Help!! My monitor's display fogged out....

[geek_kjay]

Hello techies,
Recently, my monitor (Samsung Syncmaster 591s) has got a problem with the display. The screen appears to be fogged out. I mean, it is too much brighter than earlier and screen appears to be washed out!
But the brightness n contrast settings are same as earlier. I did not change anything.
Brightnes: 50
Contrast: 90
which I had configured using Everest Ultimate editon benchmarking tool.
Also I have opened my monitor (I am an Electronics Engg BTW ) to find any loose connections and found none.
So pls help me to fix this prob. I hope u techies @digit forum will guide me through.
I'll be anxiously waiting for your reply......

I have zeroed my monitor's brightness n contrast. But instead of being blank, it displays grey colour

[geek_kjay]

Its workin okay regarding color rendition n other aspects. But its jus tat the monitor's display appears to be fogged out due to excessive brightness
Pls tell me how to troubleshoot this prob..............

[Gollum]

increase the contrast. and reduce the moir to zero.

[Faun]

It's a loosened soldering problem.

[geek_kjay]

I'll tell you the problem in detail...........

My monitor had a problem of flickering which goes out when I hit the monitor from back side
So I have opened my monitor by myself to fix this problem. Since my monitor was so dusty inside, I used a vaccum cleaner to clean the components. Later I have fixed this problem of flickering by fixing the back block of Cathode Ray Tube (CRT) firmly. Phew...

Now this flickering prob is corrected but my monitor appears to be foggy due to excessive brightness. I have lost control over brightness. Even I set Brightness n contrast to zero, my monitor displays a grey color. So I have increased contrast to 100 n brightness to 0 to compensate the excess brightness (Foggy display).


@Faun You are sayin tat this might be due to soldering problem on monitor's motherboard. But where?? In which section??? Dude I wanna correct this prob by myself. I hope you ppl can guide me

[pimpom]

Do you also see horizontal retrace lines on the screen?

[Faun]

Originally Posted by geek_kjay

@Faun You are sayin tat this might be due to soldering problem on monitor's motherboard. But where?? In which section??? Dude I wanna correct this prob by myself. I hope you ppl can guide me

It should be related to the PCB on the cathode ray tube.



http://www.thenetguruz.com/electroni...ir-diy-part-2/

My CRT had the same problem a couple of years ago. Called the LG customer care, the technician soldered the withered joints. It was normal again.

[geek_kjay]

Many thanks Faun
I'll try it n post my results.....

@pimpom Buddy, I see no horizontal retrace lines......

[geek_kjay]

Yahoo........

I got it. I have re-soldered some dry joints on the PCB connected to back of CRT to get flicker free display. And adjusted anode voltage knob on EHT to correct my monitor's brightness problem.
@Faun Ur links really helped me a lot. Thanks buddy

Thank you guys for your support

[pimpom]

Originally Posted by geek_kjay

Yahoo........

I got it. I have re-soldered some dry joints on the PCB connected to back of CRT to get flicker free display. And adjusted anode voltage knob on EHT to correct my monitor's brightness problem.
@Faun Ur links really helped me a lot. Thanks buddy

Thank you guys for your support

It's good to know that your problem is solved. But since you're an electronics engineer and not simply an end user, I think you should understand the problem and the solution more clearly.

From your description, it is most likely that the over-bright, washed-out display was NOT caused by the solder issue. That was most probably caused by you when you fiddled with the insides. You did not mention changing the EHT adjustments before, but I suspected it. That's why I asked you if you noticed retrace lines, but I didn't have a chance to follow up because my internet connection went down for some time.

What you have just adjusted was not the anode voltage, but either the grid No.2 voltage or the beam focus potential, or both. Probably both. Those two should not need any adjustments (except minor ones to compensate for tube aging) if you did not touch them in the first place.

The soldering was needed to cure the flickering, but not the "fogged out" display.

[geek_kjay]

@pimpom Ya you are absolutely correct. I have changed the accelerating grid (G2) voltage by accidentally rotating the G2 knob on EHT while cleaning my monitor insides That might have triggered the brightness issue...

Actually I opened my monitor to correct the flicker issue which was already present and irritated me a lot for past two weeks. Now with digit forum support, I got confidence that I can do it by myself and resoldered some dry joints on the PCB connected to back of CRT. This corrected flicker issue

But the brightness issue was still present back then and corrected it by after I noticed two knobs on EHT. One was labeled 'FOCUS' and other 'Screw'. I thought 'Screw' knob might be anode voltage (Now came to know that it is G2 voltage) and adjusted it. I have noticed changes in brightness levels on the display unit. Viola, my probs solved Digit forum saved me Rs.700 service charges as said by a technician nearby.

Doubt:
G2 voltage changes brightness. But what is the role of anode voltage??? Does it not change the brightness??? Pls clarify in detail.....

[pimpom]

The anode voltage is the highest in the CRT. It goes straight from the EHT transformer (via built-in rectifiers) to the tube along the thick wire attached to the tube with a suction cup. Its purpose is to accelerate electrons from the cathode to hit the fluorescent screen with high energy. It has a potential of 25-30 kilovolts!

The anode voltage does affect the brightness, but it's not practicable to make it easily adjustable. Besides, it also affects other factors such as deflection, convergence, etc.

Therefore, the anode voltage is fixed and is not controlled by the adjustment screws on the EHT transformer. OTOH, the number of electrons hitting the screen is controlled by various grids. One of these is the G2 voltage which is at around several hundred volts. Its most obvious effect is on the overall brightness. The focus electrode is at around 10 kV.

The brightness of the Red, Green and Blue pixels are separately controlled by the voltages at their respective grids (grid No.1 or G1) with respect to the corresponding cathodes. The voltage at these grids vary over a range of about 50-200V according to the display data coming from the CPU.

That was a simplified qualitative description. A detailed quantitative one would be outside the scope of a forum discussion. It will involve advanced physics and mathematical analysis of electron ballistics, electromagnetic fields, tube geometry, etc.

I hope this has helped you to gain a better concept of how CRTs work.

[Tech&ME]

^^ ya nice explanation there buddy @pimpom.

It is true that the subject is very vast and to understand CRTs a detailed study is required. But still you have covered the MAIN points very clearly dude.

[Faun]

This thread is quite informative
@pimpom thanks

Now I am waiting for my CRT to develop some problems and give me a chance to troubleshoot it myself.

[geek_kjay]

@pimpom Thank u for your explanation

[pimpom]

I've just noticed an error in my explanation. G1 is negative with respect to the cathode. The sentence above should be "The voltage at these grids vary over a range of about minus 50-200V ........"

It's important to remember that this is relative to the cathode. Depending on the design, the actual G1 voltage referenced to common circuit ground (earth) may not always be negative. For example, G1 may be at 0V and the cathode at a positive voltage. This makes the grid negative with respect to the cathode.