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

Go ahead and lift a leg and recheck.

[rddube]

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

Hi Will,

I checked again those 3 diodes that I thought were varactor diodes but I don't get any readings either way in diode mode, neither in resistance either way, it's as if there is nothing there? Should I remove one of them (or lift one leg as you showed me how to do) and test it out of circuit? If they are dead, could they be the cause of the chip getting fried? If you remember (or maybe I forgot to mention it), when I fired the board up with the new TSUM chip, the green power light came on without me touching the keypad (and the keypad wasn't responding when I tried to turn off the monitor) and a few seconds later, the screen went black and the TSUM was gone.

Attached is a photo (closer) of those diodes.

[rddube]

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

Exactly (at the input of the rails near the connector) on the 5V rail I get 520K ohms and rising, on the 3.3V rail I get 275K ohms and rising and on the 1.8V (taken on the output of the voltage regulator) I get 239K ohms and rising.

After doing those checks, I rechecked some of those small caps around the chip where I was getting 539 ohms, and now they give me anywhere from 400K ohms to 500K ohms.....maybe I didn't read those right in the first place. I tried discharging those 2 electrolytic caps to see if that would change anything but no they are still in the 400K to 500K ohms range.

[will62]

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

I believe the parts marked "93" are zener diodes (PZU6.2B2). They should check just like a regular diode. You are reading a capacitance because they are all in parallel to capacitors. I really don't think that they're a problem because we gave the keypad a pretty good checking a few pages back, and everything looked ok.

On whether the PS could cause the problem, reread post #105, item 5. It's certainly possible, but I think you would have caught it when you were working on the PS.

The 6 pin parts near your inputs are transient voltage suppressors. Pin 2 is ground, pin 5 is VDD. Pins 1,3,4 and 6 are I/O's. Just make sure the power and ground pins are not shorted with the I/O's, or to each other.

You also have some parts marked "A4". Those are common cathode diodes. I think we dealt with one of those on the PS.

Most of those small caps that you are measuring ~500 ohms on, are filter caps on the power rails. if you are reading 500 on them, that must be your new resistance from power to ground after removing the chip. I thought that you had said that it went higher than that without the chip.

[rddube]

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

Found out what those diodes are, in fact they are varactor diodes - I guess they don't work like your normal diode. I dowloaded the datasheet (SOD523), it is attached, but not sure I understand what it does.

When I check it on diode mode, no results either way, but on capacitance mode all 3 measure +-345.2 uf. So I guess they are working since I see a capacitor symbol besides the diode on the datasheet?

[rddube]

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

Ok, I actually checked everyone on the board as you can see in the attached picture...none of them show any shorts between the pins.

I also checked a few other things and am attaching another photo with some comments. I find it weird that the diodes don't do anything (if they are diodes)?

Don't forget, the photos show the chip but it's not there right now

A question Will, could the power supply be returning a voltage on one of the lines that the chip doesn't like (like on BRT_ADJ or PWR_saving?

[will62]

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

Without jumping too far ahead, I'm putting another one up so you can continue without waiting for me to post.

[will62]

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

Didn't expect to see the 3.28V on all of the lvds signal pins. Perhaps they are held high by the chip on your t-con. Everything else checks out.

Let's move on to checking the other components on the main. You may want to use a pic of your main, and mark each component as you check it, so we can be sure everything is checked.

Let's start with the transistors marked ZA. We're going to keep it simple and just check for shorts between pins. You have a couple of these on the board, so they should check the same. If one is suspect, you'll have to remove and check out of circuit.

[rddube]

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

Ok, so did the test, here are the results:

Pins 1 to 26 all have 3.28V, except for pins 7, 14, 17, 24, 25, 26, 27 that are at 0V. Pins 28, 29 & 30 have 4.79V.

[will62]

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

All boards connected. back probe the power supply connector for the 5V to the cap.

[rddube]

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

Ok, Will just to make sure I do this right....I connect the main board even though it has no TSUM on it or do I just connect the TCON and jump 5V to the cap and connect ground to the main board?

Wow, pretty nice diagrams! Tks!

[will62]

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

OK, let's check the t-con first. I don't expect that you have a problem here, because you said that you had a display before the chip fried. If you had a short on the t-con, it would probably take out the fuse on it. Anyway, let's check it so we can rule it out. See the attached pic. Jump 5V to the capacitor as shown, and measure all lvds cable pins. No need to list all 30 voltages, just tell me which ones are not 0V.

[rddube]

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

Let me try to answer some of your questions from your last 3 posts, then we'll try to come up with a step by step approach on what to do next.


6. "I didn't clean the flux"
That could have been a problem.

Oh, I like your answer on this one! When I removed the chip, I realized that there was a lot of flux there even under the chip and in behind the pins. Next time, instead of paste flux, I will try liquid flux that you dab (got a new dabber flux SRA Flux pen for rework) and will thoroughly clean right after. While at the electronics store I bought some MG Chemicals Super Wash, so it should be clean on the next go.

With the chip off, I no longer have a short on the little caps near the LVDS and neither on the 5V line on the lvds connector.

I checked the transistors near the LVDS and only one of them seems to be on the 5V rail, and it is not shorted. I tried following the schematics but there are differences in the schematics and my board. So I visually tried to trace the 5V rail and it goes to that transistor that I checked. Went over other transistors and diodes but haven't found a shorted one yet. Still searching.

[will62]

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

Let me try to answer some of your questions from your last 3 posts, then we'll try to come up with a step by step approach on what to do next.

1. "The 2 pins near the small caps have 2 ohms to ground."
Check them with the chip out. You'll see later on the schematic, that lcd_aware ties in there, so a short on the chip, will make those caps have low ohms to ground.

2. "I don't understand voltage back."
With all boards connected and powered, if something was coming from the t-con or power supply that zapped the main, then you should be able to see it when you probe the pins. An example would be if you jumped 5V to the t-con lvds cable, and a short on the t-con sent 5V(or more) back on one of the other pins on the cable, killing the chip.

3. "Is that the transistor nearest the lvds?"
Yes, you have 2 transistors in that area. I'll put up a schematic to I.D. them in a following post.

4. "How do I check the diodes?"
Yes, ~0.6V in 1 direction, OL in the other.

5. "Could it be the power board causing the issue?"
It could be, but thinking back to when we were working on the PS, I believe you checked all voltages after applying the jumper resistors. I don't remember you listing any unexpected voltages.

6. "I didn't clean the flux"
That could have been a problem.

[rddube]

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

Also, could it be the power button board that is causing the issue? How do I test that part without being connected?

Another thing I forgot to mention (blush) is that I used no clean flux (paste) and I used quite a bit to make sure to not get any bridges. I was too excited to try it out that I didn't clean the board so there was flux residue between the pins. I figured, since it is no clean flux that it is not conductive, but could that be a possible cause?

[rddube]

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

In the meantime, I removed the TSUM chip and everything is back to normal. No more shorts.

You said that I should check every transistor and diode on the board and that is what I'll do, especially the one that turns the 5v to the panel...is that the one nearest the lvds? Doesn't seem to be shorted. The one below it, I have a ground on one of the 2 parallel pins and a few others that I have a ground on one of the 2 parallel pins.

How do I check the diodes....diode mode on the meter and check one way and then the other?

I feel like a hen with no head, thank God you are there Will!

[rddube]

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

Hi Will,

Thanks for your patience. Please bear with me but I'm not sure I understand what you want me to do?

The 2 pins on the lvds connector (near the small caps) are both grounded to 2 ohms (on the main board) but they are fine on the flat cable going to the Tcon. Also, there are at least 40-50 pins on the main chip that are grounded.

When you say "both the turn on signals from the main, then measure each pin on the connectors to see if you are getting a voltage back", I don't understand a voltage back ...from where?

Or do you mean to do these tests without the main board connected? So jump 5V to the 2 connectors on the flat cable going to the Tcon and then measure each pin of the flat cable. Then do the same with the power supply?

Can you step me through this?

BTW, I reinspected the main board and the main chip (actually used a backlight behind the board) and there are no bridges on the main chip. But since it is toast and many pins are grounded, I am getting grounds almost everywhere on the chip (i.e. the 3.3V rail and 1.8V rail), but not on the 5V rail. If those 2 pins on the lvds connector (near the caps) are 5V, how come they are grounded?

Thanks for your help, we'll get there!

[will62]

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

Nothing jumps out at me with your readings. To see if the power board or t-con are zapping the main board, you need to give both the turn on signals from the main, then measure each pin on the connectors to see if you are getting a voltage back that would damage the main. Many of these ICs have a few pins that are 5V tolerable, and I think the lcd_aware is one of those on your chip. So, that would be the only one that I could think of that would be able to handle a higher voltage.

To give the turn on signals, you'll have to jump 5V to the lvds connector as I mentioned above for the t-con. You'll have to jump the same pins on the power connector that you jumped when you were testing the power supply. This time you need to jump the resistors from the back of the connector, so that it can be connected to the main. When you power on, check all pins on both to see if you are getting anything that doesn't belong on the pin. You can also probe each pin on the chip to see if you find something that doesn't belong (i.e. voltage on a ground pin, or a higher voltage than expected on other pins). You'll have to follow the schematic as you are probing the pins. I've already mentioned the transistor that turns on the 5V to the panel. You really need to make sure that it is not shorted, and sending 5V back to the chip. For that matter, I would check every transistor and diode on the board.

This is the only way that I can think of to make sure that the t-con and power supply won't zap a new main board as soon as you install it. You do have another chip, so you could try it. This time I would go through the tedious process of checking continuity pin to pin to make absolutely sure that you don't have any solder bridges below/behind the pins that are difficult to see. Once the chip is in, leave all connectors disconnected. Jump only the ground and standby voltage to the main, then check everything.

[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?

[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