HORIZONTAL THIN WHITE LINES ABOUT AN INCH SPACE ALL OVER THE SCREEN _ CRT BASED TVs
This
is a common fault to occur in most of the CRT based televisions,
regardless of which brand it is. There will be sound OK, and a bright
screen with thin white horizontal lines across; at about an inch space
in between, all over the screen area. This fault may occur along with
color shades also, like blue screen with white retrace line, red screen
with white retrace lines, green screen with white retrace lines.
Let us take the first case.
A
white raster with white thin retrace lines across the entire screen
area. These are called vertical retrace lines technically. To make it
more understand, we have to go to some picture tube electrode details.
There are three color cathodes for a color picture tube, Red, Blue and Green. Two girds are there called Focus and Screen.
Focus
grid is supplied with focus voltage which in turn determine the focus
of picture, and screen grid is supplied with screen voltage, will be
much lower than the focus voltage, which determine the emission of the
picture tube. There is a final anode supplied with Extra High Tension
voltage, can be located at the top side of the picture tube, to which
the EHT [Extra High Tension] voltage from LOT [Line Output Transformer]
is connected with the help of rubber cap and clip. Without these three
voltages, EHT, Screen and Focus, no raster will be produced. Therefore
in our case, there is a raster, so, horizontal output section including
Line Output Transformer [LOT] is functioning well, and as far as sound
is there, tuner and video IF sections are working well. In fact, all
other section except, video output section is working properly.
There are three cathode points on the base card, marked as Bk, (blue Cathode) Rk(Red Cathode) and Gk(Green Cathode). Check the voltages of these three cathodes. In our case, it will be very low or approximately equal to zero. When cathode voltages goes low, emission of the CRT increases, is the reason. There must be a voltage between 120 and 135VDC at these points. Find out the reason for this voltage drop is the next step.
There are three cathode points on the base card, marked as Bk, (blue Cathode) Rk(Red Cathode) and Gk(Green Cathode). Check the voltages of these three cathodes. In our case, it will be very low or approximately equal to zero. When cathode voltages goes low, emission of the CRT increases, is the reason. There must be a voltage between 120 and 135VDC at these points. Find out the reason for this voltage drop is the next step.
The video drive voltages in most TVs come from LOT windings, after
rectification and filtering. There will be one resistor of low value,
say one Ohms or so, will be connected series with this winding, in
between rectifier diode and winding. Check the voltage at the cathode of
this rectifier diode first. If found low replace the diode and the
smoothing capacitor. The smoothing capacitor will be between 33 and
47MFD 350VDC rating. Replace both of them. If there is voltage, say
200VDC or so, check whether this voltage comes to the CRT base board or
not, If not, check the conducting wire, serial resistors; if used for
open. There is no chance of transistor shortage at this time as the
raster we see is white. If raster we see have a specific color
predominant, that is green, blue or red, check the reverent video out
transistors and replace it. It is best to replace all the three video
output transistors at the same time with same batch and numberUsually Lines which covers parts of the screen are mostly caused by a fault on the vertical circuit particularly Capacitors ESR gone high. And for Retrace lines they may be caused by the following:
A) Blanking Circuit- As electron beam move across the screen of the television scanning from left to right and back usually causes retrace lines, but this never happens thanks to the little known blanking circuit which erase(Blank) those lines which could have been otherwise visible Very fast. But if this block fails then retrace lines will be visible on the screen.
B) G2(screen) voltage set too high- The Screen Voltage on the other hand if it is increased a lot will causes retrace lines. So next time you change a fly back transformer and find the television is up but the screen has retrace lines try to adjust the screen knob to lower the screen Voltage. The screen will be too bright also. Usually if the screen voltage increases alone it could point to fly back failure and therefore should be replaced.
C) RGB Supply too low –On the other hand if the supply to the RBG amplifier is low also usually causes retrace line on the screen and the screen will also be very bright. If this voltage is low it could point to the filter capacitor for that line being faulty(please confirm or replace it directly)
