Benq G2255 doesn't power on - amber power light when plugged in - part 2

[will62]

Re: Benq G2255 doesn't power on - amber power light when plugged in - part 2

Probably GM5766H-LF PQFP-128, by STMicroelectronics. You can practice on that one, but it's not a replacement for the TSUM.

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in - part 2

ok Will, I plunged in and took the TSUM off (will be a long wait for the new one)! Guess what, resistance on the 3V is now in the 375Ks on the 1.8V in the 180K ohms and the 15 ohm I was getting on those 2 little caps, well now were in the mega ohms!

So it does look as thought the chip was shot. One question however....pins 85, 86 and 87 don't see to have any solder or any trace on them. I checked the chip and i see the legs only...could it be that those 3 pins are not used or did I tear out the traces? All other traces look fine, I'm going to clean them up (with flux) and wait patiently for the TSUM to arrive.

I am including some photos of the board and the chip. The one upside down, pin 1 is at the top left.


I ordered another 2 by registered mail so it might go faster.

[will62]

Re: Benq G2255 doesn't power on - amber power light when plugged in - part 2

Man, you got that chip up pretty neat. A lot of people just cut the pins off and remove them 1 by 1. It is easy to lift a pad if you let the solder cool while still lifting. Also very easy lifting them when cleaning up with solder wick. If I were you, I wouldn't use wick to clean those pads until you've practiced a lot on scrap boards. You'll be missing several pads before you know it. (don't ask me how I know)

The pads that look blank to me appear to be pins 65,66 & 67. If that's the case, none of those are connected to anything. When you get the new chip in, before soldering it in, go ahead and measure the internal resistance between a ground pin and 1.8/3.3V. You'll know very quickly if there was a problem on the old chip. Back to the waiting game I guess...

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in - part 2

Ok, I'll send you a photo once I have it cleaned up. I don't use wick, I use flux with a hot iron to reflow and remove the solder on the pads. Worked very well on that Winbond, I did it a few times.

You're right, those pins are 65, 66, 67...I don't know where I got 85, 86, 87, I think it was because I read on the board 84 instead of 64.

As for the test you mention, couldn't I do it on the old chip? Which pins should I try...for example pin 1 is a ground, but would I have to check all grounds to each 3v and 1.8v pins? Not sure what to do here.

[will62]

Re: Benq G2255 doesn't power on - amber power light when plugged in - part 2

You could check the old chip, but you said the board ohms to ground went way up with the chip removed, so I'd expect the chip to show low ohms to ground. There are only 5 1.8V pins if you want to try it. You shouldn't have to use different ground pins.

1.8V = pins 21,53,74,104,126

Gnd = 1,11,17,29,52,101+++many more

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in - part 2

Can't seem to find a low ohm to ground on the old chip? Could it be that it was just soldered bad?

On the 3.3V rail I get 560ohms and on the 1.8V rail I get 575ohms, so there must have been something lowering it to 80ohms?. On those 2 little caps I get infinite ohms.

I did the check with pin 1 and 11 of the chip...so I don't get it?


Should I try and resolder it back or should I be better to wait for the new ones (they have been shipped)?

[will62]

Re: Benq G2255 doesn't power on - amber power light when plugged in - part 2

If I remember correctly, 3.3V was 85 ohms, and 1.8V was 40 ohms. It's odd that you don't find the low ohms on the removed chip. I said that you didn't need to check every ground pin, but maybe I was wrong. At any rate, I wouldn't put that chip back on. We know your ohms to ground on the pcb went up when the chip was removed.

Maybe the problem ground is not interconnected internally, so it doesn't show up until the chip is soldered. If you're curious, you can check the other grounds to 1.8V.

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in - part 2

Found the ground on the old chip, it is between pins 17 & 8. Will check the new one to make sure.

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in - part 2

Ok Will, got the chips today (very fast shipping from this store on AliExpress). I will be installing one of them (got 2) tonight. Will report back ...if you have any points you want to check first, just shout!

[will62]

Re: Benq G2255 doesn't power on - amber power light when plugged in - part 2

Great, that was quick. Did you check resistance on 17/8 pins?

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in - part 2

Yes I did and all was good!

So I changed the chip, it went very well. Had high resistance on the 3V and 1.8V (470Kohms). Installed it and when I powered up the monitor I got a green light and the monitor booted! Opening screen, backlights on, etc. It lasted about 15 seconds as I tried to power off the monitor with the power button. But it wouldn't respond, so I turned off by the power bar.

Turned it back on, all was well, I could see the message about not being connected to dvi or vga, then after maybe 20 seconds, turned itself off and I was back to the amber light. Not responding to the power button, I turned it off again with the power bar. When turned back on, same symptom as in the beginning.

So I took the board off, and checked resistance on the 3V and 1.8V rail. Bad news, the 3V rail is now 40 ohms and the 1.8V rail is at 5 ohms. I guess the chip blew up?

I did notice that the backlights appeared a bit dimmer than expected, but I don't know if it is because of brightness adjustment or if there is a problem with the backlights. Could the backlights be bad and that would fry the chip?

[will62]

Re: Benq G2255 doesn't power on - amber power light when plugged in - part 2

That's not good. Before pulling the chip, look closely for any solder bridges on the pins. I'm sure you checked it before powering up, but just make sure. You can also power it up and put your scope on the pins from the power supply. Do you see anything on bl_on, brt_adj, or pwr_sav that would be a problem for the chip. Those signals come from the main, but maybe a short on the power supply is causing a problem on them. Does your 3.3V and 5V look smooth on the scope? Go ahead and scope the 1.8V also.

If you lift pin 8, does resistance change? Going to have to spend some time checking things before trying the other chip. I'll try to come up with some other things to check.

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in - part 2

Hi Will,

What about the dimmer backlights...could that have to do anything with the problem?

[will62]

Re: Benq G2255 doesn't power on - amber power light when plugged in - part 2

Not sure about that. The brt_adj signal comes from the main. I doubt if the chip is putting out that signal now.

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in - part 2

No, the chip is bad now. One thing I was thinking is I didn't check at all the TCon board for shorts. When I plugged the new chip in, I had the TCon board connected. I should have tried it before without the Tcon connected and see if everything else was ok?

I'll be re-inspecting my soldering job (with the microscope) to see if I might have overlooked a bridge - I did inspect it carefully with a 5X loupe, but there is residue flux paste (no clean type) which is caught behind the legs and that might hide something.

I'll then run those scope tests you suggested earlier..won't be until tonight so will report end of evening or tomorrow. Tks Will!

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in - part 2

I hate to say it, but the chip was installed perfectly, no bridges, all pins soldered well. So that is not the issue.

Will, when you say scope your 5V and 3.3V (and 1.8V) you mean with the board connected? I have 2.4K ohm on the 5V rail on the board, 38 ohms on the 3.3V and 2.2 ohms on the 1.8V.

I don't think I can scope the 1.8V anywhere else than on the board, whereas I can scope the 3.3V and 5V coming from the power supply.

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in - part 2

There is a place on Aliexpress where they sell an almost identical board for 15$:

https://www.aliexpress.com/item/-/32...999.262.OWqoEa

It is the same chip, same eeprom, but the flash is a Winbond and the DVI and VGA flash chips are slightly different atmels.

Do you think if I get this board I could swap eeproms and flashes and away we go?

However, I'll wait for your reply concerning the Tcon board that might be the culprit before doing anything else.

[will62]

Re: Benq G2255 doesn't power on - amber power light when plugged in - part 2

For a new main board, the important part, is that it is compatible with the panel. There were different panels used on the various models, so you'd have to email the seller to confirm compatibility. Personally, I wouldn't take any chances. There is an exact match to your main board number available for just a little more ($17.99).

https://fr.aliexpress.com/item/G2250...662798860.html

If you want to check the t-con, you can jump 5V to the cap near the lvds connector (check earlier pics), and probe each pin on the connector. While you're in that area, check the transistor that turns on the 5V to the lvds and make sure that it is not shorted, sending 5V back to the chip.

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in - part 2

That's not good. Before pulling the chip, look closely for any solder bridges on the pins. I'm sure you checked it before powering up, but just make sure. You can also power it up and put your scope on the pins from the power supply. Do you see anything on bl_on, brt_adj, or pwr_sav that would be a problem for the chip. Those signals come from the main, but maybe a short on the power supply is causing a problem on them. Does your 3.3V and 5V look smooth on the scope? Go ahead and scope the 1.8V also.

If you lift pin 8, does resistance change? Going to have to spend some time checking things before trying the other chip. I'll try to come up with some other things to check.

I have shorts on almost every pin of the TSUM, so it's gone. I guess if I power the board now it won't give me the right readings on bl_on, br_adj or pwr_sav will it? Also the 3.3V rail is at 40 ohms and 1.8V at 2 ohms, so they are shorted too. Should I just scope without the board attached to the power supply?

Also, I guess getting a new main board is probably not a good idea, the rest might fry it, do you think?

The little cap near the lvds is measuring 2 ohms to ground, each side...so is that on the 1.8V rail, because the 5V rail is not shorted. I checked those 2 transistors near the lvds and one of them is shorted but it's probably the TSUM that is giving me that reading (I didn't remove it)?

I'm pretty confused on what to do now

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in - part 2

Took some scope readings of every wire at the power supply, without the main board connected, both on AC coupling and DC coupling.

Don't see anything special, do you?