crt deflection issues

CRT DEFLECTION ISSUES
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There are a  some  variations   when  it  comes  to  FBTs  and  their  driving
circuits. Some TVs may even have a  separated  transformer  to  produce  their
high-voltages.

However. Say the high-voltage for the TV's final anode is  generated  via  the
FBT by rectifying the (up-transformed) horizontal retrace pulses.   Then,   we
can consider this  arrangement  as  a  voltage  generator  producing  a  given
amount of kilovolts  when  being  unloaded.   This  high-voltage  is  used  to
accelerate the beam's electrons through the tube to give them  enough  kinetic
energy upon impact on the phosphor to light it up. Hence,   if  the  electrons
flow thought the tube, the generator gets  loaded  and  as  such  its  voltage
will drop somewhat due to its internal resistance. So a  high  current  within
the tube (bright parts in a scene) will reduced the high-voltage  of  the  TVs
final anode, if its not regulated properly somehow.

So what happens if this is the case? Basically, if the high-voltage  anode  of
the TV drops,  then the electrons within the tube won't  be  accelerated  that
much. A slower moving electron can be deviated from its path more  easily  and
as such the magnetic field produced by the  horizontal  deflection  coil  will
deflected the electron beam much more (than necessary) considering  all  other
things  kept  equal.   That's  because  magnetic  horizontal  deflection    is
inverse-proportional to sqrt(Ua), with Ua the TVs high-voltage anode.   So  if
Ua drops, the beam will be deflected much more to any  one  side.   That's  to
say; the displayed picture increases in width in bright parts  of  the  scene.
Funny, eh?

But that's not the end of the story. The picture  can  also  shrink  when  the
high-voltage drops due to the generator's internal resistance.  For,   if  the
voltage drops due to a high beam current,  the  generator's  driving  circuity
gets loaded. For example; say the tube's anode is at  30kV  when  no  load  is
applied (open circuit), and say the high-voltage  generator  has  an  internal
resistance of 6MOhms. Now if we let a beam current of 1.3mA flow  through  the
tube, the generator's voltage will  drop to 22.2kV  loading  the  generators's
driving circuit (the  final  transistor  before  the  FBT  etc. )  with  about
10Watts, which is the same driving  circuit  producing  the  current  for  the
horizontal deflection coil. Now if the driving  circuit  can't  generate  more
current (limited amplification) to compensate the loss, the power to  run  the
horizontal deflection coil will suffer leading  to  a  weaker  magnetic  field
not being able to fully deflect the beam to the edges.   Hence,   the  picture
will shrink horizontally somewhat in bright  parts  of  the  displayed  scene.

There are a lot of in-between cases here. Quite complicated  if  you  look  at
it. However, all these issues are solved  by  some  regulatory  circuit  which
stabilizes the high-voltage  keeping  it  constant  no  matter  what's  beam's
current. But if this circuity starts to  fail  due  to  wear  whatever,   your
picture  on   the    screen    will    start    to    show    said    effects.