Lenovo S145-14AST NM-C171 faulty RAM voltage rail

[mANgLEr]

I am attempting my first component-level board repair, on a:

FS44A&FS54A NM-C171
Rev:1.0

I have been diagnosing this board with the attached as a guide. Part PU1901, which I believe is a voltage regulator, has incorrect voltages of around 0.67V on pins 2 and 3. This is for RAM pins 257 and 259, I stopped looking after that for other components that use it. I think all the other voltages produced by this part are correct.

As I am a newbie, please can someone check that I'm correct in thinking that pins 2 and 3 should be outputting 2.5V and not 0.67V. Some of the annotations on the schematics (i.e. 'TDC=0.676A' and 'FB reference=0.75V') are confusing me.

The actual part is a Lenovo LV5075B, but its marked as LV5028RPC_QFN40_5X5 on the schematic. Don't think data sheets are available for this part, or at least I couldn't find a direct match.

If these voltages are wrong, could I safely inject 2.5V at these points with wires to test further? Looks like I can buy replacements from China for this part if I'm heading in the right direction.

Thanks.

[mcplslg123]

Re: Lenovo S145-14AST NM-C171 faulty RAM voltage rail

Pin2 should have 0.74V approx(there is voltage divider with PR1920/1921). However, pin3 should have 2.5V as output. Dont inject any voltage as it may be very risky.

Remove all power,allow it to discharge for 5 mins and then measure resistance on Pin3 of this Regulator. Meter should be in 20K or higher scale. Black probe to GND,red probe on pin3.

[mANgLEr]

Re: Lenovo S145-14AST NM-C171 faulty RAM voltage rail

Thanks for the help. I will follow your advice and see what I get.

[mANgLEr]

Re: Lenovo S145-14AST NM-C171 faulty RAM voltage rail

I have taken some readings. Pins 2 and 3 of PU1901, after unplugging the battery for ten minutes, both read 10K ohms approximately. I also checked other components in the path of these pins and all seem okay to me.

I reconnected the battery and powered it up (light is solid, not flashing). Pin 2 of PU1901 measures 0V, as do PR1921 and PR1920 it connects to. Pin 3 of PU1901 measures 0.67V, as do the connecting pin on PR1920, PC1935 and PJ1909.

I'm fairly sure when I measured these voltages last weekend, pin 2 was outputting 0.67V as well but I cannot get any variance in the readings. Mostly I was taking readings from the connected components as it is difficult to accurately hit the pins on PU1901, even with mega sharp Probemaster tips. The other voltages on PU1901 still seem okay to me, pin 1 is 3.3V for example. Not sure if that is a bit high though as it is labelled LV5028_2.5V_EN.