My LASER printer / scanner, a Brother MFC-L2729DW, would not power on. Normally something like this is due to a power supply failure, but I googled and found this: <https://www.youtube.com/watch?v=OgkHv1c01tQ> (The video is incomplete; there is another one that is linked from the first, but all the useful info is in the first.) Following his instructions, I found that I could reset the machine to work. But he said that it would fail again on a power failure or even just turning it off. Furthermore, doing this too many times breaks the printer. It turns out that settings are kept in a RAM that is powered by a supercapacitor (instead of a battery). Furthermore, that supercapacitor wears out. All you need to do to fix the printer is to replace the supercapacitor. The video claims all recent Brother printers have these supercapacitors. The video shows you how the replace the supercapacitor. You need a new supercapacitor, which he will sell you. Armed with this information, I - bought a suitable supercapacitor from digikey.ca (cheaper and faster than ordering from Australia). Actually I bought a slightly smaller one, but that seems to be OK. The original was 0.33F, 5.5V. I paid $3.48 for two, plus $8.00 shipping, plus HST. - pulled apart the printer, as per video - hacked out the old supercapacitor and soldered (badly) the new supercapacitor. (Doing the job right would have required removing the circuit board. Instead, I left the old supercapacitor's legs and soldered the new one's legs to the old ones.) - struggled to get the plastic panel back in place. It works.