I am no expert, but I will try to explain A standard TV screen is made up of 480 lines from top to bottom. Retrace lines will only happen on a TV with a regular Picture tube (known as a CRT), they will not happen on a LCD or Plasma screen. So before I say how retrace lines happen, lets first get little understanding of how the TV puts an image on a CRT. The lines on a CRT are made by a device in the back (known as the neck)of the picture tube called the electron gun. By use of magnetic fields controlled through a device called a Yoke mounted on the neck of the CRT. Now the electron beam emitted by the electron gun is made to go across the front of the CRT screen that is coated with a layer of Phosphor that glows when the electron hits it. When that beam starts at one side reaches the opposite side of the picture tube it has to quickly return back to that first side in order to make that next line. On a standard TV, in order to make the image of a single frame, it creates this line across the screen 240 times from top to bottom, it then returns to the top and starts down again with a second set of 240 lines that fill in between the first 240 lines to create one full frame image of 480 lines, and it does this 30 times a second. Now the retrace lines you are asking about happens when a circuit in the TV known as the Blanking Circuit, fails causing the Beam that is moving from the bottom to the top of the screen to become visible.
horizontal white lines problem in Hitachi CRT TV..
First, need to know the exact model of the CRT TV.
Second, a picture of the screen will help understanding the real problems.
Usually lines which covers parts of the screen are two types:
1)- Lines usually cover part of the screen but not whole screen.
mostly caused by a fault on the vertical circuit particularly capacitors ESR gone high.
2)- Retrace lines on the CRT screen refers to horizontal lines which covers the whole screen from top to bottom but they are never straight.
Retrace lines they may be caused by the:
3-blanking circuit Faulty capacitors, faulty IC circuits.
-G2(screen) voltage set too high try to adjust the screen knob to lower the screen voltage.sometimes if the screen voltage increases alone it could point to fly back transformer.
-RGB amplifier supply too low it could point to the filter capacitor for that line being faulty.
Second, a picture of the screen will help understanding the real problems.
Usually lines which covers parts of the screen are two types:
1)- Lines usually cover part of the screen but not whole screen.
mostly caused by a fault on the vertical circuit particularly capacitors ESR gone high.
2)- Retrace lines on the CRT screen refers to horizontal lines which covers the whole screen from top to bottom but they are never straight.
Retrace lines they may be caused by the:
3-blanking circuit Faulty capacitors, faulty IC circuits.
-G2(screen) voltage set too high try to adjust the screen knob to lower the screen voltage.sometimes if the screen voltage increases alone it could point to fly back transformer.
-RGB amplifier supply too low it could point to the filter capacitor for that line being faulty.
Causes of retrace lines on the CRT Television screen
In the last article by Mr Beh, you noticed that when the television came up the first time..Still there were white lines observed on the upper part of the screen.
In electronics one can get a lot of clues on the faulty block if one is keen and understand well how different blocks works and possible symptoms when they fail. The more you understand the basic functions of each block the higher the chances of localizing the circuit causing the trouble faster and hence saving on time.
Let us first start by differentiating between retrace lines and just lines:
A) Lines on Television screen refers to straight lines across the screen like the ones shown on the picture below. Lines usually cover part of the screen but not whole screen.
B) Retrace lines on the CRT Television screen refers to horizontal lines which covers the whole screen from top to bottom but they are never straight (sloping).
Usually 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)
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