The Latitude E7250 I’ve been carrying around since 2017 is one of my favorite machines I’ve ever had; it’s small, robust, has perfect hardware support under Linux… and is starting to get a little too feeble for some tasks I’d like to use it for, experienced a few spurious shutdowns, and has a screen crack causing delamination.
I continue to be a fan of having a small, relatively inexpensive machine for carrying around, and a believer in “The only Dell laptops with acceptable build quality start with a 7”, so in the tradition of the $400 for a refurb and RAM upgrade I spent on the E7250, I ordered one of its more-or-less direct successors, a refurbished Latitude 7390 on a half-off sale a few weeks ago for about $470.
After a few weeks, it looks to be an excellent successor. Nitpicky details and comparisons below the fold.
I have an Onn Surf 8 (One of the surprisingly-not-that-shitty ultra-cheap Walmart tablets) that my research group bought a couple of to use as Android dev testbeds. I’ve been occasionally using it as a normal tablet since I have it around, and have been consistently irritated by the collection of bloatware it comes with…. so I decided to hack it. To tl;dr this whole thing, ignore the collection of typically scammy Android dev forum and blogspam crud, and use the open-source mtkclient for your MediaTek Android device hackin’ needs.
I’ve wanted a hard-copy terminal for a while – both to play with and to use for explaining why serial works the way it does, but they tend to be expensive. Most of the common hard-copy terminals also aren’t really convenient objects to own: loud desk-sized machines (Teletype 33 family, most DECWriters), additionally clockwork nightmares (IBM 2741, earlier Teletype devices), which speak ridiculous protocols (…ditto). This only leaves a handful of reasonable options, the most common of which are portables like TI Silent 700s and DEC LA12s, or one of the dasiywheel-printer based terminals (which are often non-period-correct things like a WheelWriter with a modern serial interface card in it).
So, of course, I’ve been idly keeping an eye out for a deal on one on the auction sites, and mid-October last year I got lucky: I scored a TI Silent 700 Mod. 745 for $34.00+S&H (about $47 all in) from a Shopgoodwill auction, and got it working.
An old friend of mine was moving cross-country and got in touch about taking “Some of his old computers” a while ago. I of course agreed, and it turned out to be quite a growth event for my hoard. There will be several posts about machines that arrived in this process as I get to them.
The list of things to be re-homed included “an Apollo” which I was hoping was a pre-acquision DN-something or a HP 300/400 series because I’ve had a long fascination with Domain/OS. What showed up is… not that. This is a later HP Apollo 9000/735 PA-RISC workstation, ca. 1992, which is easily the most exotic piece of hardware that transaction made me steward of. The OS options are HP-UX 7-10.20, a few BSDs, or a second-class NextStep 3.3 port; I’ll probably go with HP-UX10.2. It came with the requisite HP-HIL keyboard and mouse (thank goodness) and a DEC branded 5xBNC to VGA cable.
This thing was serious rarefied-air hardware when it was new: PA-7100 99 MHz processor, 208 MB of RAM in 12 obscene proprietary 16MB RAM modules + 16 soldered to the processor board, a HP CRX-24Z video board, a full-height SCSI HDD, and an AUI Ethernet daughter card. Probably in the ballpark of $60,000 new. It is also built like a piece of high-end industrial equipment, with big sheet-metal frames with handles that pull out of the back of the system for every major component.
My first attempt to power it resulted in …a feeble blink of the power light. That suggested to me that the PSU was bad, probably due to defunct electrolytic capacitors. So, in standard “old electronics troubleshooting” fashion, I pulled the PSU, tore it down, read labels off the most suspicious capacitors, and ordered replacements. The HP Museum folk also suggest the AC line filter module is a time bomb on all of these, so since it’s still made, I grabbed one of those too.
List with Mouser links, since they were the vendor with everything I needed in-stock: 8x 2200uF 25V, 12.5mmD 40mmH, 5mm lead spacing , Nichicon UPJ1E222MHD firstname.lastname@example.org 1x 220uF 35v, 10mmD, 20mmH, 7mm lead spacing, Nichicon UPM1V221MPD1TA 1@$0.72 1x 1x 22uF, 25V, 5mmD, 12mmH: Nichicon UPS1E220MDD1TA email@example.com 1x 12uF, 35v, 5mmD, 10mmH, Panasonic EEA-FC1V120B 1@$0.39 1x AC Power Entry Module, Schaffner FN9222R-10-06, 1@$6.50 I also ordered a couple 470uF 25V, 10mmD, 20mmH, 5mm lead spacing caps, but ended up not installing them because there was no sign of damage and they were hard to get a good angle on.
I passed on dealing with a couple smaller electrolytics with no signs of damage, and also two gigantic 2x 1200uF, 250v, 35mmD, 47mmH, 10mm lead spacing input caps that cost $7.50 a piece, since they looked both fine and like a fight to get out without damaging the PCB. One of these days I really need to invest in a proper pump-driven desoldering gun to make this kind of task safer and easier.
I of course picked up an extra 1-2 of each since it was noise over paying for shipping, and it’s a good thing because I dropped one of the new 220uF/35V parts and it instantly disappeared forever, presumably to wherever my cat has been hiding toys recently. The new input filter is slightly longer than the original and required a bit of creative terminal bending to fit around the caps, but it made it back into the case. After the recap, it powered right up, and on a second try after giving the hard disc a gentle thump to unstick the heads from park, everything spun right up.
It booted to status LEDs at that point, and shows “8,6,3,1” which according to the service manual indicates “Autoselection Failure to Find Boot Device” – probably meaning the HDD is dead and/or wiped. The appropriate HP/UX media is easy enough to find. Unfortunately, the monitor on my basement bench doesn’t seem to want to sync with the presumed 1280×1024@72Hz Sync-on-Green coming out of the video card, so I’m stuck for now until I can find a workaround. There is a 4th BNC labeled “Stereo” that might somehow be useful for sync? Or I need a scan converter/sync stripper gadget? … further research required.
I impulse bought a 5 pack of tiny stepper motors off Amazon for $3 to satisfy my curiosity. A colleague showed them to me and asked if I knew anything about them and …I didn’t, but they were too cheap and interesting not to try.
I couldn’t find any documentation on the internet from the identifying marks, so I burnt an afternoon figuring them out, and I’m posing my notes in case anyone else wants to make use of them.
The labeling on the motor itself is “SRG0808 003PLK5” which doesn’t turn up anything useful in a quick search, and the bag they came in is labeled “Fashion Worlds stepper motor 9496 x5” which is also not something googlable.
The motor comes attached to a flat flex cable with various adhesive pads built in, a boardlet, and a connector at one end. The output shaft is set in a brass gear roughly 2.75mm diameter with 12 involute profile teeth, about 3mm long – I don’t know small gears well enough to infer a ton from this, but it does seem like there is a lot of compatible gearing on the market.
To get around the lack of documentation, I probed one out with a DMM then built a test rig out of a dual L9110S H-Bridge board and a little STM32F103 dev board with the AccelStepper Arduino library to figure out the details.
They appear to be 20 steps per revolution motors, though they seem to work noticeably better with a half-step drive pattern. They work nicely at 3.3V, but get a little hotter than I’m comfortable with if energized for an extended period of time; I also tried 5V and it seems to tolerate that fine as well, gain a noticeable amount of extra torque, and get appreciably louder.
I don’t have the tools around to easily test the effective torque, but was way more than I expected based on my experiences with other small hobby motors. In my little taped-to-the-table test setup (pictured), if I jammed a fingernail into the rotor when it was already at speed at around at about 1000 steps/s on a 5V supply, the motor and/or nail deflected rather than missing steps.
If you look at the motor with the output shaft facing away from you and label the four pads A,B,C,D, the phases are A-D and B-C with about 9Ω across each phase.
If you look at the attached flat flex cable with the end pointed toward you, it has 7 contacts. For reference, let’s refer to them numbered 1-7 left to right. The ribbon itself is 4mm wide, and the contacts appear to be 0.5mm pitch, so it would probably mate with any of the various “7Pin 0.5mm Pitch FFC FPC” connectors floating around on the market for cheap if you wanted to spin a driver board for it that used the included cable.
The last 4 cable pins correspond to the motor terminals 4-D, 5-C, 6-B, 7-A… but for experimentation it’s easier to just solder leads directly to the motor pads. I used two pairs out of some old stranded CAT5, visible in the top picture.
There is a bonus component on a little arc-shaped boardlet built into the flat flex. It appears to be some manner of reflective infrared optical sensor, which I assume was used to establish a home position in whatever these were designed for use in – frankly since it has convenient mounting holes and wiring it would be pretty nice to use the same way in most applications I would want one of these in.
The first three ribbon pins are attached to this part, and none of these pins are shared with the motor itself. For discussion, let’s number the pins 1,2 left to right on the side toward the flex cable, and 3,4 right to left along the other in typical IC fashion. The pins are broken out Part 1 = Flex 1, Part 2 = Flex 2, Part 3 = also Flex 1, Part 4 = Flex 3.
Two of the pins (+ on 2, – on 3) appear to be a diode with a 1V forward voltage, and after I thought about it and checked with a camera with a bad IR filter, it is an infrared LED. The other pair seem to be a phototransistor or similar; it reads about 1.5MΩ from pin 4 to pin 1 in darkness and 1KΩ across the same with an IR LED pointed at it.
I’m not sure what I’m going to do with these, but they seem promising for small motion systems, especially since (if I bought bulk packs of each from China) you could get the motor and pair of H-bridges to drive it for under a dollar. Hopefully I’ll run into something to play with them in and/or my reversing work will enable someone else’s cool project.
Yesterday I pulled my ThinkPad 560E out, dd’d a floppinux on to a real floppy, booted it up and… it traps on an invalid opcode as soon as it tries to load init.
A little thinking made me realize that the way they were building their busybox binary was contaminating it with libraries from the system they were building on (which was apparently i686), so despite all their “will work on a 486 or later” option selections, the images they produced only work on i686 or later boxes.
I opened an issue then got obsessed and decided to fix it myself, and … you can read the details in my followup to the issue.
The magic lazy out for this kind of thing now is the pre-built musl based cross toolchains provided by https://musl.cc/
I made a couple other suggestions (about using musl, about configuring the kernel for xz and using it for the initrd, etc.) while I was hacking, because putting together little cross-compiled Linuxes is something I used to know what I was doing with. It did take a couple hours to spin back up, there are always picky cross-environment things to remember, and things have changed, mostly for the easier.
I’ve posted a copy of my generated i486-clean image. (Subsequently swapped out for a rebuild with slightly more useful busybox and kernel options, but only about 450k free)
Not that I’m a frequent IRC user for the last several years, but since the freenode implosion seems to have settled out as a real thing, I’ve gone ahead and registered myself as pappp on oftc and libera.chat where the various projects I occasionally pop in on have moved.
It’s a shame the long-stable single place for all the open-source project channels has gone away, but at least IRC is an open protocol so we can all just move to different servers when shit goes down.