34461A white screen cannot start

[晓看风云]

Hello everyone, I have a 34461A that needs to be repaired. At first, its fault was a white screen, but later it turned into a black screen. After reading the forum and maintenance manual, I found that the FPGA power supply voltage was very low, and the DS200 on the motherboard kept flashing. I suspect it may be due to the power overcurrent protection caused by a short circuit in SPEAR320S-2.

We replaced the SPEAR320S-2 with a brand new one, but it turned on with a white screen and the FPGA indicator lights turned on normally. The DS200 only turned on the left half of the yellow LED.

Now, I have found that there is a CR327 capacitor behind the MCU motherboard that is short circuited. I think it is not a capacitor short circuited. I want to know where one end of the capacitor is connected to GND and the other end is connected to? If you can help me, thank you very much!

[u666sa]

It would be nice to see how this one ends ups. 


Take your meter in ohms mode and go over each and every single one of caps. Whatever looks __ off __ blow off with air and check again with LCR meter.

[coromonadalix]

once again 

sad to see this problem  ...   

Does the mcu have busted  because it was getting too hot ?

Voltage lines problems,  spikes  etc ...

or the bga's / solder balls are not fully soldered ??  but you said  there is some shorted caps ?

[晓看风云]

I applied 1.4V/1A to both ends of the short-circuit capacitor, and the results obtained from thermal imaging showed that the line from tps65070 to the short-circuit capacitor was heating up, about 30 degrees Celsius. As the voltage and current I applied were not significant, I was worried about burning the line. Upon inspection, it was found that 44/45/46 (AD_IN2/3/4) of tps65070 was connected to GND, which should be the foot position of the touch screen. However, all three outputs of tps65070 are very normal, with 1.2V/1.8V/3.3V being normal.

Thank you very much. I think I will try blowing off tps65070 when I am at a loss. I am worried that too many things will be dismantled, leading to the expansion of the malfunction. I am still a novice in the hot air gun.

[coromonadalix]

you have low temp  chipquick solder paste in a syringe

just need to order a stencil .... or not ??

http://www.chipquik.com/store/index.php?cPath=400&osCsid=vr6jdra3dqjnl7nl0p6ggget03

yeah expensive ...

[晓看风云]

No need for anything else, the new Spear320s-2 is already worth the tin ball! Of course, some soldering flux will be needed. It's best for you to have a BGA welding station or seek help, because you don't know if your own operation caused the death of the brand new MCU.

[u666sa]

When thermal imaging shows chip, usually it's something around it. Check all these caps and coils for short to ground. Multimeter in diode mode.

[coromonadalix]

diode mode is bad   and it depends of how the supply circuit is designed, in some case  you can get falses values


your best luck is try to reverse the design ... draw some schematic

the datasheet show the working principles,  the coils are used in a buck boost or buck down mode ....  at their outputs you havefiltering capacitors

you should not get a short between the voltages lines and the ground ...
if so,   problems  are further away / down the line

at least 5 voltages can come out of this ic

[jonpaul]

Get Keysigbt service may be in warranty

J

[coromonadalix]

i think i would go with the suggested values of 124k  in the datasheet  ...   

since iset2 is around the same value, i think you have your answer

just put 2 brand new ones  to be sure they are okay and equal values

--------------------------------

Dropping theur values to zero,  not sure i would do that ...

to be sure of everything, you would  need to find the lcd specs, mostly the backlight current consumption,    or using an variable psu with an ammeter, you would know how much it consumes and maybe redo resistors calculations


being in a state or urgency / emergency something ... you'll make mistakes, if you do not do things  "cold headed" 

[coromonadalix]

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 ??

[coromonadalix]

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 ..

[coromonadalix]

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.

[coromonadalix]

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

[harha]

Hi
I have an Keysight 34461A with the known white screen.Looking for spear320s-2-but can only find spear320-2!!Do any knowif i can use the spear320-2 instead?--i
Hardy