I recently visited a friend's shop. She had a blinking LED downlight (One Hung Lo at it's finest) so I took it away to see if I can fix it.
I only took the light and not the PSU which was hard wired to 240V and I had no tools with me. Secondary = 20 - 42V, 350ma. ("42" yey!) My psu only goes up to 30V, so this is what I tested with.
The leds are arranged in approx 6 banks of 10 leds in series each. (I do say "roughly" because I did not count them or pay too much attention to it either)
4 leds, 1 in each bank were dead. Shorting them lit the others ok. I replaced these with some I had lying around which looked "roughly" similar. This was not an easy job as the leds were made of melty plastic which covered the pads completely and the solder tabs were underneath. I wonder how they managed to solder these on, either with a reflow oven or hot air blowy thingy. They melted at the mere sight of my soldering iron. after attaching 4, 2 of them failed, so I just grabbed 2 regular 3mm Red leds and soldered them in. After all, all I cared about is getting the volt drop across them and not how they light up. (dim Red). Tested, returned, plugged back in and working. Don't know for how long.
OK if you are still reading this, here are some questions:
Does the voltage drop across the Red leds differ to the original ones (White phosphur), and what sort of life shortening can I expect? I know this can be a bit vague, but give me an approximation.
During testing, I noticed that some of the banks with faulty leds would light up briefly as I turned up the voltage from 0 to 30. But in the shop, all the leds were blinking. Is there some sort of weird protection cct in the psu? "Oh the current is too low - restart!" I originally swapped the lamp with a good one and the fault followed the lamp, so the psu was ok.