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Contrast Problem In Monitor

By on August 31, 2007

integrated ic

If you ever come across contrast problem in CRT monitor, go straight to the video pre amplifier IC in the CRT board. Usually it is the biggest IC in the CRT board and some typical part numbers are LM1203N, LM1281N, LM1269NA, LM1236, LM1237N and etc. Get the specification from the internet and look for the pin that have the name “ABL” or “Contrast”. Trace from there and eventually you will reach the abl pin of the flyback. Replace the video pre amp IC or check all components in this ABL (automatic blanking limiter) line if you can’t control the contrast, no contrast or even dim display. In Acer CRT monitor, a corrupted eeprom data may also caused the display to become dim and the contrast can’t go maximum. Reprogram the eeprom and you will get back the contrast.

crt monitor

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6 Comments

  1. munsif from pakistan

    December 19, 2010 at 1:35 am

    hi,

    I am munsif ali from pakistan. i have a contrast problem oupled with other problems. your above information was indeed very helpful.

    I have vl 500, crt monitor , made in thailand, from chuntax electronics pvt ltd, for which i require guidance on proper repair.

    I am using my 650 mhz processor speed computer with a CRT monitor, Its screen got to the left side leaving a one inch black blank space in the left. I got that monitor like that. its menu buttons for a series of functions such as position, size, geometry, colour etc were not working , so i decided to open the plastic case, and removed the dust etc, After no of attempts to identify any one particular variable resisitor responsible for moving the screen to the left side , i found one , after rotating it with screen driver , the screen filled the space in the left side, but when I closed the case, suddenly the screen shrunk into the middle, creating about four inch blank space on both sides, When i gave some light jolts to circuit board, the screen suddenly became normal, but it would then shrink again into the middle , situation further deteriorated when I adjusted few more variable resisitors responsible for screen adjustment. the picture finally disappeared and then zig zag colourful lines started to appear,

    I took it to a repairman, and he asked me to come after two hours, I reached him after two hours, fortunately he repaired the all buttons, which started to show menu index, but cut those earth wires which were connected between picture tube glass and a metal on a circuit board. he also removed a metal shield of the electron gun circuitory board,

    However the picture has reappeared, but not that much crisp , further on increasing brightness, the blank black space turns into darker greenish with horizantal lines, where , there is no video input signal,

    sometimes while windows xp restarts sounds like kitik are also heard.

    further if i switch off the monitor with button, the screen vanishes with a white vertical line and with a dot of white light appearing right in the middle.

    now my question is under such a situation , which parts of the circuit board will you suggest me to replace,

    regards

    munsif ali, age 40,
    commerce graduate,
    923313524436
    d-36, navy housing scheme, karachi

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  2. admin

    December 23, 2010 at 11:08 am

    I am munsif ali from pakistan. i have a contrast problem oupled with other problems. your above information was indeed very helpful.

    ---------Thanks!

    I have vl 500, crt monitor , made in thailand, from chuntax electronics pvt ltd, for which i require guidance on proper repair.
    I am using my 650 mhz processor speed computer with a CRT monitor, Its screen got to the left side leaving a one inch black blank space in the left. I got that monitor like that. its menu buttons for a series of functions such as position, size, geometry, colour etc were not working , so i decided to open the plastic case, and removed the dust etc, After no of attempts to identify any one particular variable resisitor responsible for moving the screen to the left side , i found one , after rotating it with screen driver , the screen filled the space in the left side, but when I closed the case, suddenly the screen shrunk into the middle, creating about four inch blank space on both sides, When i gave some light jolts to circuit board, the screen suddenly became normal, but it would then shrink again into the middle , situation further deteriorated when I adjusted few more variable resisitors responsible for screen adjustment. the picture finally disappeared and then zig zag colourful lines started to appear,

    -----------I suspect that is a dry joints in the flyback area. Try look for any dry joints in the modulation diode or even the B+ fet circuit

    I took it to a repairman, and he asked me to come after two hours, I reached him after two hours, fortunately he repaired the all buttons, which started to show menu index, but cut those earth wires which were connected between picture tube glass and a metal on a circuit board. he also removed a metal shield of the electron gun circuitory board,

    ------Noted.

    However the picture has reappeared, but not that much crisp , further on increasing brightness, the blank black space turns into darker greenish with horizantal lines, where ,

    ------This could be a sign of weak tube.

    there is no video input signal, sometimes while windows xp restarts sounds like kitik are also heard.

    -------Sound could be come from a flyback please check on this link:
    http://www.electronicrepairguide.com/testing-flyback-transformer.html

    further if i switch off the monitor with button, the screen vanishes with a white vertical line and with a dot of white light appearing right in the middle.

    -------This could be a sign of weak tube.

    Jestine

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  3. Anbu

    July 23, 2012 at 7:38 am

    Hi
    My display frame is smaller than monitor screen what may be the problem
     

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    • Jestine Yong

      July 24, 2012 at 12:21 am

      Hi Anbu,

      It could be low B+ supply voltage or the modulation diode/pincushion transistor have problem.

      Jestine

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  4. mario

    July 10, 2015 at 8:05 am

    hi! i am mario fom pilippines. the problem of my panasonic tv model tc-29959 is the contrast is totaly not working.if you go to the setting, all are working except contrast. is this a problem of eeprom? or defective parts and which part of board i can find this. thank you very much!

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    • Jestine Yong

      July 11, 2015 at 9:30 am

      HI Mario,

      It can be due to eeprom ic already corrupted or the contrast circuit.

      Jestine

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