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.
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