Monday, 27 August 2012

Fixing a John Lewis clock that goes black in a dark room (and changing its colour too)

Despite their faults, we like the style of these John Lewis Indigo clocks.  We have two, and one of them recently started to flicker at low light levels, and then went completely dark whenever the room was dark.  They're supposed to dim in a dark room, but not go out completely.  I took it apart to look for the source of the problem.

WARNING: if you're going to do similar, be aware that there are mains level voltages inside this clock.  Disconnect from the mains and pull out the plug before you open the clock, and don't plug it in again until it's reassembled.  And don't come running to me if you kill yourself.

Inside the clock there are four blue 3mm LEDs that poke into holes in a plastic light-spreader behind the LCD display.  A photodiode, a couple of transistors and some resistors control the LEDs' brightness, however one of the LEDs wasn't lighting up at all.  (Ok, I admit it, I turned on the power while I had the thing in pieces in order to discover this.)  I can't say I'd noticed that the backlight was uneven - the light-spreader must do a good job.

I didn't have any spare 3mm blue LEDs in stock, but I did have white ones, so I desoldered and removed all four, and replaced them with white ones, taking care to insert them with the correct polarity.  I also had some red LEDs in stock, but I chose the white because white LEDs have very similar properties to blue ones (in particular, the forward voltage), whereas red ones are quite different (a much lower forward voltage).

For some reason, the faulty LED was also causing the other three to extinguish at low light levels - with all four replaced it is working perfectly again now.

The white backlight looks really good, and matches the room better too:

The quality of the soldering inside the clock is very poor.  I should have taken a photo, but forgot.  You'll have to take my word for it.  Poor, very poor.