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

[rddube]

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

Do I need to do these tests with the CFL's connected? That would be a little tricky, because the power board needs to be side up when I connect them?

[will62]

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

Yes. Panel face down. Use a sheet of cardboard to insulate between panel and power board. I assume that you are using a power strip near your setup, so that you can turn panel on/off quickly w/o having to plug/unplug PB. After you run tests, leave the setup as is (more tests to follow).

[rddube]

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

If panel face down, then the CFL's will be inverted in the power board, is that ok (the lenght of the cfl wires are not long enough to plug them in their respective sockets)? Also Will62, the voltage measurements I did in previous post was without the CFL's connected...and you can see that pint 14 is 0V. Should I do voltage readings with the CFL's connected before doing the oscilloscope measures and report?

[will62]

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

Not sure if connector swap will create a problem, but let's not add another variable to the mix. Lay panel face down. Set power board top side up on panel. Connect your ccfl wires as normal. Now flip the other end of power board over, exposing bottom of board. Insulate with cardboard between panel & PB.

BG and TG can never have power at the same time without blowing mosfets. There is some dead time between switching.

Also, please check resistance across C808.

[rddube]

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

Ok, will try my best without blowing up the house!

Resistance across C808 is 460K ohms.

[budm]

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

The lamp connection can be used in any directions, they are just 2 two individual lamps.

[rddube]

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

Ok, tks Budm and welcome back.

Tks for the tip, will do my tests and report.

[rddube]

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

Thought I had found a resistor with wrong value, but my mistake. So this post is not important, please ignore as I can't delete it.

[rddube]

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

Ok, so did the tests with the oscilloscope as you explained Will, and this is getting to be fun!

First off, pin 11, when I put power on, I get a wave not perfectly square, but sort of conical with about 12-13v amplitude for approx 1 second and then it goes flat.

I waited a little until pin 7 & 9 were almost at 0, redid the test on pin 11 and got the exact same result.

Waited another while until pin 7 & 9 to 0 volts, then tried pin 14. Got about the same wave (conical) of about the same amplitude but for maybe 1/2 a second, somewhat shorter time than pin 11, and then flat. Did the test a few times and got the same results.

So, what is next, I'm starting to like this. I left the setup as is.

[will62]

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

The conical shape may be something related to MPS's Nu-Pulse technology, not sure.
Now we're going to bypass the chip's fault protection and take a longer look at the TG and BG signals (same as earlier). Need to jump pin 5 to ground. An alligator clip across C803 should work, if it'll hold. After the clip is on, measure resistance between pin 5 and pin 15. Should be near 0 ohms. Run the test for about 5 seconds to get a better look at the waves. Don't want to run it too long because shutdown may be due to over-current. See if you can display the frequency on your scope.

Questions
1. Do you have a hole on the back of that panel so you could see the backlight if it came on? If not maybe you could see some glow at the corners where the cables go.
2. Does your DMM have frequency function? If so, you could try touching the top of the transformer (not the pins, just the top plastic) with your red probe (black to ground) while you are running the mosfets, to see if you pick up the oscillation frequency.
3. Did you check your probe on the scopes 1kHz signal? There's a loop at the top of the scope to hook on to. Pulse nice and square, or conical? (set horizontal to 1 ms).

[momaka]

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

Regarding the logic board issue...
Try connecting only the logic board to the power supply and the power button board. Leave the T-con board disconnected. Does the power LED still stay orange only and is the monitor still not responsive? If yes, try supplying 5V and 3.3V to the monitor from a computer ATX PSU - this will eliminate noise/ripple problems from the monitor's PSU, if there are any. If the logic board is still not responsive, then the issue is probably either a firmware one (bad flash chip / firmware corruption) or the main chip on the logic board. The only other thing I can think of is that 85-Ohm resistance you measured - that could be a partially-shorted ceramic capacitor or diode/transistor. Is this resistance on the 1.8V line or the 3.3V line? The former is a bit more worrisome.

We are trying to get the inverter working, but I also have a problem with the logic board, so down the end, it might not be economically viable to repair this monitor? What do you think?

At this point, it clearly isn't economically worthwhile to repair the monitor. But the knowledge you learn from the troubleshooting process might be - so that alone could make the repair worthwhile overall. It really depends what you are trying to get out of this.

I agree that we need to get the logic board working first. Without it, the monitor is as good as dead anyways.

The one I find suspect is blue and written:
8
3KV
SEC
Not sure what I should get as I don't see 8pf 3KV capacitors on digikey?

It's probably an 8.2 pF capacitor (as that's a standard value).

[rddube]

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

The conical shape may be something related to MPS's Nu-Pulse technology, not sure.
Now we're going to bypass the chip's fault protection and take a longer look at the TG and BG signals (same as earlier). Need to jump pin 5 to ground. An alligator clip across C803 should work, if it'll hold. After the clip is on, measure resistance between pin 5 and pin 15. Should be near 0 ohms. Run the test for about 5 seconds to get a better look at the waves. Don't want to run it too long because shutdown may be due to over-current. See if you can display the frequency on your scope.

Thanks Will62 for being so kind in spending your time on my case. It is really appreciated and the knowledge that I gain is priceless. Thank you very very much!

Ok, it worked and the waves stay on.didn't go over 5 seconds on either pin. Get a conical wave and the frequency is 71.2Khz on both pins.

Questions
1. Do you have a hole on the back of that panel so you could see the backlight if it came on? If not maybe you could see some glow at the corners where the cables go.

Yes I do have some areas where I would see the lights glow, but nothing comes on.

2. Does your DMM have frequency function? If so, you could try touching the top of the transformer (not the pins, just the top plastic) with your red probe (black to ground) while you are running the mosfets, to see if you pick up the oscillation frequency.

Yes it does, but the board is upside down, so not too sure if I should poke my probe under the board and not see if I am touching metal or plastic, so I skipped this. But I did try to put my probe (black to ground) on the pcb where the transformer is and turned it on briefly, but no reading on my meter.

3. Did you check your probe on the scopes 1kHz signal? There's a loop at the top of the scope to hook on to. Pulse nice and square, or conical? (set horizontal to 1 ms).

Yes, I get a nice square wave but have to go to 50us....my tests with the pins were at 10us, because at 50us the wave is sort of flat. I did notice during the tests that the voltage range was between 12 and 13V.

Will, one question I forgot to ask...on the oscilloscope I have GND, AC and DC settings. I set it to DC settings for the tests, is that OK?

[rddube]

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

Regarding the logic board issue...
Try connecting only the logic board to the power supply and the power button board. Leave the T-con board disconnected. Does the power LED still stay orange only and is the monitor still not responsive?

Hey Momaka, nice to see your sound advice in my thread! Yes it stays orange and no response.

If yes, try supplying 5V and 3.3V to the monitor from a computer ATX PSU - this will eliminate noise/ripple problems from the monitor's PSU, if there are any. If the logic board is still not responsive, then the issue is probably either a firmware one (bad flash chip / firmware corruption) or the main chip on the logic board. The only other thing I can think of is that 85-Ohm resistance you measured - that could be a partially-shorted ceramic capacitor or diode/transistor. Is this resistance on the 1.8V line or the 3.3V line? The former is a bit more worrisome.

Ok, will try to do that once I get the power board off the bench, because right now it is connected with the oscillscope, and I will have to dismantle one of my PC's to get the ATX PSU. For your other question, it is on the 3.3V line....shucks!

At this point, it clearly isn't economically worthwhile to repair the monitor. But the knowledge you learn from the troubleshooting process might be - so that alone could make the repair worthwhile overall. It really depends what you are trying to get out of this.

You are right, I am really enjoying getting guidance from Will62 and am learning a lot along the way. I love it!

[will62]

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

Refer back to the MP10091 diagram pic on post #104. You're going to be probing pins 16, 1, 2 & 3 next. Remember to find a near component to put
the probe on. Easy to slip while you're watching the scope (pin 16 is next to ground pin). Look at the attached pic and you'll see what we're looking for.
Pay close attention to the thresholds noted on the pic. Whether or not the wave stays above or goes below the threshold is the fault trigger. Since neither of the bulbs came on, I have a feeling that I know where this is going, but let's see what your waves look like. Since the bulbs are not on, running beyond 5 seconds on fault bypass is probably not a problem.

As for your question about the scope setting, leave it on DC for these tests. After these tests, why don't you watch a couple of videos on checking for voltage ripple with a scope. If you set the scope on AC, you'll only see the ripple component. See if you can estimate peak to peak ripple on your 3.3V, 5V and 20V supplies. Some datasheets will specify a maximum tolerated ripple.

[rddube]

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

Will do I leave the shorted pins on the IC for those tests?

[will62]

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

Pin 5 remains shorted to ground.

[rddube]

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

Ok, Will this is so exciting...as I was getting prepared to do my tests, pin 5 to ground, etc. I did a quick test to see if the CFL would light up. Well guess what, they did and stayed light as long as I kept the AC on. But I didn't leave it for too long just in case. I removed the alligator clip that grounds pin 5, powered up and the CFL's came on, but out after about 2 seconds.

I have to admit that the opening to see the CFL's is very small so I might have overlooked it before.

So I put the alligator clip back on C803, put power on and they lit up and stayed lit for at least 10-15 seconds...then I powered down.

I took the attached picture of my oscilloscope on pin 14 for you to see...sorry for the quality, but one hand on the probe, the other putting power on and having to grab my camera etc. You can't see the frequency because it sort of flashes on and off, and I guess the photo snapped while it was off, but it ranges in the vincinity of 71-72Khz.

So is this good news?

[Dumah Brazorf]

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

What measure pin 1 and 16 in this case?

[will62]

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

Hey, that little $20 scope is not a bad one to learn on, is it? It looks like it'll do everything we need on the power board.

Did you notice if both lights came on equally? No flashing or anything? Jump pin 5, turn off the light in the room, then power up the board. Notice any difference on how each lamp starts up (blinks, pinkish hue... etc).

Go ahead and run the tests on pins 16, 1, 2, & 3. We need to narrow down which connector circuit is causing the problem. Then we'll swap lamp connectors, rerun the test and see if the problem moves.

[will62]

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

Oops. Forgot to mention, scope on AC.