Posts Tagged Repairs
Digital Tattoo Power Supply: Polarity Doesn’t Matter
Posted by Ed in Aphorisms, Electronics Workbench on 2018-06-19
When I rewired the guts of the digital tattoo power supply to eliminate the series foot switch, I kept the original wiring polarity, with the black wire to the sleeve and the red wire to the tip:
Tattoo Digital Power Supply – internal view
It’s the same color code I (strongly) recommend in the Squidwrench Electronics Workshops: use any color for the ground / common wire as long as it’s black, then, if you have a red wire, use it for the positive supply. You can use yellow for the higher supply voltage, but stop being clever.
I put suitably colored Powerpoles on the far end of the cable to replace the standard tattoo machine spring clip connector, so I can attach clip leads, battery test fixtures, and so forth and so on.
We wired the supply into a clip-leaded diode measurement setup with a current limiting resistor and a pair of multimeters to measure the diode current and forward voltage, whereupon we noticed all the meters displayed negative voltages and currents.
After a frenzy of wire-checking verified their setup was all good, I forced the simplest possible test, herein recreated on my bench:
Tattoo Digital Power Supply – polarity test
Which produced this display:
Tattoo Digital Supply – reverse polarity
Huh.
After a brief exploration of “Trust, but verify” territory, we swapped the clip leads from the power supply and continued the mission.
Back on my bench, I pulled the supply apart and measured the voltage at the jack terminals:
Tattoo Digital Power Supply – jack wiring
Still negative. Huh.
The bottom of the power supply PCB shows exactly what you should expect by now:
Tattoo Digital Power Supply – reversed color code
The red wire near the top of the board is, indeed, soldered to the trace labeled GND and goes to the jack’s tip terminal; the adjacent black wire goes to the front-panel LED. Similarly, the black wire just below it, soldered to the same trace as the yellow wire, goes to the jack’s sleeve terminal; that trace also connects to a resistor leading to the trace labeled LED+ and the LED’s red wire.
Although tattoo machines run from DC supplies, their motors or vibrators don’t depend on any particular polarity and will run fine with a backwards supply.
Resoldering the red and black wires where they should go produces the expected sign at the jack:
Tattoo Digital Supply – meter leads
Although measuring and plotting diode voltages and currents may seem tedious, actually wiring stuff together and taking data reveals how difficult the real world can be.
I trusted the supply’s internal color code and, although I’m certain I tested the Powerpoles, I obviously didn’t notice the meter’s sign.
Memo to self: Sheesh.
Summer Downshift
Posted by Ed in Administrivia, Home Ec on 2018-06-12
We have several high-intensity / long-attention-span home projects scheduled this summer, all of which will keep me away from the Basement Laboratory.
We’re OK, all is right with our world, but painting rooms and yard maintenance always take way more time than they should, while having close to zero intellectual content.
Like, for example, the result of a strenuous morning devoted to removing a severely overgrown holly bush:
Mother of All Holly Bush Stumps
I’ll post odd & ends a few times a week until maybe mid-August, whereupon I should get back to more usual pursuits.
Enjoy your downtime …
Tour Easy Front Derailleur Cable Clamp
Posted by Ed in Machine Shop, Recumbent Bicycling, Software on 2018-06-08
In addition to sawing through the side of the cable ferrule, the front derailleur cable began breaking at the edge of the derailleur arm:
Tour Easy Front Derailleur Cable – frayed
It wouldn’t have survived another ride!
Dan pointed out CNC machined aluminum cable clamps are a thing, but those are sized for larger frame tubes than the 1.0 inch steel used on our Tour Easy ‘bents and, although I’ve shimmed everything else on the frame, I wanted to tweak the cable angle to match the arm on the derailleur.
A bit of OpenSCAD wrangling produces a likely candidate:
Front Derailleur Cable Clamp – Slic3r
That’s a bulked-up revision of the prototype:
Tour Easy Front Derailleur Cable Clamp – installed
Done up in orange PETG, it demonstrated the idea worked, but two perimeter threads wrapped around 15% infill isn’t quite up to the task. Note the split along the screw on the far half and various irregularities around the ferrule.
The cable angle isn’t quite right, either, as the proper compound angle would, alas, aim the cable into the pedal crank. The bulky bushings get in the way of putting the ferrule where it should be with the screws aligned in a tidy manner, so I must get used to the jaunty angle.
The bulkier version, done with 50% infill and four perimeter threads, has the same tilt angle, but the ferrule sits further from the screws:
Tour Easy Front Derailleur Cable Clamp V2 – rear quarter view
The view from the left side shows the cable angles slightly to the rear, but the smaller angle should make it happier:
Tour Easy Front Derailleur Cable Clamp V2 – side view
Probably should have used black PETG. Next time, for sure!
The OpenSCAD source code as a GitHub Gist:
Tour Easy: Front Derailleur Cable Angle
Posted by Ed in Machine Shop, Recumbent Bicycling on 2018-06-03
Spotted while in the midst of replacing my Tour Easy’s rear grip shifter:
Tour Easy – front derailleur cable angle
As you might expect, the cable saws through the side of its ferrule and the brazed-on frame fitting, because it’s been basically impossible (for me, anyhow) to find a replacement derailleur duplicating whatever the good folks at Easy Racers shipped back in 2001.
On the upside, this derailleur’s cable entry has a nicely rounded ramp eliminating the need for my brass cable pulley widget.
Memo to Self: Perhaps running the cable around a bearing anchored to the frame fitting would help?
I’ve obviously forgotten to fix this for several years, so putting it here may serve as a Round Tuit.
Primo Comet vs. Green Glass Chip: Kevlar FTW!
Posted by Ed in Machine Shop, Recumbent Bicycling on 2018-05-31
The gashes don’t look like much:
Primo Comet gash – tread view
Not even from the side:
Primo Comet gash – side view
When they happened, I knew where to look, because the Kevlar-belted Primo Comet had two conspicuous bulges surrounding debris jammed between the tread and the carcass along the sidewall: the gashes were wide open!
Much to my astonishment, the tire hadn’t gone instantly flat.
Some screwdriver probing in the leftmost gash produced this nasty glass chip:
Primo Comet gash – chip side view
AFAICT, the smooth side slid over the internal Kevlar belt as the edge sliced between the rubber tread and the carcass. I think the top entered first, with the somewhat crushed end hitting the pavement on each revolution:
Primo Comet gash – chip edge view
The other gash emitted a somewhat smaller chip.
I rode over something crunchy, most likely the remains of a beer bottle, in a shaded section along Rt 376, and we stopped a few driveways later to diagnose a once-per-revolution thump from the front tire. The tube still wasn’t losing pressure, even after extracting the glass, so I continued the mission; it was a fine day for a ride!
I later filled those gashes (plus a few others) with silicone rubber to keep grit out. It’s surely a feel-good gesture, but maybe it’ll help the tire reach the end of its tread life.
You can judge our “riding environment” by the tire’s condition …
Siglent SDS2304X Screen Shot File
Posted by Ed in Electronics Workbench, Photography & Images, Software on 2018-05-30
Poking the Print button on the front of the Siglent SDS2304X scope saves the screen to a BMP file (in the /BMP directory) on a USB flash drive plugged into its front-panel port:
Siglent SDS2304X Front Panel – Print Button – USB port
Which produces files like these:
ll --block-size=1 /path-to-USB-stick/BMP/ total 2318336 drwxr-xr-x 2 ed ed 4096 May 23 13:13 ./ drwxr-xr-x 4 ed ed 4096 Dec 31 1969 ../ -rw-r--r-- 1 ed ed 1152054 May 23 13:13 SDS00001.BMP -rw-r--r-- 1 ed ed 1152054 May 23 13:13 SDS00002.BMP
The files are 1152054 bytes long, as specified by the BMP header inside the file:
hexdump -C /path-to-USB-stick/BMP/SDS00001.BMP | head 00000000 42 4d 36 94 11 00 00 00 00 00 36 00 00 00 28 00 |BM6.......6...(.| 00000010 00 00 20 03 00 00 e0 01 00 00 01 00 18 00 00 00 |.. .............| 00000020 00 00 00 94 11 00 00 00 00 00 00 00 00 00 00 00 |................| 00000030 00 00 00 00 00 00 01 01 01 01 01 01 01 01 01 01 |................| 00000040 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 |................| * 00000880 01 01 01 01 01 01 01 01 01 01 01 01 01 01 1e 1e |................| 00000890 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e |................| * 00000990 1e 1e 1e 1e 1e 1e 01 01 01 01 01 01 01 01 01 01 |................|
The first 14 bytes contain the Bitmap file header, with the file size in Little-Endian order in the four bytes at offset +0x02: 0x00119436 = 1152054.
The four bytes at offset +0x0A give the offset of the pixel data: +0x36. That’s the series of 0x01 bytes in the fourth row. Unlike most images, BMP pixel arrays start at the lower left corner of the image and proceed rightward / upward to the last pixel at the upper right corner.
The data between the Bitmap file header and the start of the pixel data contains at least a Device Independent Bitmap header, identified by its length in the first four bytes at offset +0x0E. In this case, the length of 0x28 = 40 bytes makes it a Windows (no surprise) header.
The two bytes at +1C give the bits-per-pixel value: 0x18 = 24 = 3 bytes/pixel, so parse the pixels in RGB order.
The four bytes at +0x12 give the bitmap width in pixels: 0x320 = 800. Each pixel row must be a multiple of 4 bytes long, which works out fine at 2400 bytes.
The tail end of the file shows one dark pixel at the upper right:
hexdump -C /path-to-USB-stick/BMP/SDS00001.BMP | tail 00118330 00 cc 00 00 cc 00 00 cc 00 00 cc 00 00 cc 00 00 |................| 00118340 cc 00 00 cc 00 00 cc 00 00 cc 00 00 cc 00 00 cc |................| 00118350 00 00 cc 00 00 cc 00 00 cc 0f 0f 75 1e 1e 1e 1e |...........u....| 00118360 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e |................| * 00118ad0 1e 1e 1e 01 01 01 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e |................| 00118ae0 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e |................| * 00119430 1e 1e 1e 01 01 01 |......|
Which looks like this, expanded by a factor of eight (clicky for more dots to reveal the situation):
Screenshot – upper right corner – 8x expansion
The scope can also transfer a screenshot over the network:
lxi screenshot -a 192.168.1.42 /tmp/lxi-shot.bmp Loaded siglent-sds screenshot plugin Saved screenshot image to /tmp/lxi-shot.bmp
Which has the same header:
hexdump -C /tmp/lxi.bmp | head 00000000 42 4d 36 94 11 00 00 00 00 00 36 00 00 00 28 00 |BM6.......6...(.| 00000010 00 00 20 03 00 00 e0 01 00 00 01 00 18 00 00 00 |.. .............| 00000020 00 00 00 94 11 00 00 00 00 00 00 00 00 00 00 00 |................| 00000030 00 00 00 00 00 00 01 01 01 01 01 01 01 01 01 01 |................| 00000040 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 |................| * 00000880 01 01 01 01 01 01 01 01 01 01 01 01 01 01 1e 1e |................| 00000890 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e |................| * 00000990 1e 1e 1e 1e 1e 1e 01 01 01 01 01 01 01 01 01 01 |................|
But the resulting file is three bytes = one pixel (!) too large:
ll --block-size=1 /tmp/lxi.bmp -rw-rw-r-- 1 ed ed 1152057 May 23 19:09 /tmp/lxi.bmp
The tail end of the file:
hexdump -C /tmp/lxi.bmp | tail 00118330 00 cc 00 00 cc 00 00 cc 00 00 cc 00 00 cc 00 00 |................| 00118340 cc 00 00 cc 00 00 cc 00 00 cc 00 00 cc 00 00 cc |................| 00118350 00 00 cc 00 00 cc 00 00 cc 0f 0f 75 1e 1e 1e 1e |...........u....| 00118360 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e |................| * 00118ad0 1e 1e 1e 01 01 01 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e |................| 00118ae0 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e |................| * 00119430 1e 1e 1e 01 01 01 01 01 0a |.........|
Because the file header doesn’t include those three bytes, they don’t go into the image and the resulting screenshot is visually the same.
Which looks like a picket-fence error, doesn’t it? I’d lay long odds the erroneous loop runs from 0 to NUMPIXELS, rather than 0 to NUMPIXELS-1. Raise your hand if you’ve ever made that exact mistake.
I have no practical way to determine whether the error is inside the scope or the LXI network code, but given Siglent’s overall attention to software fit-and-finish, I suspect the former.
One can convert BMP files to the much more compact PNG format:
convert /tmp/lxi.bmp /tmp/lxi.png convert: length and filesize do not match `/tmp/lxi.bmp' @ warning/bmp.c/ReadBMPImage/829.
Yes. Yes, there is a mismatch.
The space savings is impressive, particularly in light of PNG being a lossless format:
ll /tmp/lxi.* -rw-rw-r-- 1 ed ed 1.1M May 23 19:09 /tmp/lxi.bmp -rw-rw-r-- 1 ed ed 14K May 23 19:17 /tmp/lxi.png
You can eliminate the nag by truncating the file:
truncate --size=1152054 /tmp/lxi.bmp
One could wrap it all up in a script:
#!/bin/bash lxi screenshot -a 192.168.1.42 /tmp/"$1".bmp truncate --size=1152054 /tmp/"$1".bmp convert /tmp/"$1".bmp "$1".png echo Screenshot: "$1".png
And then It Just Works:
getsds2304x.sh "Test Shot Starfish" Loaded siglent-sds screenshot plugin Saved screenshot image to /tmp/Test Shot Starfish.bmp Screenshot: Test Shot Starfish.png
Test Shot Starfish
SpaceX uses Test Shot Starfish tracks for pre-launch background music; the actual test shot was spectacular.
Mower Blade Standardization, Lack Thereof
Posted by Ed in Home Ec, Machine Shop on 2018-05-27
The blade from our current Craftsman mower is on the right:
Sears Craftsman mower blades
The other two came from our previous Craftsman mowers.
Stipulated: Sears sources their mowers from various suppliers, but it’d be great if everybody could agree on a single blade mount and be done with it.
For the record, a 5/8 inch socket works fine. One could surely use a 16 mm socket in a pinch.
Wear leather gloves to prevent a nasty gash from the stamped-steel muffler shroud as you pull the sparkle plug cap to avoid an absolutely impossible engine startup while you’re wrenching under the deck.
Replace the air cleaner while you’re at it.
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