How to fix the display backlight of the MOTU 828 mkII
Last week I scored a second hand MOTU 828 mkII external soundcard for just €100. The reason why the seller wanted to part from the unit for such a low price was a “broken screen”. Initially I thought that the whole screen was broken, as in not displaying any info at all. But coming home and plugging it in, I noticed the screen still worked, but it had no backlight.
Googling for “motu 828 mkii fix screen” lands you on this Word file, posted in a Gearslutz topic, with pictures describing how to disassemble the MOTU and replace the broken LED. In case the file gets removed, I’ve saved a copy on this website. They desolder the old smd LED assembly and push in what looks to be a 5 mm LED.
It will work, but I think it’s not the most elegant solution. First of all, one should always have a resistor in series when powering an LED. An LED is a diode and should be current limited. I also decided to use an smd LED instead of a big one, and reuse the current LED assembly to have a clean fix.
So having removed the assembly, this is what it looks like.
Notice the brown burned out LED. On one side we have an smd resistor reading ‘5R6’, so 5.6 Ohm. On the other side there’s another smd component in parallel with the LED. I can’t read the markings, so no idea what it is for. I can’t possibly think which other component would be required to drive an LED.
First I desoldered all components.
Next I added a red 0603 smd LED and a 68 Ohm smd resistor (see EDIT 1 below). The resistor value was chosen measuring the voltage coming from the two wires at the back of the display. They provide about 1.4v. A red smd LED requires about 20 mA, so 1.4/0.02=70 Ohm. This also explains why the previous LED burned: 1.4/5.6=250 mA, more than 10 times the normal current.
Keep in mind to solder the resistor to the correct contacts, the ones in series with the LED.
A red smd LED typically requires 2.2v, so with 1.4v it won’t have full brightness, but the result turned out fine.
The reason why I bought this soundcard is mainly because of its ADAT ports. I want to use them with Expert Sleepers ES-3 and ES-6 modules in a modular synth yet to be started, but that will be a whole other story.
EDIT 1 (05-Apr-2016): Reading through a forum thread that links to this page, I saw there is some confusion why I chose a red LED, not a green one. If you look up the typical voltage required by a 0603 SMD LED at 20 mA, you’ll find that for a red LED it’s 2.1V and for a green one 3.2V. The luminous intensity at those voltages and current are 450 mcd and 350 mcd. Now bear in mind that the voltage to the LED is only 1.4V, which is below the typical voltage for both colors! In order to have the best brightness, I chose a red LED. The typical voltage of 2.1V to drive a red LED is closer to the 1.4V provided, than the 3.2V for a green LED.
If you choose to take a green LED anyway, the resistor value can stay the same, because also for the green LED you stay well below the typical voltage. Only if the supplied voltage was for example 5V, you would have had to take it into account.