LA76931 Ver la hoja de datos (PDF) - Unspecified
Número de pieza
componentes Descripción
Fabricante
LA76931 Datasheet PDF : 74 Pages
| |||
9
Is charged to its maximum. The current now reverses direction retracing its steps. The voltage on the S-cap is varying by just the right amount to compensate
for the geometry error.
2.5 S-correction problems
A: An open s-cap will result in no horizontal deflection - a vertical line.
B: A shorted S-cap will likely load down the +B possibly resulting in a blown fuse or other power supply components.
C: An S-cap that changed value (or selected to be the wrong value) will result in distortion at the left and right sides of the screen:
- Too low: picture will be squashed towards edges.
- Too high: picture will be stretched towards edges.
Note that this is not be the same as what is commonly called linearity which would likely affect only one side or gradually change across the screen.
2.6 EHT (high voltage ) generation
The EHT is generated from a flyback rectifier on a secondary winding of the line transformer with many many turns of very thin wire. Because the flyback
pulse is so narrow, the rectifier diode will conduct only a short time. Thus the peak current in the winding will be quite high, resulting in a significant voltage drop
when loaded. The internal impedance of the EHT source is in the order of 1 Mohm, so with a load e.g. 1 mA the EHT will drop 1000V = =3%. Usually the EHT
voltage is far from stable, 10% drop is quite normal.
If the EHT voltage drops, then the electrons will be accelerated less and will move through the deflection field at a lower velocity. As a result they will be
easier to deflect by the magnetic field, and the picture size will grow. Without special measures, brighter pictures will be larger. The measure is to feed some EHT
information or beam current information to the deflection circuits, reducing the deflection current amplitude a bit for bright pictures. For horizontal deflection
this is done by the E/W modulator. This is called anti-breathing.
Sets with raster correction free picture tubes do not have an E/W modulator. There the correction may be done by means of a power resistor in series with
+B supply. A large beam current causes more power consumption, this lowers the +B supply voltage and thus reduces the line deflection current. That also reduces
the EHT even further, but the deflection current has a stronger effect on the picture width than the EHT. Better methods exist too.
The EHT information is also used to protect the flyback transformer from overload. As the load increases, the average primary current rises. Ultimately
it may reach a level where the transformer core may go into saturation. This causes large peak currents in the HOT which might lead to destruction. To prevent
this ,some EHT information is fed to the contrast controller, to automatically reduce the picture brightness whenever the white content is too much. This is called
the average beam current limiter.
A failure in the video path, like a video output amplifier stuck at 0V, causes a high beam current that will not react to the contrast controller. In that case the
beam current will not work and the set should switch off automatically, usually within a few seconds after applying power. When the cathodes heat up ,you will
see an even picture with diagonal retrace lines and then it will switch off.
Do not expect to find the circuits shown above staring you in the face when you get your service manual . There are a semi-infinite number of variations on
this basic theme. Some of them will , to put it mildly, appear quite obscure (or to put it more positively, creative) at first.
You may see all sorts of additional passive components as well as transformers for generating additional voltages not provided by the flyback. There may be
diodes in places you would think would be impossible. Therefore , to really understand even approximately how each design works may require some head
scratching but the basic operation of them all seems to be very similar.
Share Link: 