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LCD flat panel monitor, faulty backlight

Hi,

This is a Vison F153 flat panel monitor (1024x768?)

When power is first applied (with a valid signal source), the screen "flashes" momentarily (i.e. to *normal* intensity) and then goes dark. During this flash, it is obvious that the correct image is displayed on the screen.

After the screen goes dark, it is still aparent that the display is accurately displaying the signal provided (e.g. you can see the ghost of the mouse cursor moving around in response to mouse actions; you can double click on icons and see those applications open, etc) -- but obviously the backlight is no longer on (at all!)

There are two flourescent tubes in the display (?). Both are behaving identically.

Each tube is excited from it's own inverter (i.e. the inverter board appears to have two identical circuits on it).

So, it seems logical to assume that the supply to the inverters is a likely suspect. Or, some control signal shared by both inverters (?).

Any suggestions as to where to start? I'd prefer *not* probing the output of the inverters (for obvious reasons) and suspect that would reveal very little that I can't already surmise...

Thanks!

[email address is bogus]
Reply to
Don
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Don: You already "started" in the correct place.... the inverter board and the power supply board. I would check any of the larger electrolytics on the inverter board first... if you do not have an ESR meter to properly and conclusively check them just replace them. If that doesn't solve the problem then go back to the power supply board and do the same.

-- Best Regards, Daniel Sofie Electronics Supply & Repair

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displayed

appears

likely

Reply to
sofie

Double check that all the tubes are lighting. Even though it appears to be normal brightness, one of the back light inverters might not be working (there may be two tubes at each end). Suspect any electrolytic caps on the board since they tend to run hot. Andy Cuffe

snipped-for-privacy@psu.edu

Reply to
Andy Cuffe

Hi, this symptom of lighting up for a second then poof is sign of bad lamp inverter shutting itself down due to shorted winding inside a HV transformer. Usually two or four of them depending on number of lamps used. You can see a badly degraded waveform on bad transformer (sine wave cut down to squiggles.) if the probe's body is laid on the transformer's with scope set to .1mV/div and 1usec sweep.

This trick is useful to confirm you're getting good operation from most high frequency transformers, switching power supplies, switching regulators, including flyback transformers by judging from quality of waveforms.

This what I had to replace that lamp inverter and is all well now. Have to gonna be exact replacement. If you know same models that use same design of this inverter that will help. Picture of guts (board, inverter board) etc is helpful.

Cheers, Wizard

Reply to
Jason D.

Not strictly true:

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It needs to match the lamps, certainly.

Alex

Reply to
Alex Bird

This is exactly the same symptom I have with a noname flat monitor. I took the inverter out and tested it from a known good 12V source while having it connected to the lamps - same result. Interestingly, the inverter has another (control?) cable; if I inject up to two volts into this cable the lamps stay on but flicker terribly while the inverter makes a buzzing noise. If I go up to about 3V the buzz vanishes and brightness increases, though the flicker remains. From about 4-5V onwards the inverter shuts down again.

As someone suggested previously, it might well be a broken output transformer - but as there are two identical circuits on the inverter board, why is the other one shutting down as well?

I don't see chances for successful repair here. Is there anybody who can point me to a supplier of inverters (Europe preferred)? I guess the types are quite interchangeable but if anyone can give me a hint on how to find an exact replacement, help would be greatly appreciated.

Kind regards, wishing successful repairs,

Leo

D> Hi,

Reply to
Leo Meyer

displayed

appears

likely

As suspected, the problem was in the power supply (since it would be highly improbable for *both* bulbs to fail in the same manner

*or* both inverters to fail, etc.). An electrolytic in the 12V supply to the inverter was lossy and resulting in poor regulation of that supply.

Didn't have a 470uF/25V/105C cap on hand but a 330/25/105 cap proved my point -- I'll just have to remember to order some and replace that (along with the few others onboard... a cheap prophylactic).

Thanks!

[email address is bogus]
Reply to
Don

Alex, that's GOOD.

The PROBLEMS:

- I'm NOT sure if the guy have the safety knowledge to work with these stuff.

- Not all are same. Some inverters needs digital control, others used analog to set the brightness of lamps.

- Some needs the bias voltage generated and fed back to the LCD panel, rare.

- Need to measure all the pins and identify grounds, the power supplies, there are at least two voltages, one for lamp power, one for other circuits on inverter itself. Measure for lamp brightness (analog types will be voltage levels that rise or lowers as brightness is adjusted) except digital controlled will be train of pulses. There will be one signal for turning on or off lamps much like enabling or disabling a eeprom.

This is just the surface to scratch, not just length of lamps.

Cheers, Wizard

Reply to
Jason D.

Excellent job!

But this is unusual. Sounds like this LCD using low quality caps. Most LCD panels tend to use good stuff due to space limitations.

If you don't have a cap for this value either voltage or uF, match either and upsize the other values or both one level or more for filter stuff.

Cheers, Wizard

Reply to
Jason D.

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