820-3437 Nightmare

[tlough75]

Hello. So I have an 820-3437 that suffered medium to major liquid damage. It was drawing 9mA, PP3V42 was 0 volts, PPBUS was 0 volts. I had the replace several components in the 3V42 area and I still have to replace most of if not all of the components in the area next to the LCD connector as well as the LCD connector. The SMC area looks ok, no signs of liquid but some resistors have that dull look to them. Well since I was not getting anything from PP3V42 and no light I decide to replace all of the components in that area. I now get 2.7V on PP3V42 but no green light. PPBUS is now 8.18v so I checked SMC_RESET_L and it is at the 2.7V mentioned. So I assume my SMC is not turning on. Any ideas why PP3V42 is 2.7v vs the 3.42? Also, would you fix the other areas before getting PP3V42 properly working? I'll look at it more tomorrow I just thought I would ask on here first. This is just a project board for me to practice on so it is not super critical if I get it to work I just thought it would be neat to get it running because of the bad condition it was in.

 

[2informaticos]

I would reball SMC if corrosion is so advanced.
Check R7095/96 and U7090 voltages.

Post diode mode (red probe at grounde) at 3V42.
Don't forget to check the value once SMC is removed too.

 

[tlough75]

Still working on this one. I am going to do as you reccomend and will post back.

 

[tlough75]

diode mode is .296
R7095/96 ARE 1.25V
U7090 are all ok vs working board except 3.42v & voltage on pin 3. on working board it is like 7.2v. this one is like 5.9v

 

[2informaticos]

Be sure C7094 is OK.
Check all traces in the area.
You can play with R7095/96 values to adjust output voltage for 3V42 rail.

 

[tlough75]

I am missing the end of the probe point that goes to pin 1 of R7096. Is that just a test point? or does it need to be connected to anything else?

 

[tlough75]

I did pull C7094 and tested it to be ok. I will play with the resistir values

 

[tlough75]

I replaced U7090 as well but as you can guess nothing changed.

 

[2informaticos]

1.25V at FB pins points to good U7090 chip.
If all traces are good, you must play with Ra/Rb (R7095/96) to adjust correct voltage.

 

[tlough75]

Ok, I will try that next.

 

[tlough75]

What is the threshold voltage to get the SMC to turn on? does it have to be exactly 3.4V?

 

[2informaticos]

SMC can work with lower voltage.
But it MUST start to work when power is stable.
That's the reason there is a reset circuit.
Reset threshold is the level which decides when SMC can really START.

 

[tlough75]

R7096 was reading 278k when pulled off the board. I replaced it with a 200k and now we have a working board! Other than putting on a new LCD connector we can call this one solved! Thanks again for the help.