Dell E153FPb - Screen stays white when off or in standbye

[aliasdck]

First post, but definetly not my first experience repairing an LCD monitor. Sorry to make another thread on this monitor, but I am experiencing a different problem so I figured it was okay to make a new thread.

When I received this monitor it would not stay on for more than a second. Opened it up and discovered cap c703 was bulging which I then replaced.

Now the monitor works great except that whenever the computer is turned off, put into standbye or I manually turn of the monitor by pressing the off button then the screen turns off for a split second and then goes completely white (bright) and stays that way until I turn on the computer back on/come out of standbye/turn the monitor back on.

Without a schematic I'm not sure where to start other than to desolder all the caps and transistors and check them. Before I do that, though, I was curious to see if anyone had any insight.

Thanks.

[EGuevarae]

Re: Dell E153FPb - Screen stays white when off or in standbye

First post, but definetly not my first experience repairing an LCD monitor. Sorry to make another thread on this monitor, but I am experiencing a different problem so I figured it was okay to make a new thread.

When I received this monitor it would not stay on for more than a second. Opened it up and discovered cap c703 was bulging which I then replaced.

Now the monitor works great except that whenever the computer is turned off, put into standbye or I manually turn of the monitor by pressing the off button then the screen turns off for a split second and then goes completely white (bright) and stays that way until I turn on the computer back on/come out of standbye/turn the monitor back on.

Without a schematic I'm not sure where to start other than to desolder all the caps and transistors and check them. Before I do that, though, I was curious to see if anyone had any insight.

Thanks.

It is a short in one of the components that drive/manage the BLON signal to the inverter. Post good resolution pics of you board(s) for inspection.

[PlainBill]

Re: Dell E153FPb - Screen stays white when off or in standbye

It is a short in one of the components that drive/manage the BLON signal to the inverter. Post good resolution pics of you board(s) for inspection.

I agree.

The inverter oscillators in the E172FPb and E173FPb are free running oscillators, the E153FPb seems to be the same. Here's a picture of the power supply / inverter from an E153FPb. It's likely Q743 and Q751 control the power to the inverters.

HOWEVER, given the history of the problem, it would be a good idea to start by looking very carefully at the area around C703. Odds are a stray blob of solder is causing this problem.

PlainBill

[EGuevarae]

Re: Dell E153FPb - Screen stays white when off or in standbye

I agree.
It's likely Q743 and Q751 control the power to the inverters.
PlainBill

I agree too.
See this eBay kit that contains the 4 c5707, 2 fu9024 (the Q743 & Q751 in the pic) and the picofuse. The Model and brand of the units (Dell E172FPB & E173FPB) are also mentioned. I've seen cases where all of that mentioned parts need to be replaced, and bad solder points/cold solder was also corrected.
Either a solder blob is making the BLON signal to stay always on, or one of the above mentioned components is failing.

[aliasdck]

Re: Dell E153FPb - Screen stays white when off or in standbye

I checked everywhere for any stray blobs of solder, shorts etc and cant find anything.

I desoldered the two mosfets Q743 and Q751... but I'm not really sure exactly how to test them beyond using the diode function on the multimeter for which they both tested identical (nothing obviously wrong i.e. no shorts).

Then I desoldered the four transistors Q739, Q740, Q759 and Q760 and checked the hfe's (not sure which transistors where which as I hastly got them mixed up):

392
389
393
552

Seems something could be up with that last transistors reading... but I'm probably going to have to replace atleast one of them anyways as I managed to break a pin off of the transistors whose hfe measured 389. (Doh!)

[PlainBill]

Re: Dell E153FPb - Screen stays white when off or in standbye

I checked everywhere for any stray blobs of solder, shorts etc and cant find anything.

I desoldered the two mosfets Q743 and Q751... but I'm not really sure exactly how to test them beyond using the diode function on the multimeter for which they both tested identical (nothing obviously wrong i.e. no shorts).

Then I desoldered the four transistors Q739, Q740, Q759 and Q760 and checked the hfe's (not sure which transistors where which as I hastly got them mixed up):

392
389
393
552

Seems something could be up with that last transistors reading... but I'm probably going to have to replace atleast one of them anyways as I managed to break a pin off of the transistors whose hfe measured 389. (Doh!)

You're working at it from the wrong direction. The fact that the backlights are working indicates Q739, Q740, Q759 and Q760 are good. If the two mosfets were not shorted (no need to desolder them, by the way), you should have worked back toward the control input as eguevarae indicated.

It's a little more difficult without a schematic, but you should be able to trace the signal as it goes from the connector to the FETs with a DMM by probing and watching the voltage change as you hit the 'Power' button on the monitor. It's a bit of a pain, because you have to avoid touching anything with high voltage as you do this.

PlainBill

[Bobdee]

Re: Dell E153FPb - Screen stays white when off or in standbye

I checked everywhere for any stray blobs of solder, shorts etc and cant find anything.

I desoldered the two mosfets Q743 and Q751... but I'm not really sure exactly how to test them beyond using the diode function on the multimeter for which they both tested identical (nothing obviously wrong i.e. no shorts).

Then I desoldered the four transistors Q739, Q740, Q759 and Q760 and checked the hfe's (not sure which transistors where which as I hastly got them mixed up):

392
389
393
552

Seems something could be up with that last transistors reading... but I'm probably going to have to replace atleast one of them anyways as I managed to break a pin off of the transistors whose hfe measured 389. (Doh!)

The four transistors are low in hfe, new ones range about 646 hfe
Strange on the two fets reading the same, maybe both are shot or possible one had bad solder joint causing white screen problem

[PlainBill]

Re: Dell E153FPb - Screen stays white when off or in standbye

The four transistors are low in hfe, new ones range about 646 hfe
Strange on the two fets reading the same, maybe both are shot or possible one had bad solder joint causing white screen problem

Initially I thought the symptoms described would indicate the FETs were switching off, then back on. After further reflection, I'm not sure that the brief off period isn't anything more than a delay before the cells in the LCD panel gradually return to random alignment.

PlainBill

[aliasdck]

Re: Dell E153FPb - Screen stays white when off or in standbye

You're working at it from the wrong direction. The fact that the backlights are working indicates Q739, Q740, Q759 and Q760 are good. If the two mosfets were not shorted (no need to desolder them, by the way), you should have worked back toward the control input as eguevarae indicated.

It's a little more difficult without a schematic, but you should be able to trace the signal as it goes from the connector to the FETs with a DMM by probing and watching the voltage change as you hit the 'Power' button on the monitor. It's a bit of a pain, because you have to avoid touching anything with high voltage as you do this.

PlainBill

That makes a lot of sense now that I've spent some time looking at the circuit and how everything is connected - I am working in the wrong direction it seems. I'll put everything back together and see what I can find.

Thanks.

[aliasdck]

Re: Dell E153FPb - Screen stays white when off or in standbye

I managed to solder a new pin onto the transistor so that I could put everything back together for some live testing.

Now the problem is gone... so whatever was wrong, I managed to fix it by desoldering and resoldering the 4 transistors and 2 mosfets... perhaps it was a bad solder joint or a tiny blob of solder I didn't see.

Kinda fustrating as I wanted to be able to find and correct the problem.