TPD LED Problem.

[Rowan]

I've just built a TPD for my MBSEQ and have a problem with one of the Red LED columns.

The first colum of Red LEDs on the the righthand LED matrix are always on, apart from the bottom one. Strangle, when I power on the device and the text scrolls across the matrix the problem isn't there.

I'm guess the problem is caused by a short/open connection somewhere but I really don't want to remove he LED matrix unless I have to.

Has anyone got a idea as to where the fault could be?

Edited March 15, 2015 by Rowan

[Phatline]

i had such behavior  by a led-matrix built of single leds... i plugged the pins in the false DOUT-Registers....

you cant plug it into the false doutregisters... because you have a ready built pcb... so i think:

a: wrong LED-Matrix (common katode/anode?)

b: if possible false connected LED-Matrix > solderede 180° wrong...

c: you have to change the software you use...

[Rowan]

i had such behavior  by a led-matrix built of single leds... i plugged the pins in the false DOUT-Registers.... you cant plug it into the false doutregisters... because you have a ready built pcb... so i think: a: wrong LED-Matrix (common katode/anode?) b: if possible false connected LED-Matrix > solderede 180° wrong... c: you have to change the software you use...

Thanks for the suggestions. A & B: I'm pretty sure the the anode/cathode are correct as all the other LEDs in the matrix work correctly. C: I'm using MBSEQ 4.088 that I compiled myself, I might roll back to the offical 4.087 release and see if that makes a difference.

[Rowan]

I've rolled the firmware back to 4.087 and the problem persists so I guess it must be a hardware problem.

 

Now I need to decide if I can live with the problem or remove the LED matrix and risk destroying the unit.

[ilmenator]

Hi Rowan,

 

I cannot wrap my head around this right now, so I can't tell you which joint exactly is broken, but it certainly sounds like one leg of a shift register is not making contact with the PCB. The welcome text doesn't use the lowest row of LEDs, so I guess that's why the problem doesn't show when you switch on the unit.

 

If you have proper desoldering equipment I would go ahead and try to fix the problem - without a desoldering gun I'd probably just leave it as it is now.

 

Best, ilmenator

[Rowan]

I spent some time on this tonight and I've fixed the problem.

Though I'm not 100% sure, I think the actual LED matrix itself was faulty. I removed the matrix and tested all the joints with a meter and everything was fine. I inserted the matrix back into the PCB without soldering it in to see what would happen and some of the LEDs in the column in question came on. As I had another LED matrix at hand i put that in the PCB and the fault didn't occour, so I'm putting the problem down to the matrix.

I've now replaced both LED matrices with new/better quality units and it's working perfectly.

Edited April 9, 2015 by Rowan

[ilmenator]

Good to hear that you managed to fix the problem! I was quite convinced the problem was due to a bad solder joint on one of the shift registers, but it certainly sounds like it was the LED matrix now. Where did you source the faulty LED matrix - just so we know what to avoid?