Where to find "PACIN" on the mainboard?

[DeinAbi]

Hi all,

i've got this hobby project (since 1,5 years ): reparing my old laptop (Acer 5742). The board does not recongnize when the dc jack is put in, it only runs with battery.

Thus far i found out:
- The IC PU18 has 2 pins which dont have voltage (but should): 1 and 3.
- According to schematic there are two resistors between this pin and "VIN".
- I think i may have located (through some posts from the internet, not sure if correct) those 2 resistors, they don't have voltage.
- So now i want to check if there is voltage on "VIN", but i dont know where to find that.

Since this "VIN" is all around the schematic, i guess its some kind of main power supply thing, but that does not help me.

So i got 2 questions:
1. How do you know which of the resistors on the board (without any labels) are which resistors from the schematic?
2. How can i measure "VIN" (see page 44 in schematic or search for PU18A) or should i measure something else to find out why there is no voltage on pin 3 on PU18?



The schematic:
http://www.s-manuals.com/pdf/motherboard/compal/compal_la-5893p_r1.0_schematics.pdf

[HecticZA]

I'm an absolute beginner, but Vin is 19V on your board. That voltage should be available on the charger, then the DC plug and you can follow it from there. As you mentioned, it should be everywhere and will be stepped down to lower voltages when and where needed, ie 5V, 3.3V, etc
Unfortunately I'm on my phone and following a schematic on it with my limited knowledge is a pain.

My suggestion would be to look at your charging circuit on the schematic. Somewhere the circuit will split to either use the 19V charger or the battery. The problem might be found on that circuit where the 19V charger adapter should flow.

Sent from my SM-N950F using Tapatalk

[station240]

You can figure out which resistors are the ones shown in the circuit (page 43 btw), by using your multimeter in diode check mode.
If you get a reading of 0.00 you know those two pins/solder pads connect together.

Test starting from pins on PU18
1) Resistor connected to pin 1 on one side, and pin 3 on the other = PR295
2) Other resistor connected to pin 3 = PR298
Find the 3 parts connected to PR298, one will be a capacitor connected to both ends of a resistor.
Find other parts related to this circuit, so you can test them if needed.

Make a drawing of the physical layout as you go along, with lines to show connections.
Check that ground and Vin in present by testing between components and the DC input socket.

Test that

[DeinAbi]

@HecticZA
Thanks. What you suggested was actually what I could not do with my current knowledge, because I could not find what "VIN" was

@station240
Thank you, diode check mode was the answer to one of my questions!

So just as I started looking up how to do what you suggested, I checked if there are files for OpenBoardView for this mainboard and i found them (had to pay 2€ but totally worth it, PN if you need). That way it was really easy to find out that "VIN" is actually PD2.

So then I checked PD2 and it has 19V on the appropriate pin, so it has to be PR297 which is not working on my faulty board. Luckily i have a working board so ill desolder it from the working board tomorrow, as soon as my heat gun arrives.

There is 2 more questions though:
1. On my working board I measure 26k Ohm on PR297, although according to schematic it should be 84.5k. How can that be?
2. Where can I get resistors this small if my desoldering fails?

[DeinAbi]

EDIT 3:
I have replaced another resistor and now PR299 seems to be the problem. On pin 1 i get 2.05v, on pin 2 0.92. Subsequently i get 0.93v on pin 1 on PU18. I have tried to trace ACIN/PACIN (seems like PACIN is the voltage which comes through to pin 1 on PU18) but cant seem to find it.

What i find odd is that currently the board is getting a little hot as soon as i plug it in to measure the values. Im pretty sure few hours ago that was not the case. Did I screw something up?



Old post:

ARGH....
So I desoldered the PR297 from the good board, but while desoldering with a heat gun the resistor got blown away, of course no chance to find something that small ...

So I searched in the schematic and didnt find another 84.5k resistor, but i did find a 80.6k (PR372). So I desoldered that one and soldered it in place of PR297.

Now i did get 3.75v on pin 3 (it should be 3.6v) of PU18 and I thought the problem was solved, since I though that PU18 (LM393) only compares the voltage on pin 2 and pin 3 and if pin3 is greater i get voltage on pin1.

I now do have voltage on pin 1, but only 0.03v... with that it of course does not work.... is that an issue with the resistance of the resistor (80.6k instead of 84.5k) or something else?

EDIT:
I should also note that I have bought a new LM393 (PU18) and replaced the old one. LM393 is LM393, right?

Another EDIT:
It could have happened that I switched the sides of the resistor while soldering it (PR372 in place of PR297), does that matter?

[DeinAbi]

So basically, PIN1 on PU18 is PACIN from the schematics if I understand the connections correctly. So the voltages on many resistors are not correct because of this.

Thus the new topic name should be what/where is PACIN?

I see it on page 47 of the schematics, and i measured the voltage directly on PR371 but it was under 1v. I see no other references which would help me.

Any ideas?

[HecticZA]

Please take note, as previously mentioned, Im a Noob.

I had a look and this is the path that I would take.

It is probably not the most efficient way and there are quicker ways to determine where the possible problem can be, but from looking at the schematic, these would be my options.
I apologize if this sends you totally in the wrong direction.

I see you replaced PU18? Maybe test for shorts as well as you mentioned that there seems to be some heat around that you did not notice previously.  That could potentially result in a voltage drop and a rise in current.

Here is my path that I might follow: Please note that I do skip some IC's, you are welcome to test them as well.
PQ65 - ACON Pin 1 (This should be on the other side of the board I think?)
PD15 - ACON Pin 3
Change to other side of the board
PR371 - PACIN - Pin 1
PQ61 - PACIN - Pin 2
PU18 - PACIN - On other side of the board - Pin 1
PR398 - PACIN - Pin 1
PD1 - PACIN - Pin 1
PR295 PACIN - Pin 1
PR302 - PACIN - Pin 1
PR299 - PACIN - Pin 1 - I think this is for the other part of the circuit(U18B)
PR296 - PACIN - Pin 1 (U18B )

Apologies if this is a mess and totally wrong.


Edit:
This relates to your question regarding PACIN

[DeinAbi]

@HekticZA

I did measure some of those PINs, all of the measured ones have voltage slightly under 1v (~0.85).

What I still don't understand is how/where PACIN is coming from. Which of those PINs is "the source" of PACIN for all the other PINs? I kind of think that there must be a source and them on the way some komponents which alter the voltage/current/whatever, but there must be a source, right?

The other topic was that after i plug the charger the board immediately starts getting hot. Initially it was U32 and some of the GPU RAM chips and the CPU. Then I read that the CPU can get hot if there is no RAM, so after I put the RAM sticks in the board the CPU is getting only moderately hot.

So U32 is getting reallyy hot, should I replace it? I was wondering if that is not what it is supposed to be, because (if remember correctly) this chip is responsible for things like turning the PC on, so maybe its normal that its hot?