Electronics 2000

hi guys and girls!

n00b to your forum and im slowly realising a n00b to electronics.

ive taken it upon myself to change the LED colour in my car from the horrible 'tobacco glow' orange to a nice crisp white. most things have gone swimmingly well until last night. i took apart my satnev unit and changed all the smd leds for the button illumination. i reconnected the ribbon cable and 2 of the 3 'banks' lit up.

theres a bank on the left (11 buttons, 12 LEDs), bank on the right (11 buttons, 12 LEDs), and a bank across the bottom(12 Buttons, 12 LEDs). theres also a pair across the top for CD slot illumination.

the bottom bank fully illuminates, the right bank illuminates all but one LED, the left bank is dead (unless a certain LED is pressed. i resoldered that joint and repaired a track on the PCB, it flickered and then died again... there is 2 in this bank that seem to work when they want to) and finally the CD slot pair are dead.

ive tried the dead cd slot and right bank LEDs for orientation ... i had them in the correct way.

all LEDS when tested individually light up, but now after all my fettling with the PCB, when attached as it would be installed, is dead. i think i can pretty safely rule out the feeds from the ribbon cable to the PCB, im 90% sure the issue is somewhere on this board but im not gonna rule out me frying something further back :'(

heres some info i do know the LEDs are most probably a different voltage to the originals that were fitted but in my experience with the rest of the car this doesnt seem to be a problem. the PCB in question has a L4949EP voltage regulator to run the voltage from 12v to 5v (afaik this is a pretty standard chip for automotive applications) the chip does get rather hot, i know its a voltage dropper so has to loose the difference somewhere but to me it seems a little too hot... although im probably wrong

im after some guidance really, whats the next step if i know all the LEDs are good. i can post hi-res images if it would help.

Generally White and Blue LEDs have a forward voltage of around 3.4V this varies due to temperature and the LED die itself.
Assuming the LEDs and soldering are good, it sounds like the supply voltage isn't high enough to run some of the strings.

Due to the variance during manufacture, it's common to see some LEDs turn on before others when you wire them in series and ramp the voltage up.

Personally, I would start by testing the LEDs, the diode test function on a multi-meter should be enough to light a single white LED on it's own thereby allowing you to test each on in situ.

Have you managed to identify any current limiting resistors on the board? If you are lucky you might be able to change some of the resistors but if you do only have 5V to play with then the solution maybe more complex.

Nick

there seems to be an absolute boat load of resistors on the back of the borard iirc... ill grab my multimeter and get probing...

on a similar note,,, i tend to get quite clammy hands when soldering... if i power the board from the head unit (i.e. as it would be installed in the car) and hold some of the resistors i can get different banks to light up. its not an exact science and probably a massive no-no to finger the stuff on the boards. im guessing with all my fingering and prodding ive shorted something... just hope i havent fried an important chip.