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A long time ago, there was a small Estonian website called “Mängukoobas” (literal translation from Estonian is “game cave”). It started out as a place for people to share various links to browser games, mostly built with Flash or Shockwave. It had a decent moderation system, randomized treasure chests that could appear on any part of the website, and a lot more.1 What it also had was a basic filtering system. As a good chunk of the audience was children (myself included), there was a need to filter out all the naughty Estonian words, such as “kurat”, “türa”, “lits” and many more colorful ones. The filtering was very basic, however, and some took it to themselves to demonstrate how flawed the system was by intentionally using phrases like “politsei”, which is Estonian for “police”. It would end up being filtered to “po****ei” as it also contained the word “lits”, which translates to “slut”2. Of course, you could easily overcome the filter by using a healthy dose of period characters, leading to many cases of “po.l.i.t.sei” being used. With the ZIRP phenomenon we got a lot of companies wanting to get into the “platform” business where they bring together buyers and sellers, or service providers and clients. A lot of these platforms rely on transactions taking place only on their platform and nowhere else, so they end up doing their best to avoid the two parties from being in contact off-platform and paying out of band, as that would directly cut into their revenue. As a result, they scan private messages and public content for common patterns, such as e-mails and phone numbers, and block or filter them. As you can predict, this can backfire in a very annoying way. I was looking for a cheap mini PC on a local buy-sell website and stumbled on one decent offer. I looked at the details, was going over the CPU model, and found the following: CPU: Intel i*-**** Oh. Well, maybe it was an error, I will ask the seller for additional details with a public question. The response? Hello, the CPU model is Intel i*-****. Damn it. I never ended up buying that machine because I don’t really want to gamble with Intel CPU model numbers, and a few days later it was gone. It’s 2025, I’m nearing my mandatory mid-life crisis, and the Scunthorpe problem is alive and well. fun tangent: the site ended up being like a tiny social network, eventually incorporating things like a cheap rate.ee knock-off where children were allowed to share pictures of themselves. As you can imagine, this was a horrible, horrible idea, as it attracted the exact type of person that would be interested in that type of content. I got lucky by being so poor that I did not have a webcam or a digital camera to make any pictures with, and I remember that fondly because someone on MSN Messenger was very insistent that I take some pictures of myself. Don’t leave children with unmonitored internet access! ↩︎ “slut” is also an actual word in Swedish which translates to “final”. I think. I’m not a Swedish expert, actually. ↩︎
I use Uptime Kuma to check the availability of a few services that I run, with the most important one being my blog. It’s really nice. Today I wanted to set it up on a different machine to help troubleshoot and confirm some latency issues that I’ve observed, and for that purpose I picked the cheapest ARM-based Hetzner Cloud VM hosted in Helsinki, Finland. Hetzner provides a public IPv6 address for free, but you have to pay extra for an IPv4 address. I didn’t want to do that out of principle, so I went ahead and copied my Docker Compose definition over to the new server. For some reason, Uptime Kuma would start up on the new IPv6-only VM, but it was unsuccessful in making requests to my services, which support both IPv4 and IPv6. The requests would time out and show up as “Pending” in the UI, and the service logs complained about not being able to deliver e-mails about the failures. I confirmed IPv6 connectivity within the container by running docker exec -it uptime-kuma bash and running a few curl and ping commands with IPv6 flags, had no issues with those. When I added a public IPv4 address to the container, everything started working again. I fixed the issue by explicitly disabling the IPv4 network in the Docker Compose service definition, and that did the trick, Uptime Kuma made successful requests towards my services. It seems that the service defaults to IPv4 due to the internal Docker network giving it an IPv4 network to work with, and that causes issues when your machine doesn’t have any IPv4 network or public IPv4 address associated with it. Here’s an example Docker Compose file: name: uptime-kuma services: uptime-kuma: container_name: uptime-kuma networks: - uptime-kuma ports: - 3001:3001" volumes: - /path/to/your/storage:/app/data image: docker.io/louislam/uptime-kuma restart: always networks: uptime-kuma: enable_ipv6: true enable_ipv4: false That’s it! If you’re interested in different ways to set up IPv6 networking in Docker, check out this overview that I wrote a while ago.
I love 3D printing. Out of all the tech hype cycles and trends over the last decade, this one is genuinely useful. There’s simply something magical about being able to design or download a model from the internet, send it to a machine, and after a few hours you get an actual physical object in return! I don’t own a 3D printer myself, but I’ve had access to people who are happy to help out by printing something for me. So far I’ve printed the following useful things: a Makita vacuum cleaner holder a dual vertical laptop stand it’s such a simple and cheap design, and yet it works incredibly well if you add some rubberized material to the bottom and inside the laptop holder a dual HDD adapter for a Zimaboard a stand for the Steam Deck a carrying case insert for the Steam Deck a case for the Orange Pi Zero There’s so much more that I’d want to print, like various battery holders, controller stands, and IKEA SKÅDIS mounts. There’s also the option of downloading and printing a whole PC case, which is incredibly tempting. Will I finally be able to build the perfect home server according to my very specific requirements? Probably not, given how often my preferences change, but it would be incredibly cool! And yet I don’t own a 3D printer. The main obstacle for me is the time, I feel like in order to be successful with a 3D printer, I’ll need to at the very least learn the basics of filaments, their properties, what parameters to configure and how, how to maintain a 3D printer, how to fix one when it breaks, how to diagnose misalignment issues etc. I’ll also need space for one, extruding hot melting plastic seems like a thing that I’d want to host in a proper workshop and with actual ventilation. It’s a whole-ass hobby, not a half-ass one. Durability can be problematic with 3D prints, even in my limited experience. For example, I tried positioning the Makita vacuum cleaner holder differently, but ended up putting too much strain on the design, which eventually lead to it completely failing. In other cases, filaments like PLA aren’t suitable for designs where they are attached to warm or hot computer parts, they will warp like crazy. I appreciate the hell out of anyone that shares their designs with the world, and especially those that allow remixing or customizing their designs. There are fantastic designs and ideas out there on sites like Printables, and the creativity that’s on display warms my heart.
Today I learned that Kagi uses Yandex as part of its search infrastructure, making up about 2% of their costs, and their CEO has confirmed that they do not plan to change that. To quote: Yandex represents about 2% of our total costs and is only one of dozens of sources we use. To put this in perspective: removing any single source would degrade search quality for all users while having minimal economic impact on any particular region. The world doesn’t need another politicized search engine. It needs one that works exceptionally well, regardless of the political climate. That’s what we’re building. That is unfortunate, as I found Kagi to be a good product with an interesting take on utilizing LLM models with search that is kind of useful, but I cannot in good heart continue to support it while they unapologetically finance a major company that has ties to the Russian government, the same country that is actively waging a war against Ukraine, an European country, for over 11 years, during which they’ve committed countless war crimes against civilians and military personnel. Kagi has the freedom to decide how they build the best search engine, and I have the freedom to use something else. Please send all your whataboutisms to /dev/null.
More in technology
An affordable, AI-assisted, wearable robotic arm? That’s not science fiction – it’s RedSnapper, an open-source prosthesis built by PAC Tech, a team of high school students from Istituto Maria Immacolata in Gorgonzola, Italy. Powered entirely by Arduino boards, their project won the national “Robot Arm Makers” title at the 2025 RomeCup – and we think […] The post Meet the teens behind RedSnapper: a smart Arduino-powered prosthetic arm appeared first on Arduino Blog.
Introduction The SR620 Repairing the SR620 Replacing the Backup 3V Lithium Battery Switching to an External Reference Clock Running Auto-Calibration Oscilloscope Display Mode References Footnotes Introduction A little over a year ago, I found a Stanford Research Systems SR620 universal time interval counter at the Silicon Valley Electronics Flea Market. It had a big sticker “Passes Self-Test” and “Tested 3/9/24” (the day before the flea market) on it so I took the gamble and spent an ungodly $4001 on it. Luckily, it did work fine, initially at least, but I soon discovered that it sometimes got into some weird behavior after pressing the power-on switch. The SR620 The SR620 was designed sometime in the mid-1980s. Mine has a rev C PCB with a date of July 1988, 37 year old! The manual lists 1989, 2006, 2019 and 2025 revisions. I don’t know if there were any major changes along the way, but I doubt it. It’s still for sale on the SRS website, starting at $5150. The specifications are still pretty decent, especially for a hobbyist: 25 ps single shot time resolution 1.3 GHz frequency range 11-digit resolution over a 1 s measurement interval The SR620 is not perfect, one notable issue is its thermal design. It simply doesn’t have enough ventilation holes, the heat-generating power regulators are located close to the high precision time-to-analog converters, and the temperature sensor for the fan is inexplicably placed right next to the fan, which is not close at all to the power regulators. The Signal Path has an SR620 repair video that talks about this. Repairing the SR620 You can see the power-on behavior in the video below: Of note is that lightly touching the power button changes the behavior and sometimes makes it get all the way through the power-on sequence. This made me hopeful that the switch itself was bad, something that should be easy to fix. Unlike my still broken SRS DG535, another flea market buy with the most cursed assembly, the SR620 is a dream to work on: 4 side screws is all it takes to remove the top of the case and have access to all the components from the top. Another 4 screws to remove the bottom panel and you have access to the solder side of the PCB. You can desolder components without lifting the PCB out of the enclosure. Like my HP 5370A, the power switch of the SR620 selects between power on and standby mode. The SR620 enables the 15V rail at all times to keep a local TCXO or OCXO warmed up. The power switch is located at the right of the front panel. It has 2 black and 2 red wires. When the unit is powered on, the 2 black wires and the 2 red wires are connected to each other. To make sure that the switch itself was the problem, I soldered the wires together to create a permanent connection: After this, the SR620 worked totall fine! Let’s replace the switch. Unscrew 4 more screws and pull the knobs of the 3 front potentiometers and power switch to get rid of the front panel: A handful of additional screws to remove the front PCB from the chassis, and you have access to the switch: The switch is an ITT Schadow NE15 T70. Unsurprisingly, these are not produced anymore, but you can still find them on eBay. I paid $7.5 + shipping, the price increased to $9.5 immediately after that. According to this EEVblog forum post, this switch on Digikey is a suitable replacement, but I didn’t try it. The old switch (bottom) has 6 contact points vs only 4 of the new one (top), but that wasn’t an issue since only 4 were used. Both switches also have a metal screw plate, but they were oriented differently. However, you can easily reconfigure the screw plate by straightening 4 metal prongs. If you buy the new switch from Digikey and it doesn’t come with the metal screw plate, you should be able to transplant the plate from the broken switch to the new one just the same. To get the switch through the narrow hole of the case, you need to cut off the pins on the one side of the switch and you need to bend the contact points a bit. After soldering the wires back in place, the SR620 powered on reliably. Switch replacement completed! Replacing the Backup 3V Lithium Battery The SR620 has a simple microcontroller system consists of a Z8800 CPU, 64 KB of EPROM and a 32 KB SRAM. In addition to program data, the SRAM also contains calibration and settings. replaced one such battery in my HP 3478A multimeter. These batteries last almost forever, but mine had a 1987 date code and 38 years is really pushing things, so I replaced it with this new one from Digikey. The 1987 version of this battery had 1 pin on each side, on the new ones, the + side has 2 pins, so you need to cut one of those pins and install the battery slightly crooked back onto the PCB. When you first power up the SR620 after replacing the battery, you might see “Test Error 3” on the display. According to the manual: Test error 3 is usually “self-healing”. The instrument settings will be returned to their default values and factory calibration data will be recalled from ROM. Test Error 3 will recur if the Lithium battery or RAM is defective. After power cycling the device again, the test error was gone and everything worked, but with a precision that was slightly lower than before: before the battery replacement, when feeding the 10 MHz output reference clock into channel A and measuring frequency with a 1s gate time, I’d get a read-out of 10,000,000.000N MHz. In other words: around a milli-Hz accuracy. After the replacment, the accuracy was about an order of magnitude worse. That’s just not acceptable! The reason for this loss in accuracy is because the auto-calibration parameters were lost. Luckily, this is easy to fix. Switching to an External Reference Clock My SR620 has the cheaper TCXO option which gives frequency measurement results that are about one order of magnitude less accurate than using an external OCXO based reference clock. So I always switch to an external reference clock. The SR620 doesn’t do that automatically, you need to manually change it in the settings, as follows: SET -> “ctrl cal out scn” SEL -> “ctrl cal out scn” SET -> “auto cal” SET -> “cloc source int” Scale Down arrow -> “cloc source rear” SET -> “cloc Fr 10000000” SET If you have a 5 MHz reference clock, use the down or up arrow to switch between 1000000 and 5000000. Running Auto-Calibration You can rerun auto-calibration manually from the front panel without opening up the device with this sequence: SET -> “ctrl cal out scn” SEL -> “ctrl cal out scn” SET -> “auto cal” START The auto-calibration will take around 2 minutes. Only run it once the device has been running for a while to make sure all components have warmed up and are at stable temperature. The manual recommends a 30 minute warmup time. After doing auto-calibration, feeding back the reference clock into channel A and measuring frequency with a 1 s gate time gave me a result that oscillated around 10 MHz, with the mHz digits always 000 or 999.2 It’s possible to fine-tune the SR620 beyond the auto-calibration settings. One reason why one might want to do this is to correct for drift of the internal oscillator To enable this kind of tuning, you need to move a jumper inside the case. The time-nuts email list has a couple of discussions about this, here is one such post. Page 69 of the SR620 manual has detailed calibration instructions. Oscilloscope Display Mode When the 16 7-segment LEDs on the front panel are just not enough, the SR620 has this interesting way of (ab)using an oscilloscope as general display: it uses XY mode to paint the data. I had tried this mode in the past with my Sigilent digital oscilloscope, but the result was unreadable: for this kind of rendering, having a CRT beam that lights up all the phosphor from one point to the next is a feature, not a bug. This time, I tried it with an old school analog oscilloscope3: (Click to enlarge) The result is much better on the analog scope, but still very hard to read. When you really need all the data you can get from the SR620, just use the GPIB or RS232 interface. References The Signal Path - TNP #41 - Stanford Research SR620 Universal Time Interval Counter Teardown, Repair & Experiments Some calibration info about the SR620 Fast High Precision Set-up of SR 620 Counter The rest of this page has a bunch of other interesting SR620 related comments. Time-Nuts topics The SR620 is mentioned in tons of threads on the time-nuts emaiml list. Here are just a few interesting posts: This post talks about some thermal design mistakes in the SR620. E.g. the linear regulators and heat sink are placed right next to the the TCXO. It also talks about the location of the thermistor inside the fan path, resulting in unstable behavior. This is something Shrirar of The Signal Path fixed by moving the thermistor. This comment mentions that while the TXCO stays powered on in standby, the DAC that sets the control voltage does not, which results in an additional settling time after powering up. General recommendation is to use an external 10 MHz clock reference. This comment talks about warm-up time needed depending on the desired accuracy. It also has some graphs. Footnotes This time, the gamble paid off, and the going rate of a good second hand SR620 is quite a bit higher. But I don’t think I’ll ever do this again! ↩ In other words, when fed with the same 10 MHz as the reference clock, the display always shows a number that is either 10,000,000,000x or 9,999,999,xx. ↩ I find it amazing that this scope was calibrated as recently as April 2023. ↩
