There are several symptoms that are basically similar:
|NotTaR of Television Sets : Blooming or breathing problems
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- Blooming is defined as an expansion of the raster or horizontal sections of
the raster with bright material. For example, switching between dark and
light picture causes the size of the picture to expand by 10%. A slight
change in size is unavoidable but if it is greater than 1 or 2 percent from
a totally black image to a full white one, this is either an indication of a
defective TV or one that is badly designed. The cause is poor low or high
Check the B+ to the horizontal deflection. This is usually well regulated.
If it is varying in sympathy to the size changes, trace back to determine
why the low voltage regulator is not doing its job. The reason for the size
change is that the high voltage is dropping and reducing the stiffness of
the electron beam.
- Expansion of the raster width in areas of bright imagery is an indication
of short term regulation problems. The video drive may be interacting
with the other power supplies. Check for ripple - this would be
at the vertical scan rate - in the various regulated power supplies.
The cause may be a dried up electrolytic capacitor - once you locate the
offending voltage, test or substitute capacitors in that supply.
In both these cases, if this just started after some work was done to the TV,
the brightness limiter and/or video drive may simply be set so high that the
TV cannot supply enough current to the high voltage. If the brightness
is acceptable with these turned down slightly and still have acceptable
brightness, then there may be nothing wrong.
- Breathing is defined as a periodic change in the size of the raster which
may be independent of what is displayed or its severity or frequency may
be related to the brightness or darkness of the image. This is another type
of regulation problem and may be caused by bad electrolytic capacitors or
other components in the low voltage power supplies.
If the TV uses a switchmode power supply or low voltage regulator separate
from the horizontal deflection, first check its output(s) for a variation in
voltage at the breathing rate. Test with a light bulb or resistor load to
confirm that the problem is here and not the deflection or other subsystem
of the TV.
- A condition with somewhat similar symptoms is bad focus - fuzzy picture -
but only with bright (high beam current) scenes. This could be just a matter
of adjusting the focus control but may also indicate sub-optimal filament
voltage due to bad connections or components in the filament circuit, or a
tired worn CRT. You won't get high beam current without some serious spot
blooming (a fat beam because too much cathode area is used) and you will get
cathode 'poisoning' after prolonged use.
Visually inspect the neck of the CRT for the normal orange glow of the
filaments and check for bad connections and bad parts.
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