A guide on how to troubleshoot 2 seconds to black

Re: A guide on how to troubleshoot 2 seconds to black

Originally posted by PlainBill

A fault in the sense circuitry is much more involved.

Agreed. I don't have the experience or knowledge yet in that area. I am following all those sense circuitry threads, but can't contribute anything yet.

As I commented in another thread, it is very rare that an open fuse or a shorted transistor would result in a 'two seconds to black' issue. With an open fuse you wouldn't get any backlights; with a shorted transistor at best you might get a fraction of a second flash.

PlainBill

For those wondering what thread, here is why I included the fuse part.

Originally posted by retiredcaps

I totally agree about an open fuse would cause zero second to black.

The only reason I mention is that I have a "in repair process" NEC 1700V monitor and I read this part from

http://www.electronicspoint.com/nec...tor-t75165.html

">>I checked those two fuses on the inverter
>>board, AOK. I don't think the screen
>>would even light at all
>> if those fuses were bad.

> Yes, it will if only one of them is bad. I've
> fixed several of these with that fault."

I can make revision 0.2 once I get more feedback and most importantly corrections.

REVISION 0.1 - July 23, 2010

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 over the last 4 months. 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 an image of your screen and then suddenly it turns off after 1 or 2 seconds. The power LED stays on (or green) and you can still see the 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 board and logic board are working 100%. That leaves the inverter board to check.

Tools needed to troubleshoot
==================

You will need the following tools to troubleshoot:

a) a multimeter
b) camera to take clear focused pictures
c) solder iron to replace components with optional desolder wick or pump

Personality Traits required
================

a) ability to follow directions
b) some troubleshooting skills
c) determination
d) willingness to learn
e) patience

If you don't have at least one of the traits above, sell or give away your monitor to someone else. Seriously.

SAFETY
=====

Most of the tests below require a multimeter, but all the ones that I write about are with power off and unplugged. The CCFL test does 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 knowledgable for help.

Possible causes
=========

Here are some of the possible causes of "2 seconds to black" in no particular order. Your LCD monitor may have one or all the following.

1) Bad Capacitors
--------------------

Capacitors die from age, heat, and shoddy build quality. Capacitors DO NOT have to be visibly bad in order to bad. They can be out
of tolerance uF (a 1000uF outputs 400uF) and high ESR (ohm). A multimeter will be insufficient to test for ESR. You need an ESR tester for test for ESR.

Most members here will recommend that you replace ALL capacitors with reputable brands from reputable sellers. Brands like Rubycon, Panasonic, and United Chemicon are suggested.

2) Shorted Transistors
-------------------------

On some brands (example: Benq) the transistors (C5707) are sometimes shorted. You can test for shorted transistors (with power off and unplugged)

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

If any ohm reading is less than 20 ohm you have a shorted transistor and it will need to be replaced.

3) Open fuse
--------------

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

You can test for open fuses by (with power off and unplugged)

a) put black probe (com) on one pin and red probe on other pin - read ohm

If the reading is 0.3 or 0.4 ohm you have a good fuse. Anything higher 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.

4) Bad transformer
----------------------

The transformer converts the low voltage into high voltage to drive the ccfls. These can be bad. You can test for open windings on the transformer by (with power off and unplugged)

a) Number the pins on the transformers anyway you like
b) black probe on pin 1, red on pin 2 - record ohm
c) black probe on pin 1, red on pin 3 - record ohm
d) black probe on pin 1, red on pin 4 - record ohm
e) etc
f) black probe on pin 1, red on pin last - record ohm
g) black probe on pin 2, red on pin 3 - record ohm
h) black probe on pin 2, red on pin 4 - record ohm
i) etc
j) black probe on pin 2, red on pin last - record ohm
k) black probe on pin 3, red on pin 4 - record ohm
l) etc
m) finally, black probe on pin last-1, red on pin last - record ohm

I'm guessing the secondaries should read 950 ohm (this will vary for different transformers). The primaries should read 1.3 ohm. Some will be 0L. If one secondary is 950ohm and the other is 750ohm, that would indicate a problem.

This test is not exhaustive as it does not detect all faults. For that, you will need a ring tester to test the coils.

5) Bad CCFL
--------------

Pay ATTENTION as there is a VERY HIGH VOLTAGE (600V to 1000V)! No multimeter is required for this test.

The CCFLs light up the panel and they can be bad due to age, bad solder, bad connection or a variety of reasons. The easiest way to test for bad CCFLs is to have a least one other CCFL.

Let's assume that you have a LCD with 4 CCFLs (numbered 1 to 4) and a spare.

Pay ATTENTION as there is a VERY HIGH VOLTAGE (600V to 1000V)! As a safety precaution, it might help to have someone around when you are doing this if something screws up.

a) TURN POWER OFF and unplug the monitor and wait 1 minute
b) disconnect CCFL #1 and plug in spare CCFL into spot #1
c) plug in monitor and turn monitor on - note if "2 seconds to black" occurs
d) TURN POWER OFF and unplug the monitor and wait 1 minute
e) reconnect CCFL #1 and disconnect CCFL #2 and plug in spare CCFL into spot #2
f) plug in monitor and turn monitor on - note if "2 seconds to black" occurs
g) TURN POWER OFF and unplug the monitor and wait 1 minute
h) reconnect CCFL #2 and disconnect CCFL #3 and plug in spare CCFL into spot #3
i) plug in monitor and turn monitor on - note if "2 seconds to black" occurs
j) TURN POWER OFF and unplug the monitor and wait 1 minute
k) reconnect CCFL #3 and disconnect CCFL #4 and plug in spare CCFL into spot #4
l) plug in monitor and turn monitor on - note if "2 seconds to black" occurs

If you have a bad CCFL, one of the tests above should show you which one. If you still have "2 seconds to black", then we can assume it is not due to a bad CCFL.

6) Bad Diode
---------------

Diodes can be bad. To test a diode,

a) put your multimeter in diode test mode
b) put your black lead on one side and the red lead on the other - record reading
c) reverse the leads - recording reading

A good diode should read 0.4 to 0.7 in one direction and 0L in the other. If you get a reading in both directions, you will have to desolder one leg and retest as an "in circuit" test may not be conclusive.

7) Bad/cold/poor solder joints
----------------------------------

Inspect the backside of the board carefully with a microscope or magnifying glass and look for bad solder joints.

8) other bad components
-----------------------------

Voltage regulators, mosfets, other ICs, resistors can also be bad and need to measured to verify proper operation.

This section might get flushed out better as I run into more of these issues.

Misc suggestions
==========

1) Post clear focused pictures AFTER reading

https://www.badcaps.net/forum/showthread.php?t=1868

Take your boards to a window on a sunny day and use macro mode. Take a top down view of all your boards (front and back).

2) Use clear cohert language with grammar and punctuation. If you can't articulate your problem, no one can or will help you.

3) Make an effort and do some research before posting.

4) If you are new, please read the FAQs before posting. You are more likely to get help if you do.

5) Post MORE information that little. A post that says "MY MONITOR DOES NOT WORK. HLEP (sic) ME" is likely to be ignored. The more information you give, the better.

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A guide on how to troubleshoot 2 seconds to black

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A guide on how to troubleshoot 2 seconds to black