820-00840 no image

chrislewis85

New member
Hi guys,

Newbie here still learning!

I have an 820-00840 here with no image.

Board has no signs of present fluid damage but appears to have had a previous repair to PP3V3_S4 (wire to replace broken trace).

Board boots to OS fine and video fine when connected to external screen.

Display prefs shows no internal screen detected when booted into OS. Same with known-good screen.

I have ordered a new display flex (the short one screwed to logic board) to eliminate that as I do not have a known good to test with - beyond this is there anything else I should check?

No visible damage to J8500 or surrounding components

Thanks in advance!

Chris
 

chrislewis85

New member
Update to this one.

Symptom is the same with known good Display assembly and flex cable.

System chimes, boots, get kb backlight and caps lock. Has booted into OS if connected to external display. Internal display is not detected in display prefs, no image on screen.

Where should I go from here on this one?
 

2informaticos

Administrator
Staff member
Check if you get 3V3/5V_S0SW_LCD; at least in the first seconds, when chimes.
If yes, DP_INT_HPD should go high at same time.
 

chrislewis85

New member
Apologies for the delayed response, been away and just coming back to this one!

I get a momentary 3.3 and 5v pulse on those rails several seconds after power on (with screen disconnected) but then nothing.

Will DP_INT_HPD only go high when screen is connected (and detected)?
 

chrislewis85

New member
This board is now becoming a nightmare.

While testing for DP_INT_HPD board stopped powering on and is now stuck at 5v, with a short on PP3V3_G3H. Removed R6999 and it the short is on the "right side" and so could be anywhere. Any advice for trying to locate the short?
 

2informaticos

Administrator
Staff member
Use voltage injection method; lot of info available on the forum.
Start with 1V and slowly increase, if needed; no more than 4V.
First, set amp limit of your lab PSU to max.
Will probably need a 20A PSU; few links already posted on the forum.
 

chrislewis85

New member
While waiting for del of a bench PSU to check this - tried it again and the damn thing has powered on and booted. I still have a short somewhere as it intermittently reboots. Only clue I have is that there are now no audio devices listed in output! (And no chime now whereas before audio worked fine)

Could this be a clue as to the area of the board affected or am I best using the injection method to find the culprit?
 

2informaticos

Administrator
Staff member
I doubt you have short on 3V3_G3H now; it will NEVER boot in such case.
Be aware when you measure diode mode; allow 10-15s after power was removed.

About missing chime, test doing PRAM reset 2-3 times.
 

chrislewis85

New member
Hi 2info,

Tried PRAM reset and no difference - audio hardware on the board is not detected. Having left the board overnight it is now stuck at 5V again and won't jump to 20v on either port.

Diode mode on 3v3_G3H is 0.101v so doesn't appear shorted now. Something bizarre going on because I had a 0ohm short to ground there when I last looked the day it wouldnt turn on.

So bizarre because I literally had it in the OS on an external screen yesterday!

What to investigate if its not turning 20v from charger on?
 

2informaticos

Administrator
Staff member
0.1 is not normal reading at 3V3_G3H.
Usually get 0.35 approx, red probe at ground.

Sounds like the board was liquid damaged and cleaned before arrived to you.
 

chrislewis85

New member
It has previous repair to 3V3_S4 so it is highly likely thats the case.

When I pulled R6999 the short was on the 'right' side (so a LOT of potential components)

Voltage injection best method to see what gets abnormally hot?
 

2informaticos

Administrator
Staff member
Voltage injection helps when you have short, or very low resistance to ground.
Rarely can help in other case, being lucky if some chip goes in thermal avalanche.
 
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