Dead Dell P2416db - No standby/power LED detected

[mattie47]

Hi all,

I got given a dead Dell P2416db monitor, and I've been trying to see if I can fix it (and save it going to landfill).

I get no standby LED detected once power is connected, nor any other form of light out of the screen.

- Thus far, I detect 19v on the PSU output pins, however there is a high pitch scream coming from one or two caps (see image 1 - I know these should be replaced, but given).
- Numerous step down converters around the circuit are reporting voltage
- One is not, I have highlighted this in the top right of image 2.


Most stepdown converters on the board are IACEG - Google suggests MP2144: https://www.monolithicpower.com/en/d...ocument_id/436

There's one IAGCF which google suggests to be the MP2315: https://www.openhacks.com/uploadspro...tasheet_77.pdf

As mentioned above, I was surprised to not see anything on VCC (046.9mv) or SW (012.0mv), however for SW I'm guessing this could be because EN was only 1v (below 1.5v threshold).

Any ideas for how I should process next?

Edit: I've attached pics 1 and 2 in a zip file since the forum compresses them and it's hard to read the text I've written on them. There's another zip also of other raw pics.

Thanks,

Matt

[202020]

Hi Mattie,

Based on measurement where EN line are getting 1V only.
My hypothesis would be the switching regulator MP2315's voltage divider resistor from VIN to EN are burnt or worn out. If you look at MP2315 datasheet page 16 where they show typical application circuit (Note: can ignore the Vo since we only focus on EN line), EN line would need a resistor divider R6 and R7 from Vin. Assuming our Vin is 19V, R6 is 100kohm and EN line typically need 1.4V, hence R7 would need at least 8kohm.

Of course in the dell circuitry, they can use other values of R6 and R7 to yield at least 1.4V at EN line as well.

Hence there are two approach to tackle this:
1)First probe which resistor are connected to VIN and EN line. Check if the first resistor is 100kohm or second resistor is 8kohm.Change the resistor value accordingly to make EN have at least 1.4V
2)Or external inject 1.4V to EN line see if everything works

My guessing would be the first resistor resistance become something bigger since it ties to VIN where inrush current or voltage spike might potentially kill the resistor in the long run.
Of course there might be other factory as well and I would be wrong also, lets try this to see if it works.

[mattie47]

Hey thanks for that suggestion.

I tried measuring the resistors but found there values jumping all over the place (I'm guessing because they're in-circuit?).

Regardless, I opted for the "let's see if this works" approach, and connected enable to a 1.5v source on the board.

This can be observed in the attached pic.

While I now get 10v out of the mcp2315, unfortunately still nothing on standby led or screen itself.

Any ideas for what I should try next? I had hoped the mcp2315 would have been the "smoking gun".

Thanks,

Matt

[mattie47]

Okay, some more thoughts towards a next step.


1. There's an ldo with marking GH16D. A google says this is really a AZ1117 - http://www.wsecl.com.tw/prd6/AZ1117C...8%20121130.pdf

Interestingly, there's no voltage with this on input. I'm not quite sure where perhaps it should be getting voltage either :-/

I'm trying to understand the pcb layout but I'm more or less a determined novice.

2. There's another part (I'm not sure what it is), but it seems to be reading 3.3v on both sides of it?? This doesn't sound right does it? Update: I see other ones of these components on the board are doing a similar thing.

Update: In my last pic, I highlighted a component with 5v on 2 of the 3 pins. I see the other pin on the left of it has continuity with the LDO's input pin. That component itself has marking "N5K1" (can't find anything on google for it).

It looks like it has D B G around it, which I'm guessing are Drain, Base, Gate ? In which case, this is a transistor. If that's correct, does it make sense that the Gate and Base are both seeing 5v?


Appreciate any thoughts on what I should consider doing next.

Thanks,

Matt

[202020]

Please help to disconnect the 10V out of mcp2315. I dont think thats the right voltage and might have potential to damage the board. Possible to trace which circuitry do the mcp2315 connected to? But i think its tough

I guess the more common voltages would be 5v, 3.3v, 1.8v, 1.1v, no idea what we gonna do next, perhaps need other more experience member to comment.

[mattie47]

Unfortunately this story has a sad ending.

I managed to borrow a working monitor of the same model to try narrow things down.

It turns out, after plugging in the "dead" monitor's panel, that not only would it get no power, but the person I got it off damaged the panel

Despite this, I thought I'd check a few things on the "working" monitor.

Turns out with the "working" monitor:
- mcp2315 also has no power coming out of it. This only would enable power when the power button is turned on. Secondly 10v *is* correct for this rail.
- The LDO mentioned previously with 3.3v also has no VOUT when the monitor is off. Only when turning monitor on, does this have 3.3v coming out of it.


Regardless - I think I've found a short to ground. I noticed the "working" screen 's heatsink didn't appear to be heating up in standby like the "dead" screen.

I then had a look on the other side of the board.

See attached pic "short". There's a red line of drawn on a VIN pin which is hitting continuity when I attach to ground. This wasn't the case with the "working" screen.

Right in the center, I noticed some of the caps seem to get a little bit warm.

Correct me if I'm wrong, but using a bench power supply set to say 3v, 3amps, and injecting power somewhere on the board should cause the problem component to heat up right? I.e. could then use some IPA to see where it dissolves? I don't have my bench PSU handy, so haven't tried this yet.

[SMDFlea]

Desolder that 24C16RP chip first and see if the short is gone.If it is then you`ll need to source another chip and copy the contents from the working one to the new one using a programmer.If the short is still there after desoldering 24C16 then you can use your bench PSU to find the shorted component.Solder a wire to pin/pad 8 ,its the VCC. Start at 1V 1 amp and gradually increase the amperage,lower the amps down again if you find nothing heating and up the voltage gradually to a max of 5V .

24C16RP Datasheet https://pdf1.alldatasheet.com/datash...ICS/24C16.html

(4.5V to 5.5V for ST24x16 versions)