Refurbished Dell Latitude E7290 for Teaching with Linux

My Dell Latitude E7270 laptop had developed the disconcerting habit of blanking out the external HDMI signal at random. This 12.5" laptop was a premium product in 2016 but after four years of intensive use in my hands (second hand), it needed replacing. Perhaps I could have had the unit repaired, but the simplest option was to buy a newer model, the E7290, refurbished from the same place as the previous model, Australian Computer Traders. This time I purchased direct, not through Kogan. 

The package arrived on time, looked brand new (but with a HP USB power supply). One catch was that when I booted the machine it had the logo of a mining company on the startup screen. It looked as though Microsoft Windows was still registered to the previous owner. This was not a great problem as I was installing the Ubuntu version of Linux and erasing Windows. There were a few difficulties with Ubuntu: as per Dell's instructions I turned off the BIOS security. The install stopped part way through. I then found the suggestion online to switch off WiFi in the BIOS, install Ubuntu, and then switch it back on. This worked fine. 

The e7290 is a slim sleek minimalist machine, which looks almost the same as the e7270. It worked so well I bought another one for a friend and installed Ubuntu on that. The second unit had an illuminated keyboard, and a different generic USB charger. After installing Ubuntu, the computer would only recognise the charger intermittently. There is much written online about problems with USB chargers not being recognised by laptops. The charger worked fine with my other Dell, but neither charger would not work with the new laptop. After much head scratching I decided to work around the problem, old school. The e7290 has an old style Dell DC socket, as well as a USB C socket, for charging. So I ordered a generic DC charger ($22 from MSY), which worked fine (just make sure it has Dell's unusual 7.4 X 5.0mm plug).