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

  • Debugging Tube Circuitry: Open Resistor

    Open 2.2 meg resistor
    Open 2.2 meg resistor

    I dropped in to mooch some female header strips from my buddy Eks (which is not nearly as obscene as it sounds) and got the story behind this innocent-seeming 2.2 megohm carbon-composition resistor.

    It seems he was debugging a defunct tube-based audio amplifier. He’d probed everything and discovered that the grid bias on one of the tubes was totally wrong, which caused protracted headscratching over the associated circuitry.

    Now, in semiconductor work, a 2.2 meg resistor is an open circuit compared to the other circuit impedances. In fact, you can use pretty nearly any resistor with green or blue in the third band as a standoff in Manhattan-style construction in place of those small insulated pads.

    Megohm-value resistors are actually useful in tube circuitry; you’ll see plenty of green and blue bands sprinkled around those sockets. Although we didn’t get into details, I suspect this one was part of a grid-leak bias circuit that holds the grid voltage just a bit below the cathode; the bias comes from the few electrons that whack into the grid wires rather than passing through, so the total DC current is in the microamp range.

    After more headscratching, Eks yanked this resistor, measured it, and found it was a completely open circuit. A 2.2 meg resistor isn’t all that much different from an open circuit (it’s hard to tell the difference with an in-circuit measurement) when used in a transistor circuit, but the difference separates correct function from failure for a tube amp.

    Eks swapped in a new resisistor and the amp worked fine. Case closed!

    The digital multimeter in my desk drawer tops out at 2000 kΩ, which shows you just how much demand there is for high-value resistors these days…

  • Nichicon SMD Electrolytic Capacitor Polarity

    Nichicon electrolytic capacitors
    Nichicon electrolytic capacitors

    This should be obvious, but isn’t. The black bar marks the negative terminal and the corner-cut side of the base marks the positive terminal.

    How much would it cost to put little hyphens down the middle of the black bar?

    The data sheet for a related, not identical, series of caps is there.

  • Slitting Brass Tubing

    Casting Wood's Metal in brass tube
    Casting Wood's Metal in brass tube

    I needed a brass tube with a lengthwise slit to serve as an electrostatic shield around a ferrite bar antenna. There are many wrong ways to do this, all of which produce terrible results, pose a serious risk of personal injury, or both. I say that with some confidence, having tried some of them over the years.

    Here’s one right way: fill the tube with Wood’s Metal, thus turning it into a solid rod, then cut the slit with a slitting saw.

    Wood’s Metal is a moderately toxic alloy that melts in hot water, which turns casting into a simple workbench operation. You might not want to cast it in the kitchen, but that’s your call. Clean up the scraps, wash the counter even though you used newspaper, wash your hands, and don’t suck your thumb.

    As shown, I just poured the molten metal into the brass tube atop a steel block, broke off whatever seeped out, and remelted the scraps. Turns out I had just barely enough for the job.

    Slitting brass tubing - overview
    Slitting brass tubing – overview

    My buddy Eks gave me a stack of slitting saws a while ago and I modified a standard Sherline holder to fit them. Turns out there’s just barely enough room for everything within the mill’s working envelope; the saws are a bit over 3 inches in diameter.

    So I cut the back of the tubing, making the pictures somewhat disorienting.

    The tubing fit neatly into an old V-block (evidently homebrewed by a better machinist than I), held down by ordinary Sherline clamps on perilously long studs screwed into the tooling plate. The saw had just enough reach to clear the rather broad V-block’s shoulder.

    The tubing is 0.630 OD with a 15-mil wall and the saw blade is pretty nearly 32 mils thick. I touched off Z=0.331 (630/2 + 32/2) with the blade atop the tubing, then jogged away to Y=+1 and drove down to Z=0 to cut exactly through the middle of the tube.

    Slit 0.015 inch deep
    Slit 0.015 inch deep

    The V-block is aligned with the front of the table, but I did a bit of nudging to persuade it into final alignment. Of course, the saw wasn’t quite centered on the holder, so a blade or three tinged on the tubing when I did a Y=0 trial pass at low RPM.

    For lack of anything smarter, I cut at 500 RPM and fed at 5 inch/min. That’s painfully slow, but correspondingly boring… remember, in machine shop work, boring is good.

    I did five passes: one trial at Y=0, three cuts at 5-mil steps, and a cleanup cut. The picture shows the 15-mil pass left a very thin web at the far end. A final 2-mil cut removed that web, leaving only a few burrs. You could do it in one pass, but I wanted to minimize the depth-of-cut into the Wood’s Metal.

    Unclamp, discover that the cast metal rod slides right out, touch up the edges with a file, and it’s all good. A lovely slit, perfectly aligned, without bent metal or bloodshed.

    As a bonus, I get a nice Wood’s Metal ingot out of the operation. The line along the rod is just barely perceptible with a fingernail; it’s more of a polished line than an actual cut.

    Slit tube with Wood's Metal ingot
    Slit tube with Wood's Metal ingot

    Turns out the shield works a bit too well: it cuts out the WWVB signal, too. I think the tubing is too close a fit to the ferrite rod and detunes the winding. More experimentation is in order…

  • Salvaging Old Photos: Gamma Boost

    We’re scanning a bunch of really old photographs to assemble a book of memories for Mary’s father. Most of the images are about what you’d expect for old photos: bad exposures, poor focus, and scratched emulsion. There’s not much you can do to save ’em, but one image really surprised me.

    Scanned at 600 dpi with the black and white points set to maximize the dynamic range, we got this image (reduced resolution for display here).

    Original
    Original

    The original image is somewhat brighter than that: there’s a figure visible in the upper-left, but you can’t see much more.

    After dramatically adjusting the gamma and switching to grayscale mode, her father popped out of the shadows.

    Gamma = 3
    Gamma = 3

    Now, it’s not a great picture, but it’s one of the few we have from that era… and it’s a much better picture than no picture at all!

    A similar trick can recover dull gray snow pictures, as mentioned there.

  • EAGLE vs Sherline CNC Mill: Maximum-size PCB Platen

    I use the Standard edition of Cadsoft’s EAGLE schematic capture & PCB layout program, which puts a 160×100 mm upper limit on circuit boards. That meshes nicely with the capabilities of my Sherline CNC mill, which I use to drill component holes.

    I’m currently making a set of PCBs that are pretty close to that maximum size. They’re awkward to clamp, difficult to peel off from double-sided tape, and require careful positioning to ensure they don’t hit the mill column. Been there, done that, time for something better.

    The simple acrylic sheet platen shown here seems to work well. The PCB is a 5×8-inch sheet, clamped along three sides with some aluminum U-channel from the heap. That’s why two of the rails have random holes: it came pre-drilled for something else.

    Platen with 5x8-inch PCB
    Platen with 5×8-inch PCB

    The rear edge (closest to the mill column) has three screws that serve multiple purposes:

    • They clamp the edge of the sheet firmly to the platen
    • The two end screws protrude through the platen and align it along the rear edge of the mill table
    • The middle screw is an origin alignment marker
    Rear clearance
    Rear clearance

    My mill has slightly less than the absolute maximum Y-axis travel because I added a bushing to capture the end of the leadscrew, as described there. The picture shows the clearance between the back of the platen and the mill column: 2 mm, more or less. The 6-32 screw head is flush with the rear edge of the platen.

    Alignment along the Y-axis is easy: jog rearward until the stepper motor stalls, ease away a smidge, then touch off at Y=3.8 inches. Stalling the motor is bad practice with servos or husky steppers, but on this sort of low-power machine it’s perfectly OK. (One could argue for limit switches, but in vain.)

    Slap the platen on the mill table tooling plate (turns out that the Z-axis reach is marginal for the shortest carbide drill when it’s in a collet, oops), adjust more-or-less to the middle of the X-axis scale on the front of the table, line up the hold-down clamps, then crunch the U-channels down on the circuit board. That holds everything in place very firmly; the front overhang doesn’t get much torque because the mill can only reach 4 inches from the rear edge, just beyond the mill table underneath.

    That center screw is eyeballometrically in the middle of the platen’s width, so X-axis alignment is also easy: put the laser dot (visible in the top picture if you squint) on the near-side edge of the screw and touch off X=3.2 inches.

    That alignment puts the X=Y=0 origin at the front-left corner, about 1/4″ in from the left-side clamp and an inch behind the front clamp.

    The mill’s X axis reach goes beyond the clamps, but the 160 mm = 6.30 inch extent of an EAGLE board fits neatly inside.

    The Y-axis reach is barely over 3.8 inches, just shy of EAGLE’s 100 mm = 3.94 inches, but that’s close enough for what I need to do. Getting that last 0.14 inch would require a very, very thin clamp at the rear, minus the Y-axis bushing. There wouldn’t be much clearance from the holes to the edge of the board, either.

    The generous Y-axis clearance on the front allows for the trickery needed to run toner-transfer sheets through the fuser; you want margins all around the drilled area. More about that there, plus search for PCB to unearth other posts.

    Remember that the way I make PCBs, the holes act as alignment points for the toner transfer sheet. That means I don’t really care about absolute alignment with respect to the raw PCB sheet: just clamp it down and start drilling.

  • Holding Machine Screws for Trimming

    Screw in slotted nut
    Screw in slotted nut

    Some years back, I bought a lifetime supply of stainless steel machine screws in the usual sizes, all in 1- and 2-inch lengths. I was always cutting the things to length anyway, so why not start with nice screws?

    The problem with cutting a screw is holding it securely enough that it doesn’t fly off into a far corner of the shop, but without goobering either the threads or the head.

    The secret, at least as far as I can tell, is slitting a nut to make a secure clamp for sawing, filing, and grinding. I ran a slitting saw through a nut to get the result you see here. Although it’s awkward, a slit through a point means grabbing the nut on two parallel sides squeezes the slot closed: exactly what you want.

    Screw firmly under control
    Screw firmly under control

    Slit a bunch of nuts whenever you get set up to do this, because those ugly thread ends on the cut screws tend to chew ’em up. If you have any foresight, you’ll thread the nut on the screw before you cut it, but that doesn’t work for really short screws.

    Yeah, a lifetime supply of all different screw sizes and all different lengths would be nice, but I really don’t spend a whole lot of my life cutting screws…

  • Hammered Solder Ribbons

    I needed more solder ribbon for resistance soldering, so I figured I should make a batch of the stuff. Put a length of silver solder between folded paper, hammered it on the vise anvil with a polished brass hammer, and it worked fine.

    Flushed with success, I did the same with some ordinary rosin-core lead solder for the next time I must solder a shield or some such.

    Solder Ribbons
    Solder Ribbons

    Just snip off the appropriate length and fire up the iron…