Lenovo Pro2840mD monitors power down after a while

[EvilDeece]

Hi folks,

I have a couple of Lenovo Pro2840mD monitors that I pulled out of an e-waste pile.

Both monitors work initially, but after a while, power down hard (no standby light, buttons unresponsive), and will not come back up unless power is cycled at the wall. Once power cycled, they last less time than when brought up cold.

I've hunted around but can't find replacement PSU boards for them.

I've buzzed out all the electro caps in the PSU, and they all appear to be good (capacitance in spec, no leaking, low ESR).

There are 2 control chips on the back of the PCB:
LD7790GS (https://datasheet4u.com/datasheet-pdf/Leadtrend/LD7790/pdf.php?id=770292 )
GB98AEGN  - backlight inverter? (https://www.datasheetq.com/datasheet-download/142142/1/O2Micro/GB98A )

I'm guessing it's a temperature related issue based on the behaviour, but I need assistance in narrowing it down.

It would be tricky to bring the monitor up with the board exposed, due to how it's mounted (screwed into a metal shield which is then bolted to the back of the panel), otherwise I think using a thermal imager might reveal some candidates.

Does anyone have advice on how to investigate the problem further?

[EDIT] I forgot to mention, both these monitors are circa 2015

[james_s]

Problems when cold that get better when warm is a classic symptom of bad electrolyic capacitors. My first suspect would be that small one near the middle, that's probably part of the startup circuit.

[EvilDeece]

Thanks for the tip, it's the other way around though, the shutdowns become more frequent when warm. I'll re-check that cap anyway.

[james_s]

In that case try poking around with an insulated stick and see if you can find a ticklish spot. Cracked solder joints are a common cause of intermittent problems. A can of air duster held upside down to freeze parts can also be helpful.

[tonycrouch]

Hey there,
You are certainly not alone in your plight with these monitors.

I use a dual setup of these 2840md's @ work (1 works perfectly, the other has the same symptoms you describe).

I've recently purchased a 2nd unit (with the same auto-shutoff fault to hopefully work out the problem).

I had originally suspected LD7790GS IC to be the culprit as a result of high - temperature but, like your picture, mine also has no heat marks / damage.

My CAPs are also within spec and show no physical signs of problems.

My only thought is to replace LD7790GS IC and hope for the best. However, I cannot be certain that there is not something further down the line that is not causing an over-voltage which in turns causing the IC to shut off power.

[tonycrouch]

Also, on closer inspection the only physical defect I could see was the solder joint between C822 & D851 - as shown in the attachment.

But given the fact this monitor initially powers on, and takes 10 odd minutes to shut off then I highly doubt this is a problem.

[amyk]

Use freeze spray (a can of "duster air" upside-down will work) to find the heat-sensitive component.

[tonycrouch]

Thanks for that :-)

I don't have any air in a can to use. However, I do have this from loctite - freeze & release - would that work?  It feels just like an upside down air can on my arm at least.

https://www.autobarn.com.au/loctite-freeze-and-release-aerosol-310g-far

Apologies for the n00b questions. But, if the IC was over - temperature, would that not burn / mark the PCB?

Given, the PCB is so clean (in both our cases) would that not mean it's something else causing the IC to fail for another reason (ie. no heat - maybe over-voltage or something else?)

[amyk]

That contains R134a which is a fine refrigerant, but also flammable butane, propane, and also the oil which might be problematic.

A heat-sensitive component doesn't have to be itself overheating, but just contain something like a tiny crack that opens with heat.

[tonycrouch]

Hi All,
I’ve ordered a replacement LD7990GS chip last night - but sadly won’t be delivered until the beginning of March.

I’ve put this monitor back together this morning and it’s currently working (which is good means I didn’t break it getting it apart) - but I suspect it’s only a matter of time before it randomly powers off.

Will report back should there be some amazing improvement.

[EvilDeece]

I've set the machine up with the PCBs exposed, and tapped vigorously on every through-hole component, as well as the PCB wherever there was space (to the point of introducing minor deflection in the PCB), and couldn't get the problem to reproduce. I might try warming up areas with an SMD rework gun and see if I can find an area that is heat sensitive.

[tonycrouch]

As an aside, not sure if this will help at all - but I’m currently running the monitor without it’s additional peripherals (ie speaker / menu buttons etc) - no improvement as yet - and I highly doubt it will fix the problem but I figured it was worth a shot

UPDATE - no improvement - shut off after approx 3 mins

[tonycrouch]

Decided to check out the GPU board - there’s only this pink pad between the GPUs and heat sinks. Is this normal?

[tonycrouch]

what I’m assuming to be GPU chips ...

[tonycrouch]

Ok - another update - I’m starting to think the GPU IC chips (or at least that’s what I’m calling them) are the cause of this problem.

I have disconnected all internal cables on the monitor except for the secondary board and the backlight (as it won’t turn on without the backlight).

I placed a fan I stole out of my young fellas bedroom aimed at the GPU heat sink on its lowest setting - the monitor worked flawlessly for  30 mins (25 mins longer than the screen has ever stayed on before). To clarify the monitor DID NOT turn off - I just got bored waiting for it to turn off

I turned the fan off and within 4 mins the monitor turned off - the GPU heat sinks were very hot to the touch.

I’m thinking of pulling off the pink pad underneath the heat sinks and applying thermal paste and seeing how we go.

[Rasz]

The Scaler chip looks BGA, probably cracked solder joint underneath, warms up, pcb flexes, loses connection, crashes, turns off. This is how Xbox 360 RROD worked. Reflowing with lead balls will fix it.

[EvilDeece]

Hmm, the thermal pads are pretty thick, they may need to be replaced with a shim instead. I measured mine at 2mm.

I'm thinking the easiest option would be 1.6mm aluminium flat bar from Bunnings: https://www.bunnings.com.au/metal-mate-20-x-1-6mm-1m-aluminium-flat-bar_p1064257

In the meantime, I'll try reflowing the video ICs - there's 3 of them on the board, each with their own 50x50mm heatsink.

[tonycrouch]

Good idea on the BGA needing a reflow.

My thermal pads were very, very thin (far thinner than 2mm) - and turned into mush when touched. So I reckon they’ve had it.

I’m gonna order some new heat sink pads (I’ll get a few diff thicknesses) and see how we go.

But I suspect the reflow might be the ultimate fix. I don’t have access to a heat gun - so @evildeece if you are able to give it a whirl, I’d be much appreciated.

[EvilDeece]

I'm just reassembling after reflow (it's entirely possible I botched it as I bumped one of the chips and had to nudge it back).

I did notice that some of the screw holes on the scaler board sit flush, while others don't. This may affect the heatsink contact. I'm going to dill those holes out to ensure better contact.

[Update] Ignore this, the screw mounts on those holes don't rise up to penetrate the board.

[tonycrouch]

Fingers crossed for you mate - let me know how it goes 👍

[Rasz]

I'm just reassembling after reflow (it's entirely possible I botched it as I bumped one of the chips and had to nudge it back).

I wouldnt turn it on after that, have to remove, reball, and solder again

[EvilDeece]

Yeah, definitely botched it... On to unit #2...

(I don't have the time & patience to learn how to reball it for the sake of a 5 year old monitor)

[EvilDeece]

Botched the second screen too - nothing was disturbed, but maybe overheated? The screen comes up white, and the power LED does not light and none of the buttons respond.

I guess these are going back into the e-waste pile.

[tonycrouch]

Ah mate that’s so sad - good on you for giving it a go 👍

I have bugger all BGA experience, and worried I will stuff it up - might be time to call in a pro - sadly though the nearest BGA place to me is 4+ hours away.

[tonycrouch]

On second thoughts mate, those chips are about $30 each - I’m thinking of ordering some - I can ship you some if you want to give it another go 👍

[EvilDeece]

I'm good, these 2 no longer boot, so there's not much point putting more effort into them for me.

They're a lot cheaper from China, AliExpress has them for <$10 for 5 pieces: https://a.aliexpress.com/_mqB1Ty5

I suspect I overheated the chips with my crude reflow setup (hot air gun), but I'm not sad, these were already slated for e-waste.

[tonycrouch]

I’m hearing ya mate - top work giving it a go. 👍

[Pctronics]

I thought I’d share my supposed fix for this. All 3 chips were reflowed ramp up temperature of 120c stage 2, 180c @2mims stage 3, 280c @1min stage 4 @400c @30seconds ramp down stage 1 280c 2mins stage 2 120c @1mins. Bit of a weird ramp but it was based on the flux bubble activity. I used amtec NC-559-V2-TF and did not clean it off. While still warm I added general thermal paste and let it sit in drier for a few hours. The thermal paste was enough to keep the heat sink sticky to the chips. Reassembled the unit and low and behold it hasn’t shut down after streaming OBS and I’ve been testing it for the last week. I couldn’t even get 15mims out of it without it powering down. And now I’ve saved $400aud and don’t have to buy a new monitor.

[EvilDeece]

That's good to hear - what did you use to reflow it? An SMD oven or a rework setup?

[Pctronics]

although I do have a reflow oven this board is too big, I just used my hakko rework station. I rigged it to a microphone stand so I don't have to hold the handset. I also use aluminium cans and a little captan tape to make mini chambers around the chip/work area to direct the heat more effectively. I use the same method when removing GPU/CPU and it works really well.

[EvilDeece]

Ah, thanks. I've been thinking of attaching my rework gun to a scissor arm to do similar, after seeing the combined IR heater + rework stations in action. Time to go and 3D print a mount, I guess

[perieanuo]

I thought I’d share my supposed fix for this. All 3 chips were reflowed ramp up temperature of 120c stage 2, 180c @2mims stage 3, 280c @1min stage 4 @400c @30seconds ramp down stage 1 280c 2mins stage 2 120c @1mins. Bit of a weird ramp but it was based on the flux bubble activity. I used amtec NC-559-V2-TF and did not clean it off. While still warm I added general thermal paste and let it sit in drier for a few hours. The thermal paste was enough to keep the heat sink sticky to the chips. Reassembled the unit and low and behold it hasn’t shut down after streaming OBS and I’ve been testing it for the last week. I couldn’t even get 15mims out of it without it powering down. And now I’ve saved $400aud and don’t have to buy a new monitor.

you're lucky, 400C is a bit much for the peak
but 400C with airgun is much less below chip, depends on your airflow setting
I never had more than 300-330C on metcal 5000, and the temp was the real temp of the chip, we had thermocouples on top/bottom side of pcb. the air flow and heating power is gigantic for those machines comparing with airgun. of course, it was solder without pb, the laptop/desktop nvidia cards sh...ty years rememberings
I tried for some pcb's up to 400C, the result was a dead or semi-dead video chip
good work, you have 'good hand'

[chilobo]

I own a Lenovo 2940. Since about one year my monitor about once a week makes a short knacking noise, goes dark, but comes up again after about 3-5 seconds.
I suspect some caps are bad, thinking about replacing them.
But: How do I open the monitor? There seem to be no screws. Do you have to pry the casing open with some plastic tools? Where do I start?

[EvilDeece]

The 2840 had clips around the edge of the frame, use a plastic pry tool, guitar picks, or credit cards to separate the catches without damaging the case.

[transistor-man]

Hi all, I came up with a pretty simple workaround to get these lenovo's running, I documented it here: https://transistor-man.com/lenovo_4k_repair_update.html

I found just having some air making its way through the back of the case would keep the temps down enough to prevent the thermal issue with the graphics processor part. I realize it's a bit goofy but it works.

Cad files for printing are included in the writeup