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.

Author: Ed

  • Optiplex 980 Power Supply: Capacitor FAIL

    Came up from the Basement Laboratory to find my Dell Optiplex 980 PC had failed, with the power button and diagnostic 1 + 3 LEDs blinking amber. They built it back in June 2010, so section 3 of the Dell reference applies, the power supply status LED on the back panel was off, and, going straight to the heart of matter, I popped the top, disconnected the internal power supply cables, and poked the power supply test button:

    Optiplex 980 Power Supply - rear panel test button
    Optiplex 980 Power Supply – rear panel test button

    … and it’s dead.

    Inside, the system board sports a Mini-ATX power supply connector:

    Optiplex 980 - Mini-ATX power connector
    Optiplex 980 – Mini-ATX power connector

    I originally hoped to swap a supply from an Optiplex 755 (also in a Small Form Factor case) residing on the recycle heap, but it has an ordinary ATX connector:

    Optiplex 755 - ATX power connector
    Optiplex 755 – ATX power connector

    So I moved the 980’s SSD and dual-Displayport video card into the 755, fired that devil up, and … it worked!

    With my desktop back in action, albeit somewhat slower, I popped the dead supply’s case by violating the Warranty Void If This Label Removed sticker to unscrew the last screw:

    Optiplex 980 Power Supply - overview
    Optiplex 980 Power Supply – overview

    Notice anything?

    The electrolytic capacitors over on the left look like this:

    Optiplex 980 - Good capacitor
    Optiplex 980 – Good capacitor

    The cluster of caps on the upper right have bulged pressure-relief lids, like this:

    Optiplex 980 - Bulging capacitor 1
    Optiplex 980 – Bulging capacitor 1

    And this:

    Optiplex 980 - Bulging capacitor 2
    Optiplex 980 – Bulging capacitor 2

    And this:

    Optiplex 980 - Bulging capacitor 3
    Optiplex 980 – Bulging capacitor 3

    None had ruptured, but they’re obviously feeling a bit nauseous.

    Given the 980’s mid-2010 manufacturing date, this probably isn’t capacitor plague, just simple overheating from operating in a dead-air zone amid all those heatsinks and wires. Some of the Usual Unnamed Sources suggest overheating the capacitors is how manufacturers ensure their hardware doesn’t last forever, without being obvious about planned obsolescence; I’m loathe to ascribe to malice what can be explained by design desperation.

    A Genuine Dell replacement supply from eBay ($25 delivered) came from yet another “small form factor” Dell chassis, so it isn’t quite the same size, lacks a supply test button / LED status light, and doesn’t quite fit:

    Optiplex 980 - replacement supply misfit
    Optiplex 980 – replacement supply misfit

    Nothing a sheet metal nibbling tool can’t fix, though, given I haven’t developed a deep emotional attachment to the chassis. I gnawed off the left side of the frame and squared up the rim around the lower screw, after which the opening fit the supply pretty well, although the latching tab bent up from the bottom of the chassis didn’t quite engage the far end of the supply. No big deal: it’s not in a high-vibration environment.

    The new-to-me supply also carries an ATX connector, but the eBay seller included a Mini-ATX adapter. Jamming the adapter + wires into the space available required concerted muttering, assisted by tucking the SSD under the DVD-RW drive. No pictures, as it’s a classic seven pounds in a five pound box situation.

    And then It Just Worked again.

  • AT26 / TF26 Quartz Resonator Identification

    There’s not much room on an AT26 / TF26 can for a readable label, unless one owns a metal-marking laser, but a simple bar code should let me identify each one:

    Quartz Resonators - binary marking
    Quartz Resonators – binary marking

    The empty “0” slot down at the bottom will hold the crash-test dummy resonator I’ve been using to get the tester working.

    The red-and-blue stripes from plain old fine-point Sharpie pens will rub off under duress, which I hope to avoid. After finishing up, I’m still not sure blue makes a better zero than red; you can make a convincing argument either way:

    Binary marked AT26 Quartz Resonators
    Binary marked AT26 Quartz Resonators

    The bag allegedly contained 25 resonators, although I’m willing to agree the last one escaped into the clutter on or under the Electronics Workbench.

  • Crystal Modeling: Parasitic Capacitor Values

    A Circuit Cellar reader asked for a better explanation of the parasitic capacitors inside a quartz crystal can than I provided in my April 2017 Circuit Cellar column. Here’s the schematic, with values for a 32 kHz tuning fork resonator and the original caption:

    Quartz resonator circuit model
    Figure 1 – The mechanical properties of quartz resonators determine the values of the “motional” electrical components in the bottom branch of its circuit model. These values correspond to a 32.768 kHz quartz tuning fork resonator: the 10.4 kH inductor is not a misprint!

    I wrote this about the caps:

    The value of C0 in the model’s middle branch corresponds to the capacitance between the electrodes plated onto the quartz. The 3.57954 MHz crystal in the title photo, with two silver electrodes deposited on a flat insulating disk, closely resembles an ordinary capacitor. You can measure a crystal’s C0 using a capacitance meter.
    However, each of those electrodes also has a capacitance to the resonator’s metal case. The top branch of the model shows two capacitors in series, with Cpar representing half the total parasitic capacitance measured between both leads and the case. Grounding the case, represented by the conductor between the capacitors, by soldering it to the ground plane of an RF circuit eliminates any signal transfer through those capacitors. They will appear as shunt capacitors between the pins and ground; in critical applications, you must add their capacitance to the external load capacitors.

    And this about the measurement technique, using a fixture on my AADE LC meter:

    With the resonator case captured under the clip on the far right and both its leads held by the clip to the upper left, the meter measures Cpar, the lead-to-case parasitic capacitance. The meter will display twice the value of each parasitic capacitor, at least to a good approximation, because they are in parallel. The five resonators averaged 0.45 pF, with each lead having about 0.25 pF of capacitance to the case. Obviously, measuring half a picofarad requires careful zeroing and a stable fixture: a not-quite-tight banana jack nut caused baffling errors during my first few measurements.
    With the resonator repositioned as shown in Photo 2, with one lead under each clip, the meter measures C0, the lead-to-lead capacitance. After careful zeroing, the resonators averaged 0.85 pF.
    Although the parasitic lead-to-case capacitors are in parallel with C0, their equivalent capacitance is only Cpar/4 = 0.1 pF. That’s close enough to the measurement error for C0, so I ignored it by rounding C0 upward.

    He quite correctly pointed out:

    With both leads connected together on one side, then that essentially constitutes a single electrical element inside the case. And with the case being a single element, this configuration in the test fixture seems like a single capacitor with one lead being the case and the other lead being the pins, with a vacuum dielectric.

    I would think the meter would display the total capacitance rather than twice the value … It makes sense to me to later say Cpar/2 when the leads are not connected together.

    Here’s my second pass at the problem:

    … the two-capacitor model comes from the common-case condition, where each lead displays a (nominally equal) parasitic capacitance to the case, because the crystal mounting is reasonably symmetric inside the can. It’s easiest to measure the total capacitance with the leads shorted together, because it’s in the low pF range, then divide by two to get the value of each lead-to-case cap.

    […]

    For “real” RF circuits with larger (HC-49 -ish) crystals in parallel-resonance mode, you ground the case and subtract the parasitic capacitance at each lead from the external load capacitors. That’s the usual situation for microprocessor clock oscillators: the crystal sits across the clock amplifier pins, with two more-or-less equal caps from the pins to ground. You should subtract the internal parasitic caps from the clock’s specified load caps, but in practice the values are so small and the cap tolerance so large that it mostly doesn’t matter.

    […]

    Un-grounding the case puts those two parasitic caps in series, just as with two discrete caps, so the lead-to-lead capacitance is (or should be!) half of each: 1/4 of the both-leads-to-case value.

    Re-reading yet again says I glossed over the effect of having C0 in parallel with the Cpar/2 caps, but methinks dragging those complications into the model benefits only the theoreticians among us (or those working very close to the edge of the possible).

    To make it worse, I also botched the QEX reference, which should be Jan/Feb 2016, not 2017. Verily, having a column go read-only makes the errors jump right off the page. [sigh]

    At least I can point to this and amend as needed.

  • Amazon EK3211 Scale: Re-Footing

    Our long-suffering kitchen scale lost a pair of feet, most likely because those two feet do most of the skidding as we slide it onto a shelf below a cabinet. The scale has (well, had) six silicone rubber feet:

    Amazon EK3211 Scale - OEM foot
    Amazon EK3211 Scale – OEM foot

    The vagaries of color photography turned a neutral-gray silicone disk into that weird blue.

    A pair of ¼ inch disks punched from non-skid textured rubber tape fit perfectly into the recesses:

    Amazon EK3211 Scale - tread foot
    Amazon EK3211 Scale – tread foot

    Now, we’ll see how tread adhesive withstands the same abuse.

  • Drying Rack Re-Repair

    Despite having sworn a mighty oath to the contrary, I found myself doing this again:

    Clothes Rack - end clamp
    Clothes Rack – end clamp

    A strut on the other end of the dowel split across its face:

    Clothes Rack - split clamp
    Clothes Rack – split clamp

    The white stuff is wood-filled epoxy, normally used to repair rotted wood, left over from another project. I’ll claim this tests its mechanical strength against peeling forces.

    Easily determined by inspection: a sensible person would toss the rack, but …

  • AD9850 Module: Oscillator Thermal Time Constant

    With an LM75 atop the 125 MHz oscillator and the whole thing wrapped in foam:

    LM75A Temperature Sensor - installed
    LM75A Temperature Sensor – installed

    Let it cool overnight in the Basement Laboratory, fire it up, record the temperature every 30 seconds, and get the slightly chunky blue curve:

    125 MHz Oscillator - Heating Time Constant
    125 MHz Oscillator – Heating Time Constant

    Because we know this is one of those exponential-approach problems, the equation looks like:

    Temp(t) = Tfinal + (Tinit - Tfinal) × e-t/τ

    We can find the time constant by either going through the hassle of an RMS curve fit or just winging it by assuming:

    • The initial temperature, which is 22.5 °C = close to 22.7 °C ambient
    • The final temperature (call it 42 °C)
    • Any good data point will suffice

    The point at 480 s is a nice, round 40 °C, so plug ’em in:

    40.0 = 42.0 + (22.7 - 42.0) × e-480/τ

    Turning the crank produces τ = 212 s, which looks about right.

    Trying it again with the 36.125 °C point at 240 s pops out 200.0 °C.

    Time for a third opinion!

    Because we live in the future, the ever-so-smooth red curve comes from unleashing LibreOffice Calc’s Goal Seek to find a time constant that minimizes the RMS Error. After a moment, it suggests 199.4 s, which I’ll accept as definitive.

    The spreadsheet looks like this:

    T_init 22.5
    T_final 42.0
    Tau 199.4
    Time s Temp °C Exp App Error²
    0 22.250 22.500 0.063
    30 25.500 25.224 0.076
    60 28.000 27.567 0.187
    90 30.125 29.583 0.294
    120 31.750 31.317 0.187
    150 33.250 32.810 0.194
    180 34.375 34.093 0.079
    210 35.375 35.198 0.031
    240 36.125 36.148 0.001
    270 36.813 36.965 0.023
    300 37.500 37.668 0.028
    330 38.125 38.273 0.022
    360 38.500 38.794 0.086
    390 39.000 39.242 0.058
    420 39.500 39.627 0.016
    450 39.750 39.959 0.043
    480 40.000 40.244 0.059
    510 40.250 40.489 0.057
    540 40.500 40.700 0.040
    570 40.750 40.882 0.017
    600 41.000 41.038 0.001
    RMS Error 0.273

    The Exp App column is the exponential equation, assuming the three variables at the top, the Error² column is the squared error between the measurement and the equation, and the RMS Error cell contains the square root of the average of those squared errors.

    The Goal Seeker couldn’t push RMS Error to zero and gave up with Tau = 199.4. That’s sensitive to the initial and final temperatures, but close enough to my back of the envelope to remind me not to screw around with extensive calculations when “two minutes” will suffice.

    Basically, after five time constants = 1000 s = 15 minutes, the oscillator is stable enough to not worry about.

  • AD9850 DDS Module: 125 MHz Oscillator vs. Temperature, Quadratic Edition

    I let the DDS cool down overnight, turned it on, and recorded the frequency offset as a function of temperature as it heated up again:

    125 MHz Osc Freq Offset vs Temp - Quadratic - 29 - 43 C
    125 MHz Osc Freq Offset vs Temp – Quadratic – 29 – 43 C

    The reduced spacing between the points as the temperature increases shows how fast the oscillator heats up. I zero-beat the 10 MHz output, scribbled the temperature, noted the offset, and iterated as fast as I could. The clump of data over on the right end comes from the previous session with essentially stable temperatures.

    I only had to throw out two data points to get such a beautiful fit; the gaps should be obvious.

    The fit seems fine from room (well, basement) ambient up to hotter than you’d really like to treat the DDS, so using the quadratic equation should allow on-the-fly temperature compensation. Assuming, of course, the equation matches some version of reality close to the one prevailing in the Basement Laboratory, which remains to be seen.

    In truth, it probably doesn’t, because the temperature was changing so rapidly the observations all run a bit behind reality. You’d want a temperature-controlled environment around the PCB to let the oscillator stabilize after each increment, then take the measurements. I am so not going to go there.

    The original data:

    125 MHz Osc Freq Offset vs Temp - 29 - 43 C - data
    125 MHz Osc Freq Offset vs Temp – 29 – 43 C – data