Acer x223w first white screen, now no backlight

[retiredcaps]

Re: Acer x223w first white screen, now no backlight

Okay, here is what we have tested so far and looks okay.

1) n channel mosfets do not seem shorted
2) picofuse is good
3) inverter transformer readings (secondary) are good
4) secondary inverter voltages are present (the 17.x V DC seems correct - each manufacturer/board varies).

The only thing I can think of is the 16 pin IC on the back of the board. What is the part number of that chip?

[teri3s]

Re: Acer x223w first white screen, now no backlight

Okay, um.............tried a few times to make sure......and I'm not seeing anything displayed on the screen while shining the flash light on it in the dark.

[retiredcaps]

Re: Acer x223w first white screen, now no backlight

Okay, let's do another simple test.

Hookup a working computer displaying your favourite wallpaper to the lcd. Do you see the wallpaper even briefly? If you turn off all the lights and shine a flashlight onto the lcd, do you see the wallpaper?

[teri3s]

Re: Acer x223w first white screen, now no backlight

Not even sure how to explain the testing of the mosfets. Pretty much every combination has the display unchanging from '1' except with ohms set at 200k, with the black probe at S1 and the red probe at D1 I get a number that keeps ascending. I tried discharging the gate with my finger first, but right now the number is 33.5 ohms ascending.

The behaviour of both mosfets is identical, seemingly synchronized.

[teri3s]

Re: Acer x223w first white screen, now no backlight

I just rechecked CN801 (plugging in the logic board this time) with the digital multimeter against the ground where the plug is and I get

While off
1) 5.05v (VCC 5V)
2) 5.05v (VCC 5V)
3) 0V (GND)
4) 0V (GND)
5) 0.03V (on/off)
6) 2.56V (Brightness)
7) 0.04V (Vol) -----no speakers on this monitor
8) 1.75V (Mute)

While on
all other pins are the same values except
5) 3.12V (on/off)

Brightness voltage is the same

[retiredcaps]

Re: Acer x223w first white screen, now no backlight

Scale was set to 10x

I'm curious. Can you remeasure the n channel mosfets "in circuit" with your digital multimeter?

I have nothing against analog multimeters, but I'm curious if you get the same 50 ohm reading on the digital.

[retiredcaps]

Re: Acer x223w first white screen, now no backlight

It looks like there may be a voltage missing on the signal card, for the backlights to come on the great majority of monitors there will be around 3 volts on the backlight on pin.

Good catch. I totally missed that.

[alexanna]

Re: Acer x223w first white screen, now no backlight

remaining 4 pins (in order, on/off, Brightness, Vol, Mute) read zero volts.

)

It looks like there may be a voltage missing on the signal card, for the backlights to come on the great majority of monitors there will be around 3 volts on the backlight on pin.

[retiredcaps]

Re: Acer x223w first white screen, now no backlight

With Rx10 setting on analog, about 500 ohms at both the red and yellow dots.

Both those readings look good.

The 50 ohm reading is still suspicious to me. You may want to desolder and retest them just to be sure.

[teri3s]

Re: Acer x223w first white screen, now no backlight

With Rx10 setting on analog, about 500 ohms at both the red and yellow dots.

[retiredcaps]

Re: Acer x223w first white screen, now no backlight

Hmm, I wondering about the 50 ohms and wonder if it could be shorted? If you can, desolder the chip and retest out of circuit.

Before you do that, please measure the resistance between the yellow dots and the red dots. Report both.

You are measuring the resistance of the inverter transformer secondaries. Do this with power off. I don't need any voltage readings from this area.

[teri3s]

Re: Acer x223w first white screen, now no backlight

Scale was set to 10x

The "2's" displayed the same pattern.

[retiredcaps]

Re: Acer x223w first white screen, now no backlight

Am I supposed to desolder these devices to test them?

Generally in order to get an accurate reading, yes. However, I usually test "in circuit" first and if the readings suggest a short, then I desolder to verify.

S1 and G1, the needle stays at about 50 ohms (or the middle of the scale).

What about the "2"s? S2-D2, S2-G2, D2-G2?

On the analog meter, what scale is your dial set to? 1x? 10x?

[PlainBill]

Re: Acer x223w first white screen, now no backlight

Thanks for that PlainBill. Yeah, I got an inkling that "across" would cause confusion.

The picofuse is not open. It reads nearly 0 ohms when the probes are placed at opposite ends. No voltage registers if probed "across".

If I go from a ground point to the picofuse I get the 5V reading.

Yeah, that makes more sense. It's a nasty area, because when I see 'At the fuse' I wonder 'which side of the fuse?'

PlainBill

[teri3s]

Re: Acer x223w first white screen, now no backlight

Thanks for that PlainBill. Yeah, I got an inkling that "across" would cause confusion.

The picofuse is not open. It reads nearly 0 ohms when the probes are placed at opposite ends. No voltage registers if probed "across".

If I go from a ground point to the picofuse I get the 5V reading.

[PlainBill]

Re: Acer x223w first white screen, now no backlight

Not sure what you mean pedro?

This may be a communication problem. When I hear 'Across F802 I get +5V', I interpret it to mean that you put one probe on one end of F802, the other probe on the other end of F802, and read 5 volts. The only way that can occur is if F802 is open. And that is a problem, because when a fuse goes open it is usually because something has failed (shorted).

PlainBill

[teri3s]

Re: Acer x223w first white screen, now no backlight

I am not sure what I'm looking for exactly in terms of ohmmeter readings while checking for shorts so I'll just post what I found and if I say anything stupid.......well please bear with me lol.

Am I supposed to desolder these devices to test them?

But in the meantime, using an analog meter, when I touch the probes to S1 and D1 it goes from near 0ohms and then gradually falls back to infinite ohms.

D1 and G1 it goes from about 50 ohms and the needle falls back to infinite resistance.

S1 and G1, the needle stays at about 50 ohms (or the middle of the scale).

This goes for both mosfets.

When the power button is off, I get 17.7V at U501 and U502.
When the power button is on, I get 18.0V at U501 and U502.

[teri3s]

Re: Acer x223w first white screen, now no backlight

Not sure what you mean pedro?

[pedro]

Re: Acer x223w first white screen, now no backlight

Across F802 I get +5V
The jumper above, "J808" I get +17.6V

Did you really mean "across F802"? If so, that's a problem ...

[teri3s]

Re: Acer x223w first white screen, now no backlight

Alright retiredcaps, will do. Also, a sincere thank you for all the help thus far.