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The picture tube is right on the edge of working with the set's AKB, or auto cathode balance. All Sony tube TV's use AKB. AKB is a circuit that attempts to compensate for aging of the picture tube (among other things), and if the tube isn't conducting enough (due to age), it will intermittently blank the screen or, in severe cases, not allow the picture to come on at all. But if you are comfortable taking the back off of the set and making a minor adjustment, you can probably restore the picture for a while longer. Take the back off, turn the set on to a blank channel (receiving snow). You will see a large gizmo with a thick red wire coming out of it going up to the glass on the picture tube. That's the high voltage (a.k.a. flyback) transformer. On this transformer will be a small knob labeled "Screen." If you turn this control ever-so-slightly counterclockwise, the screen will blank off immediately. But if you turn it just a bit clockwise, it may fix your problem. Don't go too far with this control, just enough to make it work. This is DANGEROUS FROM HIGH VOLTAGE, so make sure you have a qualified tech help you do this.
Could be a weak Picture tube . You can try turning up the screen control but don't turn it up too much you'll get retrace lines in the picture . Turn it alittle at time until picture comes on then turn it down until the retrace lines go away . Then try turning the tv on and off to make sure it comes back on. Be careful you can get shocked if you do this while it is on and you touch one of the circuits . The control is on the flyback or the module thats on the picture tube it will say screen by it . If this does not work you could have vidoe problems or just the picture is no good . Good luck
I have studied the internet with your problem for about 5 hours and find that 75% vote for replacement rather than repair, but it can be fixed for some time and some for quite some time. I am choosing to replace after I find someone who can walk me through the on-screen diagnosis, (just for fun). Jimmy Wayne
It seems as though from your description it is just the brightness changing.
The Cathode Ray Tube has three electron guns at the back end.
These are for green, blue, and red.
Each gun has a cathode and a grid.
Nearly all control the brightness by having the grids tied to a variable voltage.
And nearly all control the colour and luminance by an amplifier connected to each cathode.
As you do not say the colours change the the amlifiers are OK.
I would suggest the voltage to the grids ( or cathodes in some), is unstable due to a bad connection or a dry joint, or a faulty resistor in that part of the circuit.
Very old sets without remote control used a potentiometer to vary the brightness.
A voltmeter is all that is needed.
When the picture goes off try holding the increase brightness for some time, to see if it reappears.
Right now it should not be expensive to repair,the symptoms point to loose solder conection and it has gradually become worst,if you take the set to the shop it should cost less.What happens is the 10 legs the vertical ic has get overheated and loose,to prove this tap on the side of the set just to see if it reacts,it should also be check for other overheated areas while in service.If the quality of the picture is good then is worh repairing.