Help a newbie with his first project

[jacobtc]

Re: Help a newbie with his first project

Hi, the code '3H =' comes from Richtek, the chip is a RT6585B, it is compatible with the chip TPS51285B.

Sounds good, why would my Alienware use the RT6585B, when the schematic says TPS51285BRUKR?

[caspian]

Re: Help a newbie with his first project

It is possible that somebody replaced it, or sometimes the manufacturer changes his mind to use another model of IC.

[jacobtc]

Re: Help a newbie with his first project

It is possible that somebody replaced it, or sometimes the manufacturer changes his mind to use another model of IC.

Weird :O But back to the problem at hand, how would I go about finding the culprit?

[JaroTech]

Re: Help a newbie with his first project

Remove the inductors (PL101 and PL102), check if any of the MOSFETs become hot (PQ100, PQ101, PQ102, PQ103).

[jacobtc]

Re: Help a newbie with his first project

Remove the inductors (PL101 and PL102), check if any of the MOSFETs become hot (PQ100, PQ101, PQ102, PQ103).

The Mosfets get a little warm, but nothing special, PU100 still gets burning hot like before. It might just be the PU100 heating up the mosfets from the underside...

[JaroTech]

Re: Help a newbie with his first project

Measure all the voltages from the pins of PU100, like
pin 1 = ?V
pin 2 = ?V
...

[jacobtc]

Re: Help a newbie with his first project

Measure all the voltages from the pins of PU100, like
pin 1 = ?V
pin 2 = ?V
...

I'm really new at measuring in circuits, is it okay to probe ground in some random place on the board when measuring? I did that, and here are the results:

Pin 1 = 0V
Pin 2 = 0V
Pin 3 = 1.2V
Pin 4 = 0V
Pin 5 = 0V
Pin 6 = 0.8V
Pin 7 = 0V
Pin 8 = 0V
Pin 9 = 4.72V
Pin 10 = 0V
Pin 11 = 0V
Pin 12 = 19.5V
Pin 13 = 5V
Pin 14 = 0V
Pin 15 = 0V
Pin 16 = 0V
Pin 17 = 4.76V
Pin 18 = 0V
Pin 19 = 0V
Pin 20 = 0.8V

[JaroTech]

Re: Help a newbie with his first project

Sorry I did not mention that before, did you put back the coils? If not, put them back and remeasure the pins, I don't think that will change anything. For the ground, any place on the motherboard is good.

[jacobtc]

Re: Help a newbie with his first project

Sorry I did not mention that before, did you put back the coils? If not, put them back and remeasure the pins, I don't think that will change anything. For the ground, any place on the motherboard is good.

No problem! No I didn't, so I will do that later today and measure again! Also I was wondering, now that my measurements seems completely different than what the schematic says, is that due to not having anything connected to the motherboard, or is it due to not being turned on? Also I've found that the coils I took off are 2.2uH, while the schematic says they should be 1uH.. Are the schematic wrong, or has someone decided to replace more components on the board? And does it matter? Thanks again for all your help!

[JaroTech]

Re: Help a newbie with his first project

Thanks to Johndill who provided the schematics for the LA-B753P, the PL101 is a 2.2μH inductor and the PL102 is a 3.3μH inductor. The voltages are different because the PU100 can be defective.

[jacobtc]

Re: Help a newbie with his first project

Thanks to Johndill who provided the schematics for the LA-B753P, the PL101 is a 2.2μH inductor and the PL102 is a 3.3μH inductor. The voltages are different because the PU100 can be defective.

PL101 and PL102 are both using 2.2uH inductors on my board

[JaroTech]

Re: Help a newbie with his first project

Maybe they run out of 3.3μH inductor!

[jacobtc]

Re: Help a newbie with his first project

Maybe they run out of 3.3μH inductor!

Oh well, does it have any negative effect?

I just got done measuring PU100 again, and it has exactly the same voltage on the pins... Is it defective, or is something else causing it to output wrong voltage?

[JaroTech]

Re: Help a newbie with his first project

When you have the VIN=19.5V, you should have the output VREG3 at 3.3V to power the EC chip and the ON/OFF button, now you have 1.2V. Check the resistance on the VREG3 (pin 3) to ground to see if you have a heavy load, otherwise the PU100 is defective.

[jacobtc]

Re: Help a newbie with his first project

When you have the VIN=19.5V, you should have the output VREG3 at 3.3V to power the EC chip and the ON/OFF button, now you have 1.2V. Check the resistance on the VREG3 (pin 3) to ground to see if you have a heavy load, otherwise the PU100 is defective.

I just measured the resistance, it is coming in at 12 Ohm on VREG3... I measured with no power attached, is that okay?

[JaroTech]

Re: Help a newbie with his first project

When measuring the resistance, always remove all the power from the motherboard, if you can, change PU100.

[jacobtc]

Re: Help a newbie with his first project

When measuring the resistance, always remove all the power from the motherboard, if you can, change PU100.

Okay good, thought so as well

Ordered some RT6585B's from China, and have to find my hot air station...

So it is certain that PU100 is defective? It makes sense, as nothing lights up, and the power button does nothing, when it is only receiving 1.2V, however can anything else on the board affect PU100, so that VREG3 only outputs 1.2V and not 3.3V as it should?

[JaroTech]

Re: Help a newbie with his first project

Yes, you could have a leaky capacitor like PC101 and CU26, remove the PC101 and measure the resistor on pin 3 again, you can continue with the components that are connected to the ground.

[jacobtc]

Re: Help a newbie with his first project

Yes, you could have a leaky capacitor like PC101 and CU26, remove the PC101 and measure the resistor on pin 3 again, you can continue with the components that are connected to the ground.

With only the schematic and not the board view, I have a hard time finding PC101, escpecially because it is a 0603 cap. Is there a easy way to find it? Could it be the one right above Pin 3 on PU100?

[JaroTech]

Re: Help a newbie with his first project

You put a probe on pin 3 of PU100, and the second probe you put it on the capacitor, when you have zero ohm, it is the one, and at the terminals of the capacitor you find the 12 ohms.