Dell 7280 Power Issues (LA-E122P)

[reid]

Hi, this is my first new thread and I am hoping to get some assistance troubleshooting a Dell Latitude 7280 (LA-E122P). I have fixed a few MacBook logic boards before, but I'm not familiar with Dell boards or schematics, so I don't really know what things are important to know or check first.

When I first got the laptop, it would not turn on. I'd plug it in and press the power button, the power LED would come on for a second and then turn off and the laptop wouldn't do anything. This was fixed by flashing the UC5 EEPROM, so I don't think there was any hardware issues when I first got the board.

But, before I figured out it was an EEPROM issue, while testing voltages and troubleshooting, I did something that caused the behaviour of the laptop to change - when the DC adapter is plugged in, the power and front LEDs pulse on and off, and I can hear a clicking sound coming from near the DC in barrel jack (about 1 click per second). I can't find any components that were visibly damaged, either by myself or before I started working on the board.

All the voltage rails are pulsing, and fluctuating around their normal voltages, for example +PWR_SRC is around 13.4 V, and drops to about 12 V every second before jumping back up.

The strange part is that the board boots to BIOS fine when connected to a Dell USB-C dock (after the EEPROM flash). Connected to the USB-C dock, there's no voltage fluctuation on +PWR_SRC. The voltage issue with the DC barrel jack is the same before and after the EEPROM flash. I have also verified that the UT6 EEPROM seems to be okay - it matches the dump I found here: https://www.alisaler.com/dell-latitu...e122p-bios-ec/

If anyone can provide some assistance with troubleshooting the power issue with the barrel jack input, it would be greatly appreciated!


Some additional information/guesses:

• From what I can see on the schematics, the SDC_IN rail is shared between USB-C power and the DC jack, so I'm guessing the problem is somewhere before SDC_IN, or with some sensing circuit that's shutting down and restarting, or maybe it's related to the PS_ID line.
  
• PS_ID is pulled up to 3.3 V, but I don't see any communication on my scope.
  
• I have tested two known good Dell power supplies and it's the same issue with both.
  
• When I touch near crystal YE1 for the EC MEC5105 chip, the pulsing stops and PWR_SRC stays around 12.3 V.
  
• +3.3V_ALW and +5V_ALW both seem to be around the correct voltages, but pulse along with all the other rails.
  

Thank you!

Schematic and boardview ---> https://www.badcaps.net/forum/showth...41#post1252941

[mon2]

Re: Dell 7280 Power Issues (LA-E122P)

Hi. See attached. Post the voltages for the requested areas.

Dell uses a one-wire communication (Dallas / Maxim) to authenticate that the adapter is genuine so the ID communication must exist.

https://hackaday.com/2014/03/03/hack...dentification/

[reid]

Re: Dell 7280 Power Issues (LA-E122P)

Thanks for the response and information! Here are the voltages those areas:
1: 3.4 - 3.5 V
2: 0.44 V
3: 5 V
4: 3.4

[reformatt]

Re: Dell 7280 Power Issues (LA-E122P)

Usual place to start is to check the first two FET's PQ9 and PQ4. The laptop should still work with no valid PS_ID, however it will throttle the CPU to it's slowest speed and it won't charge the battery.

PS_ID comms traffic is only visible briefly after plugging in the adaptor. The host drives the data line low, the one wire chip in the charger feeds out the data then stops.

[reid]

Re: Dell 7280 Power Issues (LA-E122P)

Thanks, that makes sense. I’ve used Dell power adapters in the past that have had a broken ID pin/wire and the computer has always booted and displayed a warning about the wattage being unknown.

Here are some readings from those two MOSFETS. Does anything look out of the ordinary? I'll try to dig into that area a bit more, but I thought I'd post just in case something jumps out as being obviously wrong.

PQ9:
Pin 1, 2, 3: 19.5 V
Pin 4: 9.3 V, fluctuating
Pin 5: 19.5 V

PQ4:
Pin 1, 2, 3: 19.1 - 19.5 V, fluctuating
Pin 4: 18.3 V on multimeter, but unstable
Pin 5: 19.5 V

Resistance to Ground, Diode mode reading:
PQ9:
1, 2, 3: ~200k, 0.635 V
4: 1.2M, OL
5: 94k, OL

PQ4:
1, 2, 3: 70k , 0.562 V
4: 600k, OL
5: 94k, OL

My uneducated guess is that it there might be a protection circuit kicking in? It seems like the voltages are around where they should be (e.g. 19.5 V), then briefly cut out to 0 V or a lower voltage and immediately jump back up, about once per second.

[reformatt]

Re: Dell 7280 Power Issues (LA-E122P)

Ahh, Dell. When you can achieve something with just a few components, lets add 100 more and complicate the shit out of it. Compare a TI BQ charger design with this nonsense.

Because a lot of these signals are coming from the EC (and it's working when powered by the USB C), I would be still be looking in the PQ4/PQ9 area still. Faulty feedback and/or timing in this area will result in the fluctuations.

My approach would be to get the boardview and schematic, and examine each passive component in this area. Check for knocked off components and measure each resistor. Especially around the biasing for the FET's.

[reid]

Re: Dell 7280 Power Issues (LA-E122P)

Looking at the biasing for the FETs was very good advice!

USB Type C uses a similar transistor configuration, seemingly with PQ1213 = PQ9 and PQ1202 = PQ4.
On USB C, the gate pins are both 1.77 V, which appears to be correct based on the resistors used in the voltage divider. For the DC jack, the same voltage divider resistors are used, so I think I should also see 1.77 V on the PQ4 and PQ9 gates.

PQ9 has 9 V on the gate, and PQ4 has 19 V. When I measure PR16 and PR11 for PQ4, they seem to be the correct resistances.

Looking at PQ9, both PR12 and PR18 read around 1M. That probably explains why I was reading around 9 V on the PQ9 gate.

I just removed PQ9, PQ6, and PC6 with no change in the resistance readings. I don't see anything else on the schematic that could affect those resistances... I'll keep looking at that area, and try removing the resistors themselves to measure out of circuit.

[mcplslg123]

Re: Dell 7280 Power Issues (LA-E122P)

With dell , gate voltage for 1st dc-in fet is 3.3V and 2nd fet is 1.7V. there are exceptions but it apllies to most of the dell laptops.

[reid]

Re: Dell 7280 Power Issues (LA-E122P)

Interesting! So the 9 V seems abnormal since it's definitely not 1.7 or 3.3V.

I just removed and measured PR12 (499k) and PR18 (49.9k) and they both measure 1M on two different meters. I know I haven't changed those resistors.

The schematic I have is for a Rev 1.0 A00 LA-E122P, and the silkscreen on my board says the same thing. I've attached the schematic and boardview I'm using.

EDIT:
I looked at the schematic for the newer 7290 (LA-F312P), and PR12 and PR18 are 1M on that model, so I guess it might not be abnormal on this 7280...

[reid]

Re: Dell 7280 Power Issues (LA-E122P)

Looking closer at the circuit, +DC_IN_SOURCE connects to "DCIN" of the ISL88738 (PU901) chip through a SDMK0340L Schottkey diode, PD904. This reads as OL on both my meters.

There are two other diodes that also connect to "DCIN" of PU901, another SDMK0340L (PD901) that connects +PWR_SRC to DCIN, and a different type (PD903) that connects +VBUS_DC_SS (from USB-C power) to DCIN. When the USB-C power is connected, the cathode side of these diodes reads 19.5 V, and when the DC supply is connected, it fluctuates from 5-9 V.

Unfortunately, I don't have a donor board I could get new diodes from to explore this more.

Any thoughts on whether these diodes could be the issue?

[reid]

Re: Dell 7280 Power Issues (LA-E122P)

Good news!! I replaced the two diodes with some B0530W's that I found on an old 2008 MacBook donor board - they seemed to be close enough in spec. The laptop is now powering up successfully from the DC barrel plug, and the behaviour and voltages are pretty much the exact same when comparing the DC jack and USB-C power.

I'll probably order some RB751V-40WS diodes on Digikey, which seem to be closer in spec to the originals that are out of stock.

Thank you all very much for your help!!

For the sake of troubleshooting, I just flashed a BIOS that I found on Alisaler.com, but it takes a long time to boot and shows up as a 7380. It seems that the slow boot is an issue with the ME region, but there's a bunch of other posts here related to that. I'll read up on those and hope to get the software side of things fixed on this laptop.

[PowerAmpFreak]

Re: Dell 7280 Power Issues (LA-E122P)

Interesting! So the 9 V seems abnormal since it's definitely not 1.7 or 3.3V.

I just removed and measured PR12 (499k) and PR18 (49.9k) and they both measure 1M on two different meters. I know I haven't changed those resistors.

The schematic I have is for a Rev 1.0 A00 LA-E122P, and the silkscreen on my board says the same thing. I've attached the schematic and boardview I'm using.

EDIT:
I looked at the schematic for the newer 7290 (LA-F312P), and PR12 and PR18 are 1M on that model, so I guess it might not be abnormal on this 7280...

Hello where did you find the motherboard partnumber in silkscreen ?

[reid]

Re: Dell 7280 Power Issues (LA-E122P)

I don't have the laptop anymore to check, but the part number should be printed inside the RAM slot area. You might have to remove some black plastic tape to find it, I can't remember for sure.

There's a few pictures that come up when you Google LA-E122P that show where in the RAM slot it is. Here's one from NewEgg: https://www.newegg.com/p/2S7-07JP-19NM3

[PowerAmpFreak]

Re: Dell 7280 Power Issues (LA-E122P)

Thanks a lot!
Will check! 😊

[rumensat]

Re: Dell 7280 Power Issues (LA-E122P)

thanks