Hi all. This is my first post here. I won't bother you with my history as far as laptop repairs are concerned, but I want to mention that I have some basic electronics knowledge and just learned how to somewhat understand laptop motherboard schematics.
The specific issue I wanted to ask about here concerns an HP 550:
The issue is that the laptop has no backlight. Its not an obvious fix, I've made sure that both LCD and inverter are working properly.
I found that the problem is on the motherboard.
I'm in the fortunate situation to have three identical motherboards. One with a functional backlight, the other with a defective backlight and the third with serious moisture damage. I can use that last one to scavenge components.
Now, I started off by comparing voltages on the LVDSB connector on the motherboard with functioning backlight to those on the defective one.
I found that one of the pins had a 3.3V on the functional motherbaord, but 0V on the defective one. Using my lab power supply I applied 3.3V to the pin on the defective mobo and got the screen to light up. Bingo.
The problem is, I want to actually fix the issue and I can't for the life of me figure out which component went bad. It may be a schematic issue, but I can't find a more appropriate one.
Here's the schematic I'm using:
The component names match up to what is on the motherboard itself. That is to say - they mostly do. This schematic is for a HP 540, but I tried hard to find one for a 550 and I just can't. To make things more interesting, sometimes I see this same schematic listed as applying also for the HP 550.
Using the HP 540 schematic I see that the pin with the missing voltage is pin number 9 on connector CN17. This is shown on page 30.
Looking at where pin 9 goes, it should lead to a Q61. Now that's the issue. There is no Q61 on the motherboard. Trust me, I didn't do a half assed check to find this thing. I looked over the entire motherboard for well over an hour as I was convinced it had to be there. It isn't. I searched for the resistors connected in series with Q61 (R9613 and R9614) but those were nowhere to be found either.
I did find U28, but its fourth leg is connected to R15, not R9613 like the schematic shows.
Do I really have the wrong schematic? So many other parts match up perfectly.
Additional info: The motherboard has the following printed on it:
6050A2216301-MB-A02
DYnamic KM-VO
E25540094V-O
If I do have the wrong schematic and can't get one, then how should I approach this? I tried checking for continuity between Pin 9 and a stupid amount of nearby and far away components and I couldn't get anything anywhere. I suppose I could find a spot where 3.3V is available and solder a jumper wire to pin 9, but I'm pretty sure that qualifies as a hack job and am uncertain it would work as a long term solution. Besides, it might put too much strain on other components and would keep the lid switch non functional.
I'm all open for ideas!
The specific issue I wanted to ask about here concerns an HP 550:
The issue is that the laptop has no backlight. Its not an obvious fix, I've made sure that both LCD and inverter are working properly.
I found that the problem is on the motherboard.
I'm in the fortunate situation to have three identical motherboards. One with a functional backlight, the other with a defective backlight and the third with serious moisture damage. I can use that last one to scavenge components.
Now, I started off by comparing voltages on the LVDSB connector on the motherboard with functioning backlight to those on the defective one.
I found that one of the pins had a 3.3V on the functional motherbaord, but 0V on the defective one. Using my lab power supply I applied 3.3V to the pin on the defective mobo and got the screen to light up. Bingo.
The problem is, I want to actually fix the issue and I can't for the life of me figure out which component went bad. It may be a schematic issue, but I can't find a more appropriate one.
Here's the schematic I'm using:
The component names match up to what is on the motherboard itself. That is to say - they mostly do. This schematic is for a HP 540, but I tried hard to find one for a 550 and I just can't. To make things more interesting, sometimes I see this same schematic listed as applying also for the HP 550.
Using the HP 540 schematic I see that the pin with the missing voltage is pin number 9 on connector CN17. This is shown on page 30.
Looking at where pin 9 goes, it should lead to a Q61. Now that's the issue. There is no Q61 on the motherboard. Trust me, I didn't do a half assed check to find this thing. I looked over the entire motherboard for well over an hour as I was convinced it had to be there. It isn't. I searched for the resistors connected in series with Q61 (R9613 and R9614) but those were nowhere to be found either.
I did find U28, but its fourth leg is connected to R15, not R9613 like the schematic shows.
Do I really have the wrong schematic? So many other parts match up perfectly.
Additional info: The motherboard has the following printed on it:
6050A2216301-MB-A02
DYnamic KM-VO
E25540094V-O
If I do have the wrong schematic and can't get one, then how should I approach this? I tried checking for continuity between Pin 9 and a stupid amount of nearby and far away components and I couldn't get anything anywhere. I suppose I could find a spot where 3.3V is available and solder a jumper wire to pin 9, but I'm pretty sure that qualifies as a hack job and am uncertain it would work as a long term solution. Besides, it might put too much strain on other components and would keep the lid switch non functional.
I'm all open for ideas!
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