Electronics > Repair

34461A white screen cannot start

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anything in the 120k 100k ...   not going down to zero  for sure,   maybe 10k  ....

unless this ic is busted ??

or something is opened in the supply path ??

if you want to put zero values   it's your call,  for me it's a no,  you could really have backlight problems  and the ic may or could fry ??

even on the spear mcu  there could be some backlight pin control / contrast ... output .... a simple bad solder could be a problem

To be sure
Try to supply the bakclight with an external current controlled psu     if it lit's up and you see the meter display / digits etc ....  if it is okay, the schematic show an serial parallel diode array on the 2x Isink pins, and the resistors does current limitations for the ic, it as 25 ma max output per rail, you could be in an overload situation ?

If not you have to follow this circuit, understand it / repair it

small story : i have dealt with lots of fakes ones recently, even huge fgpa's ...  micro power ic's  etc ....  a nightmare, i have one 10x10 cm pcb who cost as much as the 34461a meter loll,  has 11 supply lines in it for fpga, memory, i/o's  and 1 huge (16 channels) supply supervisor ic, when this one decide to play with you .. you're up for the long run

For me changing twice an ic, is still possible : Simply overheating it, could have damaged it ...still unresolved issues around it .... many factors can or could happen, or happened ??  manupulation(s)  etc ...

Brand new is not a garanty of resolving your problem, and sadly keysight is not helping

If other(s) have repaired the unit with zero values its their problems if it blows up again ...  the circuit has been deisgned a certain way ...  they would not put parts who serve  / do nothing ??

And this ic has many supply lines,  dont forget that some meters have problems reading the inductors voltages if it is on the ic side / frequency switching ... once it's filtered, you can read good values, you have to understand the topology used in this case, feedback lines,  powergood, circuit biaising .... reset line, enable line

changing a part is sometime easy, sometimes not ..

if i understand their calculations, the strip seems to consume slightly higher current,   

could be the leds over time as they get used / heat  etc ... their properties have changed a little, that could be a problem in the design ???  i dont know

in your case  if im not wrong

to give the maximum output (25ma) on each strip ouputs, you could put resistors around 47k - 49k  each value,  that should  pimp the output current a bit,
that would give a total of 50ma  total / 25 ma on each output

124k  was a max of 10ma per ISINK output = Total of 20ma    wich apparently you slightly go over ??
60k - 62k   would double the value 20ma per ISINK output = Total of 40ma

Not sure i would push the ic to the max of 25ma per ISINK output, (total of 2x 25ma  for the entire IC  led section) it would need around not lower than 50k ohms
Maybe the ic could heat a little ??    but it's an PWM output ??  and you should keep some safety margins

at 0 ohm, you disable all curent protection given by the ic  ...

i would keep a eye on it

TERRA Operative:
I have a working 34461A, if the two resistors are not hard to access and can be measured in-circuit (or have markings) I can take a look at mine.

My original 34461A failed with white screen a while back but it was sent back under warranty. It came back with a new serial number, so I assume the main board was probably replaced (or I got a new unit...)
Unfortunately I have no idea what the issue was, Keysight wouldn't tell me.

glad it worked   :-+

for sure knowing theses 2 resistors values  would be a help for future repairs ???

keysight for sure know something,  but simply change a pcb ??? 

wow  are they afraid to do a recall of some sort ??  even theses who past the warranty period,  what a let down


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