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820-2610 C7771 voltage still .3V with C7771 replaced.

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Fizzl92

Member
Hello,
working on this 17" 820-2610 came to me not starting with C7771 issue (.3V). I replaced C7771, but still the same. I also replaced U7750, but still .3V. Does that point to the MCP itself, or is there anything else that could cause this and that I am missing here?
 
2informaticos

2informaticos

Administrator
Staff member
I hope you didn't use hot air to replace it.
Did you use same kind of capacitor or bigger one (in size)?
Bigger caps, even removed from scrap boards, have much better ESR than the small new one.

Check all resistors and traces in the area.
 
F

Fizzl92

Member
I used only my Iron for that. U7750 I replaced with hot air, but I removed the heat sink from the board and shielded the surrounding area before. Also pointed hot air towards the edge of the board...
I checked everything around U7750 before I decided to replace it. Everything seems fine as far as I can tell. So I thought it just could be the chip and replaced it.
 
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Fizzl92

Member
I measured the same voltage with no cap installed at all. I don't have the same board to compare to. My reading is 0.251V to GND on C7771. Maybe somebody has a value/experience to compare it to?
 
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Fizzl92

Member
I checked everything in that area. Here is a list of all voltages at U7750 Pins. I also replaced U7750 with brand new one but still the same.

Pin1 8,4V
Pin2 0V
Pin3 0V
Pin4 1,5V
Pin5 0V
Pin6 0,87V
Pin7 0V
Pin8 0,32V
Pin9 0,34V
Pin10 0V
Pin11 0,07V
Pin12 0V
Pin13 0,2V
Pin14 0,34V
Pin15 0,34V
Pin16 3,4V

Datasheet says enable (Pin4) should be >2V. R7801 is fine (5K1). I get 3,38V at R7801 Pin 2 and 1,5 at Pin 1. Is that normal? I removed C7801 for testing but same thing. Other signals (Pin2, 12, Output, Feedback, etc.) could be messed up if the enable is missing or not high enough I think.
 
2informaticos

2informaticos

Administrator
Staff member
2.8V is good enough for enable signal.
Then you have another problem on U7750 area.
Possibly broken/corroded trace, or component.
 
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Fizzl92

Member
OK, I'll look more into that. But you think that it's not the GPU causing this? Could the two transistors controlled by U7750 cause this?
 
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Fizzl92

Member
Sorry, I mixed that up with 820-2850. I mean MCP as it receives the voltage from U7750 and I don't have a reference to my measured resistance on that line. So MCP could pull that voltage down, maybe? I'd try lifting L7770 and see if U7750 can produce its voltage then?
 
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Fizzl92

Member
I don’t have the scematic in front of my eyes right now. Without the coil I thought the voltage would be not as steady as with a coil in place. But I would suspect it being higher than a steady .3V. If the multimeter would not be capable of measuring this I would have used my CRT. My plan was to lift the one pin off the coil and put it at an angle (on the MCP side) so that I can measure the voltage from U7750 after the coil with no load attached to it that could pull it down. I will check C7770 tomorrow and report back. As I don’t have the schematic with me right now I can’t tell for sure but I probably did already check and maybe replace it. Before opening the threat I looked at pretty much every component and voltage in that circuit…
 
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Fizzl92

Member
Sorry. I had no schematic on hand and wrote that post from my phone in a hurry. You are totally right. There are important signals after the coil that U7750 needs, like FB, so it can't work the way I suggested. I thought from my memory of the schematic that lifting L7770 would be equal to removing F7040 on PPBUS_G3H... My bad. I'll look at the schematic first next time.

To answer your question: I replaced C7770 and still only 0.32V at C7771.
 
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