How to fix the display backlight of the MOTU 828 mkII

Niek

I'm a pharmacist who likes to travel, design electrical stuff, airsoft and sail...

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6 Responses

  1. Peter Wood says:

    Thank you very much for this post. I just replaced the faulty LED in my MOTU 828 Mk2. I didn’t need to add a resistor, but simply used a 3mm Ultra Bright LED from UMT Media on eBay (part number 262440490767).

    So grateful for your help!

  2. Tudy says:

    Thank You very much!
    I used 5mm white LED WW-M05A34WUQ4-B1-Q without added resistor. Current is 16mA and display is very nice now.

  3. Richard Alton says:

    Dear Niek,
    After scratching my head a while, I just realised that with a 12V supply, my changing the onboard resistor from 7R5 to 8R2 would have made almost no difference to the current through the LED.
    I failed to mention before that I noticed a 220 Ohm 1W resistor on the main board which is connected in-line with the supply to the backlight feed wires. This is most likely the main current reducing resistor for the LED.
    I suspect therefore that the extra components on the backlight pcb are a reverse diode and accompanying inline resistor connected across the LED. This arrangement is typically used to protect the LED in case the supply wires are reversed.
    Also note – this is also why the original idea of discarding the backlight pcb and replacing it with just an LED works fine as well as the current reducing resistor is still in circuit on the main board.

    Warm regards
    Richard

  4. Richard Alton says:

    Many thanks for your wonderful post. I managed to repair the backlight in an 828 mkii which was given to me for free! with your help!!
    One thing to note – The off-load voltage to the led PCB is actually around -12V. I believe the 1.4V you measured would have been with the damaged LED still in circuit which was probably dragging the supply down. I measured 2.2V with my damaged LED. For this reason I was able to refit a blue LED which when in circuit brought the supply down to the forward voltage level of 3.4V as expected. Also my original inline resistor value was 7R5 Ohms which I increased still further to the next preferred value of 8R2 Ohms in hope of easing the strain on the new LED. It still looks plenty bright enough to me.
    Warm Regards
    Richard

  5. Martin Persson says:

    Thanks for the nice description! I just fixed my 828 mkII using this guide (combined with the original Word document). I left both driver components on the PCB and used a green 0603 LED from Everlight as replacement. The backlight now works but is very dim. Should probably have done some calculations before to get the right current for my replacement LED.

    Some observations:

    The LED on my unit looked completely different from yours. The housing was completely transparant and it was embedded in some kind of transparrent rubber.

    The unknown komponent on teh PCB was marked “C1”. After some research and measurements I concluted that it’s most likely a Zener diode connected in parallell with the LED. Don’t know enough about LED deriver circuitery to tell if it makes any sense or not.

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