i’m using a mb64 firmware to control some leds via midi…
it’s an artistic project.
i noticed that sometimes when i turn on the system all the leds get bright. it happens even on another mb64(a complete midi controller).
so i turn off and on again untill the panel is powered with all the leds off…
is it an electrical problem? is it related to the multiplexers?
do i have to add some capacitors somewhere?
i need my system to start working at power on with all the leds off!!
Are you using all the standard parts including the bypass capacitors? Do you have short neat cabling from core to DOUTs? They are 74HC chips, not HCT?
the connection to the core is correct, becuse the machine works correctly… the problem is at power on.
another fact is that i’m using ultra bright leds, and some of those (60 leds for this project) now are fired…
i’m using 74hc595, without any bypass cap…
what are htc?
do i have to use 100uF for the bypass caps?
what exactly these caps do?
the machine works correctly… the problem is at power on.
LOL. “it’s not broken, but it’s broken” 
another fact is that i’m using ultra bright leds, and some of those (60 leds for this project) now are fired…
Maybe you haven’t used the correct resistors for your LEDs… This could certainly cause power supply issues at startup.
what are htc?
Logic levels. Search the forum, I’ve explained this before someplace, including links to more info, etc. HCT are more susceptible to noise due to looser levels.
do i have to use 100uF for the bypass caps?
what exactly these caps do?
Another one for the forum search. I don’t wanna start that argument again 
Sidenote: The bypass caps are ~100 n F not 100 u F
i added all the bypass caps and still all the leds are bright at power on.
it happens in 3 different midiboxes
but i repeat, sometimes at power on all the leds are normally off…
I am not an electronics engineer, but I do know that LED’s especially the bright ones, work as solar collectors producing electrostatic energy on incoming light. You may want to discharge them first before the power on by creating a shortcut first. The caps only backfire on you on this moment, as they are working as a battery.
But again, I maybe all wrong.
When they do not go on at power up, was that shortly after power down? What about running it in the dark for a try…
The fact that they got fried even with proper resistors means we’re dealing with abnormal voltages here.
All the best…
Gio.