready to rock

    It still has those original huge electrolytics, though. Eks says the best test comes after half an hour: if the cans remain cool, the leakage and ESR will be good enough.That’s the case, so we’re rolling with them. However, the amp has some residual hum that the Hum nulling pot can’t remove, plus a bit of noise, which means those ‘lytics probably hover at the bare minimum values required to keep it going.

    I discovered (inadvertently, of course) that swapping the two identical 6D10 triple triode tubes killed the Vibrato oscillator. That triode would oscillate for a few seconds after the footswitch grounded the cathode, but one tube didn’t have enough gain to keep it going. More likely than not, the feedback resistors have increased in value, too. Swapping the 6D10s restored it to operating condition.

    My Shop Assistant compared her tiny DSP Fender amp with this monster and concluded that DSP effects only sound good when you don’t have the original for comparison. Of course, you could lose that tiddly amp inside the Ampeg’s speaker case.

    I should’a learned to play the guitar…

  • Ampeg B-12-XY: Tin Whiskers!

    After Eks set me straight on cleaning the contacts involved with the Ampeg’s Echo circuitry, we emerged from his shop into brilliant sunshine. He looked into the thing and shouted “Tin whiskers!”

    It turns out the Hammond folks made the outer frame from tin- (or, shudder, cadmium-) plated steel that has grown a dense crop of whiskers on its interior surface. They glittered in the sunlight like a carpet of crushed glass, with the longest ones maybe 3 mm tall!

    This view looks nearly parallel to the side of the channel (upward as it mounts on the speaker box), with the steel wall to the bottom of the image. I applied gruesome contrast stretching to make the whiskers more visible:

    Ampeg Spring Echo Unit - Tin Whiskers
    Ampeg Spring Echo Unit – Tin Whiskers

    This is the first time I’d ever seen a tin whisker in person and there’s a bazillion of ’em in there!

    If that Ampeg had transistorized components, it’d be dead as a doornail! Fortunately, a tin whisker doesn’t stand a chance in an analog vacuum-tube circuit. The power supply puts 400-ish V into 40 μF caps, providing plenty of energy to vaporize the errant whisker; all you’d hear is a pop.