Question about Sony SDM-HS74P/S 17" LCD Monitor

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Same issue. 5VDC & 15VDC OK even when CCFL's go off. New 1,000uf/16VDC cap. Just prior to going out screen starts to flicker slightly. Can't see any bad solder joints on the power board. Any hints?

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The 1000uf/16vdc caps are for the 5vdc logic, there should be at least tow caps rated at 25vdc for the 15vdc inverter circuits. Try using 1157 12v car lamp attache to the 15vdc, the power switch will have to be off (the 5vdc and 15vdc are always on) so the inverter circuits will not be drawing the current from the 15vdc power supply, the 15vdc should be at 15vdc +/-1~2v with the lamp load. If it is steady, then you still have problem in the inverter circuits which can be the lamp assemblies, inverter transformers, Transistors/IC. I use test lamps set up to narrow down the problem to find out if it ia the lamps or the inverter circuits.
Failed TV and Monitors: http://s807.photobucket.com/home/budm/allalbums

Service manual: http://elektrotanya.com/?q=showresult&megnev=Sony%20&megnev2=SDM-HS74P&kategoria=&kat2=all

Posted on Sep 10, 2010

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  • Bud Martin
    Bud Martin Sep 10, 2010

    I looked at the service manual (it does show the sch for the power/inverter circuits), it looks like the DC voltage for thr inverter circuits is 12vdc, so the 1000uf 16vdc may be for the 12vdc power supply.

  • 2fornow Sep 17, 2010

    Yes it is and the 12VDC is good and solid. Tried to get the manual from your link, but could not. Could I use a resistor as a dummy load for the CCFL's to see if inverters are OK? I don't know if the lamps are wired in series or parallel to each inverter output.

  • Bud Martin
    Bud Martin Sep 17, 2010

    The lamps are wired in parallel, two lamps per set.
    Typical Sch (75k resistor will have to be at least 10watts) for testing the inverter:http://s807.photobucket.com/albums/yy352... testing/. I use spare lamp for testing. You may have bad lamps also. http://s807.photobucket.com/albums/yy352... backlight lamp/ http://s807.photobucket.com/albums/yy352... lamp tester/

  • 2fornow Sep 18, 2010

    Thanks. I'll try testing the inverter outputs with the resistive loads and see if the inverters don't shut down. If OK I'll get replacement CCFL's. I did finally get the service manual copy today from the link you included.

  • Bud Martin
    Bud Martin Sep 18, 2010

    Inverter tester: http://www.lcdparts.net/IT01.aspx

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Samsung 2233RZ going black after 1-2sec


First if you can, try the monitor with other PC first to make sure it is not the PC problem.

If it behaves in the same fashion, then most probably has dying power supply due to bad DC filter caps, just see the top side of the caps to ensure it has not bulged up distorted.See the images bellow
12_24_2012_5_33_15_pm.jpg OR


12_24_2012_5_34_01_pm.jpg

Then there is the logic board. Plug the monitor in but do not activate the power switch so the backlight inverter circuits will be off. Check the 5vdc and the 12~24vdc to make sure they are OK. They should be tested with the load, you can use 6V 1A (6watts) lamp for the 5vdc, and car lamps such as 1157 (12v 8watts lo/26watts high) turn signal brake lamp using high filament connection for testing the 12~18vdc (or use two 1157 in series for 19~24vdc) for the backlight inverter circuits.
If the power supplies are working, the output voltages should be steady at the rated voltages. The power supply will go into shut down if it detects too much current draw due to false in the power supply or short circuits in the backlight inverter or in the logic board.

The backlight inverter circuits:
It takes the 12~24vdc and converts it to high frequency AC to drive the inverter transformers CCFL (Cold Cathode Fluorescent Lamp) assemblies. The transformers will drive the CCFL by applying the start up voltage (around 1500~2000v), when the CCFL start conducting, the voltage will drop down to about 500~800v.
The inverter has detection circuits to detect open circuit if the lamp is not attached or does not fire up after the start up voltage is applied, it will go into shut down. It will also shut down if the lamps draw too much current due to ages (when lamp gets old it will draw more current).
The inverter gets two signals from the logic board, one is the backlight ON/OFF signal, the other one is the Dimming signal for the lamps.
Common problems:
Bad filter caps, resonant caps (in the inverter output circuits), blown transistors/IC, shorted or open transformer winding.

If you are confident with your skill then go ahead else go to an authorized outlet for repairing
Do write back if you face any difficulty while working on the monitor

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1 Answer

Hi, I have a big problem with my lg L194WT,when I press power on button nothing happens. I open'd up the display case nothing looks wrong except a bad cap 1000 micro F/16V I replaced that. DC...


For 19" monitor, I would expect more tahn 12vdc for the inverter circuits, some runs as high as 19vdc, but since you are getting 5vdc OK, more likely may be OK but should be check out, that 1000uf 16vdc may be used for the 5vdc power supply section, the inverter power supply section will usually uses 25vdc caps.
Do you see the backlight lamps flash on for second at all? You may need test lamp to verify if the inverter circuits are OK or not, since bad lamp can cause the inverter to go into shutdown also.
There are also BL_ON (about 3.3vdc = ON) signal and PWM_DIM signal feeding the the inverter circuits.
I would attach a12v car lamp such as 1157 to the inverter 12vdc power supply section to see if it can maintain 12vdc with load (do not turn on the power switch during this test since the power supply is always on even the power switch is not activated).

Service manuals: http://elektrotanya.com/?q=showresult&megnev=LG&megnev2=L194&kategoria=&kat2=all

Basic LCD monitor and TV troubleshooting guide:
http://www.fixya.com/support/r6150077-basic_lcd_monitors_troubleshooting
http://www.fixya.com/support/r5093881-lcd_flat_panel_tv_troubleshooting_guide
http://www.fixya.com/support/r7406380-tv_training_manuals
Failed TV and Monitors pictures: http://s807.photobucket.com/albums/yy352/budm/

Learn about bad caps: http://en.wikipedia.org/wiki/Capacitor_plague

Capacitors kit: http://lcdalternatives.auctivacommerce.com/ he can make you a set of caps for you.
Or www.digikey.com just make sure to use caps with low ESR, 105c, high ripple current, long life rating such as PANASONIC FM or FC series.

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My Princeton 19 inch LCD monitor keeps turning off and I have to un-plug it, wait, and plug it back into the wall socket for it to go back on. I replaced the multiple box and the power cord and am using...


bsims77010, Tell me brand name of electrolytic filter capacitors on the large "switch-mode-power-supply / CCFL inverter supply" printed circuit board. Are any of them SWOLLEN on the end that is up ??? You can see typical "failed units if you visit YOUTUBE for video clips that show owners/tech's that replace them with new ones and now have working units again. You must tell me more about this "keeps turning off" thing???? Is unit going to "black screen" only?? Or is it going to "black n blank" screen"??? There is a difference!!! If black only then 5vdc might be ok! If "black n blank then +12vdc & +5vdc sources could be in shut down mode. Do you see screen data with aid of bright flashlight held 1 inch from the "black" screen?? Once you have knowledge of what makes "raw white light" on reverse side of your lcd panel, then you will have clue to how the new screw-in, curlie-Q style CFL light bulbs work and what an "electronic ballast" circuit is. 12fixlouie
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The standby 5vdc power section is working but the rest of the power sections (5vdc, 12~16vdc, and 24vdc) may not be working.
That will be the first place to look, look for caps with leaking/bulging tops.
Parts: www.shopjimmy.com
http://s807.photobucket.com/home/budm/allalbums
http://en.wikipedia.org/wiki/Capacitor_plague
http://www.badcaps.net

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Check out LCD Parts for service, parts, and DIY info Use this link to their WEB site :-
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1 Answer

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You could have a defective screen - a defective inverter or a CCFL lamp.
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