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Old 12-17-2011, 12:42 AM   #19
retiredcaps
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Join Date: Apr 2010
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Posts: 9,078
Default Re: A guide on how to troubleshoot 2 seconds to black

REVISION 0.3 - Dec 16, 2011

More revisions are sure to come. Make sure to read the latest posts for corrections or input from others.

I wrote this to help others by sharing what I have learned. This information is nothing new, but it is scattered all over different threads.

This is NOT a definitive guide as others have far greater knowledge and experience.

A guide on how to troubleshoot 2 seconds to black

Description of problem

You turn on your LCD monitor and it displays a computer image on your screen and then suddenly it turns off after 1 or 2 seconds. The power LED stays on (or green) and you can/may still see the computer image by shining a flashlight on it. This is called "2 seconds to black".

Where the problem lies

Since you are seeing the image, but with no backlight, we will assume for now that the power and logic board are working properly.

Tools needed to troubleshoot

You will need the following tools to troubleshoot:

a) multimeter
b) camera to take clear focused pictures
c) solder iron to replace components

Personality Traits required

a) ability to follow directions
b) troubleshooting skills
c) determination
d) willingness to learn
e) patience
f) ability to articulate your problem clearly

If you don't have at least one of the traits above, sell or give away your monitor to someone else. Seriously. Don't dump in the garbage. Recycle it properly please.

SAFETY

Most of the tests below require a multimeter, but all the procedures that I write about are with power off and LCD unplugged. The CCFL tests do not require a multimeter, but there is VERY HIGH VOLTAGE (600V to 1000V).

I take no responsibility for any of the tests below that may cause you or your monitor any harm. If you are not comfortable, do not try it. Ask someone knowledgeable for help.

Multimeter

If you have a manual range multimeter, set it to 200 (two hundred) ohms. Touch the black and red probe together. It should read between 0.1 or 0.5 ohms. If it is higher than 1.0 ohm, there is something likely wrong with your multimeter. Either the test leads are frayed internally and/or the battery is dying which will result inaccurate readings.

If your multimeter reads "1" or "0L", it means the measurement is outside your chosen range. Don't confuse "1" on the left hand side of this display with 1.0 on the right hand side. The first means out of range and the second means 1.0 unit of your measurement.

Do NOT use the continuity or "beep" feature of your multimeter for measurements. Some multimeters "beep continuous" for resistance readings less than 1.5k ohms.

Always post the actual results of your measurements when asking for help.

Possible causes

Here are some of the possible causes of "2 seconds to black". Your LCD monitor may have one or all the following. I suggest reading all the possible causes and then applying the ones that make the most sense to your situation.

1) Bad Capacitors

Capacitors (C designation on the PCB board) die from age, heat, and shoddy build quality. Capacitors DO NOT have to be visibly bloated in order to bad. They can be out of tolerance uF (a 1000uF measures 20uF) and/or have high ESR (ohm). A multimeter will be insufficient to test for ESR. For that you need an ESR tester which costs between $50 and $300.

Bad capacitors will not provide stable reliable power which may cause "2 seconds to black".

Most members here will recommend that you replace ALL capacitors with reputable brands from reputable sellers. Brands like Rubycon, Panasonic, Nichicon and United Chemicon are suggested. A list of recommended caps can be found at

http://www.badcaps.net/forum/showthread.php?t=2280

The one exception might be the largest capacitor on the power board which rarely, but not never, fails.

Here is an example of how to choose capacitors with respect to capacitance, voltage, diameter, height, ripple and ESR.

http://www.badcaps.net/forum/showpos...4&postcount=19

2) Shorted Transistors/MOSFETs

On some LCD brands (example: Benq) the transistors (C5707) are sometimes shorted. Transistors are usually marked with a "Q" designation.

If you have a manual range multimeter, set it to 200 ohms (two hundred). You can test for shorted transistors "in circuit" (with power off and unplugged)

a) put black probe on pin 1 and red probe on pin 2 - read/record ohm
b) put black probe on pin 1 and red probe on pin 3 - read/record ohm
c) put black probe on pin 2 and red probe on pin 3 - read/record ohm

If any reading is less than 30 ohms you might have shorted transistor. Remove the transistor and repeat the tests out of circuit to verify.

Note: a shorted transistor would likely cause a very brief flash of the backlight or no backlight.

Mosfets

If you have a manual range multimeter, set it to 200 ohms (two hundred). This is for 3 pin ICs only. You can test a mosfet (Q, IC designation) "in circuit" by (power off and unplugged)

a) black on pin 1- red on pin 2 - record ohms
b) black on pin 1- red on pin 3 - record ohms
c) black on pin 2- red on pin 3 - record ohms

If any reading is less than 30 ohms you might have shorted mosfet. Remove from circuit and repeat the tests to verify.

Some mosfets are more than 3 pins. To test those, identify the part number and search for its datasheet. Once you find the datasheet, the pins will be designated source (S), gate (G), and drain (D). It will probably be documented as S1, S2, G1, G2, D1, D2.

Simply test

a) black on pin S1- red on pin G1 - record ohms
b) black on pin S1- red on pin D1 - record ohms
c) black on pin G1- red on pin D1 - record ohms

Repeat for the "2" pins. That is S2-G2, S2-D2, G2-D2.

Note: a shorted mosfet would likely cause a very brief flash of the backlight or no backlight.

Here is a case study.

http://www.badcaps.net/forum/showthread.php?t=17659

3) Open fuse

Some boards will have a fuse (F designation) or picofuse (PF designation) and they may be open because a transistor was shorted or some other component went bad.

If you have a manual range multimeter, set it to 200 ohms (two hundred). You can test for open fuses "in circuit" by (with power off and unplugged)

a) put black probe on one pin and red probe on other pin - read/record ohm

A reading of less than 1.0 ohms indicates a good fuse. Anything higher than 1.0 ohm is a sign of an open fuse. Do NOT replace fuses and turn on the power back on without checking for what caused the fuse to go open otherwise you will just be replacing the fuse again.

Note: In older monitors, some inverter boards have 2 fuses. One fuse could be bad and one could be good which results in 2 seconds to black. In newer LCD monitors, there is usually only one fuse protecting the inverter section. If that fuse is bad, you would get NO backlight flash (i.e. no 2 seconds to black).

Here is a case study.

http://badcaps.net/forum/showthread.php?p=197027
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Last edited by retiredcaps; 12-17-2011 at 12:54 AM..
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