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too bright CRT tv picture

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yaaku

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i have a be problem with my samsung 29inch CRT tv, the video brightness goes up and down intermittently. holds in one state too much bright(low contrast, excess white light) for some time 20mins or more .. then similarly it goes to normal for some time and then becomes too dark (high contrast). brightness flickers up and down.

I set the screen brightness to zero from my remote but still video is too bright white. brightness goes down to normal sometime.

can someone suggest which part of the chassis i should test for faulty component.

background: I had recently changed the HOT (2 transistor). I changed the snubbing caps after which HOT stopped blowing.
 

-Check CRT socket..
-Check DST/EHT Transformer ( Screen Adj.)
 

crt socket looks good, swapped few components but no luck. changing screen adjustment has the usual effect but fluctuation is still there
 

Something near to the HOT generates the high voltage for the first CRT anodes, if the voltage rises it makes the picture too bright. I suspect a resistor is going open circuit or there is a bad joint in the vicinity. We need to see a schematic, or at least know the model number to identify the part location.

Brian.
 

crt.JPG my chassis resembles closely to samsung KS3A and CRT base board is from KS2A whth 5pin connector to chassis (high voltage pins heater, etc) and 6pin connector to (chrome IC)
 

My best guess would be something between the point marked 'SCREEN' and the CRT itself -or- a fault around 'B+200V'. If the picture size stays fairly constant it is unlikely to be a fault to the right side of D411. Unfortunately, we can't see what B+200V feeds but it is probably the video output transistors. Depending on the output stage configuration, it is possible a reduction in the 200V could make the CRT conduct higher current and increase the brightness, probably causing 'crushing' of the contrast (white out) as it does so.

Brian.
 

B+ 200 feeds to the RGB amplifier STV5109 it reads 190v few days back it was giving 215v when the green color was missing intermittently. even after disconnecting the base board from CRT it was giving same voltage. can i remove the VDD connection to measure the voltage to rule out any fault in the amplifier chip ?

- - - Updated - - -

is it okay if the secondary side of the smps shows slight different voltages than those marked in the schematic. like 130 showing 132, 16.5 showing 18. 12 showing 11.3 when standby.

12 showing 13.3, 130 as 132.1, 16.5 as 17.7 when turned on.
 

You can remove the voltage to isolate the loading effect of the amplifier but do it cautiously. You might find the CRT runs at full current (max brightness) and overloads the EHT generator circuits. I would expect the voltage to rise with no load but it should be stable. Certainly, change of 215 to 190V would account for brightness variation.

Brian.
 

Hello yaaku,
Please refer to the link below:

Samsung

Could you please check the spark-gaps as shown with the RED arrows?
What you may have to do is grab an old blanket and cover yourself and the rear of the telly and
just watch those spark-gaps to see if they discharge or not.

The components marked with the GREEN boxes, could you please lift one leg on each and measure their
resistance values.

The capacitors shown with the BLUE boxes, could you please inspect them very closely. Preferably with
a good light and magnifying glass. You need to look for any cracks on their surfaces.

Regards,
Relayer
 

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