Lenovo L490 Ram voltage circuit malfunction

[rddube]

Hello friends,

Looking for some guidance here. The RAM voltage circuit on this motherboard keeps burning out PU301 which is a RT8231AGQW. Copy of the board with the burnt chip attached with some voltage measurements and copy of the schematics also included.

I think I've checked almost every component in the circuit, but when I change the RT8231, the laptop starts I have 1.2V on the inductor for about 10 seconds and then the RT8231 goes south and no more voltage on the inductor. All other voltages on the laptop are good, this is the only one conking out for a reason I can't pinpoint. Any advice would be appreciated and if you need me to take other voltage measurements (keeping in mind that the RT8231 is out) I will be please to provide.

I only have one RT8231 brand new left, so the next test would have to be the good one (already toasted 4 other chips trying different things). The 2 mosfets in the schematics are good, I've changed them AON7408 and AON7380.

[rddube]

Anyone?

[SMDFlea]

Anyone?

If you have a DC bench PSU inject 1V on pin 16 and check caps PC311, PC312, PC313 ,and post resistance measurements on all pins.

[rddube]

If you have a DC bench PSU inject 1V on pin 16 and check caps PC311, PC312, PC313 ,and post resistance measurements on all pins.

Hello SMDFlea and thank you for your suggestion. It was a challenge to inject on pin 16, but I did it and got approx. 90 ohms reading on all 3 caps. However cap PC313 looked discolored so I changed it for for another cap. Resoldered carefully the RT8231 and bingo, I have my 1.2V and it is steady.

Thank you SMDFlea, you put me on the right track! May I ask, what kind of technique is that, is it that when you apply voltage, some capacitors could increase or decrease their resistance? Again, many thanks!

[SMDFlea]

Hello SMDFlea and thank you for your suggestion. It was a challenge to inject on pin 16, but I did it and got approx. 90 ohms reading on all 3 caps. However cap PC313 looked discolored so I changed it for for another cap. Resoldered carefully the RT8231 and bingo, I have my 1.2V and it is steady.

Thank you SMDFlea, you put me on the right track! May I ask, what kind of technique is that, is it that when you apply voltage, some capacitors could increase or decrease their resistance? Again, many thanks!

What i meant was post resistance measurements for RT8231 ,sorry i wasn`t clear about that. All resistance measurements should be taken with no power to the board. When you inject voltage you can see which part is shorted and getting hot with a thermal imaging camera, or if you havn`t got one pour some IPA on the board, the bad cap will evaporate the IPA quickly if it`s fully shorted. Measuring the resistance of the cap while its on the board is no good as it`s in parallel with some others. I`m glad you got it fixed