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820-3115 - No picture. Lower resistance to ground on CPU rails

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phuketmymac

New member
Hello,

I have that mainboard which had Q7802 ripped off and which has been obvisouly reworked near the jack connector.
The board would not start until I re-install Q7802.

Now the board has the fan spinning but doesn't seem to get a hot enough CPU and has no picture either on the LCD or the Thunderbolt display.
A USB mouse will light on.
I am getting no beep if the RAM in unplugged.

I am though getting a 1.1V on PPVCORE_S0_CPU but 0V on PPVCORE_S0_AXG which makes sense since I don't get a picture on the screen.

Measuring the resistance to ground on these 2 rails, I am getting 4ohms compared to 11 ohms on a good working board.

I am guessing that the CPU got a hit but I am wondering if replacing U7400 could help in case the chip could be the reason of the low resistance to ground.

Any help would be appreciated. Thanks.
 
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dukefawks

Administrator
If USB mouse lights the board is probably working. Do a PRAM reset with an external keyboard and see if there is a chime from the speaker in the topcase.
 
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phuketmymac

New member
Hi Duke,

I did that but I couldn't hear the chime anyway + please note that the system won't beep without RAM inserted.
I tried to swap the Bios with another board just to see if it would make any difference. None so I put back the original Bios chip.

It really sounds like the CPU is not doing its job.

Any other thing I could try?
 
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dukefawks

Administrator
Do you have the speaker in the case connected, the beeps/chime will only come through that speaker. Any signs of liquid damage?
 
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phuketmymac

New member
Yes I did plug the speakers to the board, not to the jack connector.

Board has a bit of corrosion here and there but I would say, normal signs of corrosion, no liquid damage imo.

What is the resistance to ground on the CPU rails on that board please? Can anyone give me his reading?
 
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dukefawks

Administrator
4 Ohm is in the "ok" range for Vcore. As the board has been messed with it could be any knocked off component at this point. U7400 certainly won't fix anything. Try a BIOS from a good board for testing.
 
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