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

[rddube]

Hello everyone,

This is the continuation of a former thread where we worked on the power board. This is about the main board photos attached of everything, but especially higher resolution photos of the main board.

Thread part 1:
https://www.badcaps.net/forum/showthread.php?t=66796

As I had mentioned, when plugged in this monitor shows the amber light on the power switch and there is no response to keypress on the switch panel. Someone mentioned that it could be a problem with a corrupted eprom, but for some reason I am getting 80 ohms resistance on the 3V rail. Also checked the switch panel and it looks to be OK.

I removed the 2 electrolytic caps and checked them - their values and ESR are ok. Checked the voltages on the voltage regulator on the main board, and I get 3.3V on left leg, and 1.8V on right leg, so that looks ok.

Not sure what to check next. Any ideas?

[will62]

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

Look who's back. I was able to find a schematic that may help on this one. Not your exact model, but similar chip on main board. You'll notice that the power board and inverter are different. Take a while and look over the main board section. Is it a close match for your's? Component numbers the same?

[rddube]

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

Hi Will,

It very close, however slightly different on mine the large chip is TSUMU58NWHJ-LF-1, which is close. It doesn't seen to use the same eeprom chip...mine is a 24C16 chip.

I checked the voltage on VCC of the pin and am getting 2.9V (specs say 1.7 to 5.5v) so I guess the eeprom is getting voltage. I doulbed cheched my voltage readings at the voltage regulator - left pin 3.270V right pin 1.81V. That looks OK. I still hae 80 ohm resistance on the 3.3 volt rail which bugs me, but maybe that is ok also.

Momaka in another thread said to try and feed it 5V and 3.3V from an ATX power supply and see if that helps. I have an old PC I haven't used for years with an iffy power supply, I wonder if it would do more damage than good.

Well, will keep on searching and thanks for the schematics!

[rddube]

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

Does anyone have any experience with flashing an eeprom?

I have the firmware file (.bin), I have the way it must be connected (photo attached), and I think I have the way of programming it with an arduino uno

https://learn.sparkfun.com/tutorials...serial-eeproms

But I have never done it before, and I wonder if I must take the chip out to program it, or can I do it on board while supplying the appropriate voltage to the chip?

Or am I going at this backwards, should I try to locate a fault in the hardware first?

Any help is welcome!

[will62]

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

I've never done it, but there are a lot of other things you can check before going down that road.

So I can stop asking what your test equipment can do, please give me the model numbers for everything you are using.

[rddube]

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

Hi Will,

Ok, here we go:

Fluke model 87V DMM
Oscilloscope DSO138
Transistor Tester M328 (2 line display) but with probes and version 1.12K firmware
Desoldering station Aoyue int 474A++
Soldering station with hot air GQ 5200
Frequency generator FG085
UT33 DMM
Arduino Uno

That's pretty much it!

[rddube]

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

I've been taking voltage measurements on the chips on the main board (moreover the VCC pins to see what kind of voltage I am getting).

On the 2 ATMLH134 (AT24C02), pin 8 vcc I get 2.6V and pin 3 Vout .08V on both of them.
On the 25L2026E, pin 8 vcc I am getting 3.2V
On the 24C16 (which I think is the EEprom), pin 8 I am getting 3.25V

[will62]

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

The amber led indicates that the chip is in sleep mode. The first thing it should do when power is applied, is load the source code for the MCU from your flash chip. The EEPROM holds DDC data such as panel info, user adjustments... etc.
There are 2 ways to activate the chip, but it appears that it is not responding to either:

1. When power is first applied, The 5V from your power board goes to C48,C49 & R45. This is a capacitive divider with a pull down resistor. C48 will let a brief high pulse through, but then becomes a DC blocker. The high pulse goes to pin 108, which is your chip reset pin. That should wake the chip up to load the flash.

2. Pin 125 is pulled high to 3.3V. When you press your power button, the switch is grounding pin 125. That brief grounding should also wake the chip up.

The chip is not responding to either, so you probably have a power to the chip problem, a chip problem, or a flash problem.

You could start by checking power to the chip. Go back to the schematic. There are 18 power pins, and 14 ground pins. Make sure that all are what they should be. I guess we'll find out here if your chip matches schematic.

I hope you have a sharp tip probe. If not, improvise something with a pin or needle. Some of the power pins are right next ground pins. If you are not extremely careful, you'll smoke that chip (if it has any left inside).

(added a pic. zoom in and I've labelled a few things)

[rddube]

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

Hi Will,

Wow, that is amazing guidance!

Just to make sure I am understanding right, you want me to measure the points in the diagram (copy attached)? I count 17 of them, where would be the 18th?

Also, I'll measure the points you indicated for 5V and 3.3V.

Yes I have needle type probes, so is it ok if I position myself on the pin and then turn the power on, then off, then next pin?

Tks Will for all your help. If we get this guy going, I'll owe you quite a few beers!

[will62]

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

Pin 7 (REXT) has an external resistor going to AVDD_33. I believe it is used to set the internal clock rate. No need to power up and down, just be careful. As before, if you see an obvious external component, probe there.

[rddube]

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

Ok Will, here are the results:

First thing however, is that I get 5V on the opposite side of the capacitor you marked in the pic (left side, where as you indicate right side).

The Mosfet measures 3.28V on the left pin and 1.82V on the right pin.

Now for the chip:

Pin 7 REXT: 3.29V
Pin 108 RESET: 0V

Pin 8 AVDD33 : 3.25V
Pin 14 AVDD33: 3.25V
Pin 20 AVDD33: 3.26V

Pin 21 AVDD18: 3.26V

Pin 53 VDCC: 1.8V
Pin 74 VDCC: 1.82V
Pin 104 VDCC: 1.8V
Pin 126 VDCC: 1.8V

Pin 40 VDPP: 3.26V
Pin 54 VDPP: 3.28V
Pin 58 VDPP: 3.28V
Pin 59 VDPP: 3.28V
Pin 71 VDPP: 3.25V
Pin 87 VDPP: 3.25V
Pin 107 VDPP: 3.256V
Pin 112 VDPP: 3.25V
Pin 114 VDPP: 3.25V

Looks like the schematic is accurate.

[will62]

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

All that looks good. How about your ground pins. All 0 (or very low) ohms to a good ground?

Going to have to look into the 85 ohms to ground also. Is that only on 3.3V?
How about 5V and 1.8V (all connectors removed).

[rddube]

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

Ok, checked all grounds on the chip and they are very good, very low ohms.

3.3V rail is 81.5 ohms
1.8V at the mosfet is 41 ohms
5 V is in the kiloohms going up

All connectors removed.

[will62]

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

Yeah, the 3.3V and 1.8V are way low. I don't know what the minimum for that board is, but I've seen other main board troubleshooting guides that used 100 ohms as the cut-off. Below that, guide said change board.

We can try something else before we dig into the low ohm issue. Take a look at your flash. Let's try to determine if the main chip is even trying to access the flash when you turn on the power. When not being accessed by the main, the flash should have about 3.3V on pin 8 (VCC) and pin 1 (chip select). When pin 1 is pulled low, the flash can start communication with the main. Use your scope to probe activity on pin 1. When you turn power on, pin 1 should go high. If the main chip is attempting to access, pin 1 should go low during communication. The changes will happen quickly, see if you can catch it on your scope.

Datasheet for the flash attached.

[rddube]

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

Ok, did the test. When I apply power, pin 1 goes high (approx 3V) and immediately (within a fraction of a second) drops low. I did the test 3 or 4 times to make sure.

I also probed pin 8, and when I apply power it goes high and stays high (3.8V).

[will62]

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

OK, well that means the main chip is attempting comms. You don't really have anything that will measure frequencies as high as we're dealing with on the main board. Your Fluke tops out at 4kHz, and your scope at 200kHz. Go ahead and probe the crystal (probe one side, probe gnd other. What do you see? Just a solid bar ~300mV high? (AC)

[rddube]

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

Probed the crystal. On one side I see a straight bar 1-2v high no frequency reading. On the other side I see sort of a wide wave with frequency reading of 60hz.

[will62]

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

Ok, that solid bar is because of the limited bandwidth of your scope, the wave swings appear to be touching, making a bar. Recheck it and try to remember the intensity for reference. Now power down, and probe pin 6 of the flash. It is the clock for the data in and data out of the flash. When you power up, do you see a bar that is more intense (it'll have ~4-5 times the number of swings vs. the crystal). Power down again. Probe pin 5, then pin 2 (data in, data out).

[rddube]

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

Ok Will..on the crystal I get 2.01V, at pin 6 I get .92V and pin 2 and 5 very low .2 .3V. However on 2 occasions, while probing pin 6 and 2, I saw square waves and they were on steady, then I couldn't replicate it. On pin 6 the square waves were much larger than those on pin 2, but they were nice and regular. Never saw any square waves on pin 5.

[will62]

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

Sorry, I told you AC on the crystal, and didn't tell you to change to DC on the flash. See if that makes a difference. I'd be surprised if the data signal was more than ~300mV. Data in (p5) activity should be brief. Short instruction to read code and send on data out (p2). The data out should run longer. The clock will run during the entire process. Pin 1 will go high again when the chip finishes loading the code. I don't think that you should be able to see the square waves with your scope. They're as high as 85MHz. Maybe you are picking up extended high or low signals that indicate something in the process.

With your scope, you can't see much. We are just trying to confirm that the flash is accessed (p1 low). There are very high speed signals on pins 6, 5, and 2. Then pin 1 goes high again when process is finished.