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From your detailed description, it sounds that the main chroma processor IC has damaged. Raster means nothing, but the lighted screen that appears, when a TV is switched ON. For some brands, there will be a blue screen; when there is no signal input, and some others will have a bright white screen with some snow.
I advice you to contact any service technician, to get this TV repaired. If you wish to get some details; check the site linked here. Viewing it in "Mosaic" or "Magazine" will make surf easy. Pull up older posts. Surf the site with patience. Do a search by type in the brand name of your device at its search box and press "Enter". http://electronicshelponline.blogspot.com/
First make sure that the brightness and contrast are turned up.If the screen is lit up (that is you have a raster) then the problem would be in the video circuits.If the screen is not lit then you have a problem in the deflection circuits.A good possibility,with no raster,would be the Horizontal output amplifier.If you are not familiar with circuitry you would need a tv tech.to do the repair.
what kind of display have, totally black or only a raster?. if you are watching a local channel then only a raster with sound, it could be a tuner problem. now if totally black screen you need to check if the flyback transformer has high voltage coming out at the anode cap. and then the tube has heater light. this case can be hand held with a trained technician if you don'y have an idea on how to deal with it just leave it to the tv technician for safety reason. however at least you have an idea what to expect if you decide to hire for somebody to fix.
This should be a quick fix. Try resoldering the vertical IC. This is a small flat IC attached to a heat sink with about 11 pins. It should be marked LA7837 or something similar. That should resolve the problem.
You have an intermittant vertical sweep circuit! It could be the vertical out put IC solder connections on the printed circuit board, or maybe the IC itself has poor connections between the IC and the contact legs that mount the IC, or another componet in the vertical sweep circuit. If you know how to solder and unsolder, work on printed circuit boards, then you shoul have no problem replacing the vertical sweep circuit. Good Luck
do you get sound while screen does black? what do you mean by "until eventually it has turned black and will not come back"? is it black from the minute you turn the tv on untill you turn it off? if sound is good but picture is black, it may be the resault of a fried monitor tube, in this case replacement will cost alot.
if sound is dead also, them it is probobly power supply related issue. get out your DMM and measure the voltages in the power supply. When the screen goes black, is there still regulated B+ going to the circuitry? Do you still have HV? Is the filament of the CRT lit? Does the set still have a raster (you may have temporarily increase the screen setting to see it)? Do you measure any of the flyback derived B+ voltages that go to the video processing, tuner, Vertical Deflection, and other circuitry?