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Mid 2012 model, A1278 / 2.5 GHz i5 or 2.9 GHz i7 processor.

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No keyboard backlight and no bottom ram slot

Working on a MBP 820-3115-B without keyboard backlight. The board has another issue, it will produce a 3 beep RAM error if the bottom RAM slot is populated. It boots and works fine with a single RAM module installed in the top slot. Even though a faulty bottom RAM slot is a fairly common issue on this model (the legs of the RAM slot detach from the board, but I checked them on my board and they're solid), I believe the 2 issues might be related. After checking the schematics, I noticed that R5852 is completely missing from the board. There are some very faint signs of corrosion in the area, but from the look of it, it seems like this component was never installed (for the record, it's a 10k size 402 5% resistor). According to the schematics this component connects to both SMC_SYS_KBDLED and to pin 174 (MEM_B_DQ<55) of the bottom RAM slot. I probed around and all values of components on the KB backlight circuit seem perfectly fine, no shorts or anything. I'm planning to source a replacement and (re-)install the resistor, I'm just trying to confirm that this has the potential to fix the issue. I also noticed that several other components that should be there according to the schematics are missing and it looks like there was never anything soldered to the pads. Why is that?

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Apple sometimes takes out unneeded parts. This is the case here.

I’m away in VT so I don’t have access to the schematics so I can’t point you to what you need to check physically. What you need to probe the power rails to trace out the keyboard backlight power.

I suspect you likely have a damaged keyboard backlight unit or the cable connection.

Your memory slot issue is likely a warped logic board. Loosen the mount screws slightly to see if that corrects things.

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Thank you for the answer. I kept probing around the keyboard backlight circuit and everything tested ok. I was able to pull a known good keyboard backlight from another machine and it actually worked with the mid 2012 MBP. Due to the mild corrosion on the board and the fact that the KB backlight connector looks fine, I think that the original KB backlight unit got liquid damaged at some point. Thank you for pointing me in the right direction, it helped me fix this issue.

As for the bottom RAM slot, I tried what you suggested, but unfortunately nothing changed. This system is running on a single 8GB RAM stick, which is perfectly fine, but if you have any other suggestions on how to fix the bottom RAM slot issue, feel free to leave them here, I'd still like to try and fix it.

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Happy to hear the backlight was the issue

See if a small wedge under the lower SODIMM latches offers a bit more pressure.

Sadly, that’s all you can do other that replacing the logic board.

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b266 will be eternally grateful.
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