Fault: Set switches ON, audio Ok. But after showing picture for some time, the brightness of the picture goes down slowly, and finally complete dark screen remains, sound OK.
The fault is due to loose solder points at the picture tube base socket. Take a close look at the solder points at the picture tube base socket, fitted at the video out board [picture tube neck board]. Normally, loose solder points are very difficult to detect, as they may be so small to see by naked eye. With the fault described above, the filament voltage [soldering at pin number 5 & 6} should be noted. When we switch on the set at cold state, the loose soldering on these points may be in good touch, but when filament heats up, the solder joints will expand a little, by disconnecting the supply voltage to filament. You can see it by looking at the neck of the picture tube, as the glow of the filament goes slowly down, as the brightness of the picture too. It is best to resolder all the solder points of the picture tube base socket, by applying a little more solder.
If you have a Desolder pump with you, just Desolder out all the solder from these solder terminals, and resolder them with fresh solder.
This is a common fault to many CRT based TVs.
