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820-3437-B no post chime, good USB, no boot, no video, fan speed good

M

mainmacman

New member
Good day, I am a newb. liquid damaged board, CPU not getting warm. while reading forums for some help I looked at this section of the board to check for corrosion around U5110. I noticed these resistors are gone. R5116 and R5101. I believe this would explain why I cannot reset SMC but do not know. I looked at schems and do not believe that the lack of these resistors would cause no chime. no boot. I am looking for advice on best way to check for missing voltage to allow MBA to post. Any advice appreciated, please. ALL_SYS_PWRGD is present 3.3v
 

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M

mainmacman

New member
on L7310 I get 1.698V. U1950 does not look corroded. C1950 and R1960 when checking crumbled to dust. next up will try and find a donor board with similar components. At the risk of sounding stupid, do they need to be exact replacements? what is the most efficient way to find these parts on other Macs? Have plenty of dead ones here...
 
M

mainmacman

New member
I am curious, it appears that many of the Main Logics have the same power circuitry. Is there a defined efficient way to check in order power rails quickly to determine what exactly is missing? On U1950 on this board after looking at the logic breakdown of the chip and then looking at schematics I noticed many of the power rails were the same throughout the board. Is there a voltage divider that takes the AC in and splits all these power rails? There has to be some power regulation/creator circuitry that pumps out all needed voltages... Sadly, Because I am a newb and learning slowly I cannot find this in the schems... is there test point locations on the board that breaks out these voltages?
 
D

dukefawks

Administrator
If CPU Vcore is present all other rail will be OK. Is a USB mouse lighting up then plugged in? Does the board turn on/off 3 times before staying on?
 
M

mainmacman

New member
Yes, Mouse does light on both USB ports. Board stays on, does not turn on/off 3x.


looking forward to review of worthy hot air station, tried to purchase Crest cleaner however site states still out of stock.
 
D

dukefawks

Administrator
Do a PRAM reset with and external keyboard. If a USB mouse lights up it means the board is working somewhat. After PRAM reset it should chime, IO board needs to be connected of course.
 
M

mainmacman

New member
installed board into chassis, connected speakers. with AC adapter plugged in fan spins automatically normal speed, when I do connect the AC adapter the fan automatically spins without hitting power key. With Apple external keyboard and Apple mouse connected on both left and right side no mouse light. Mouse lights if only it is connected to both sides. Could not complete PRAM reset. To me it seems like it is getting some power, just not enough. I have not replaced C1950 and R1960. What should be correct voltages on U1950.? What is good voltage for CPU Vcore?
 
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