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TV Overscan

Discussion in 'Electronic Repair' started by Engineer, Mar 5, 2005.

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  1. Engineer

    Engineer Guest

    Our den TV, an RCA 20 inch Colortrack, series CTC158/159, manufactured
    in August 1989, has more horizontal overscan than I would like, i.e.
    some extreme L & R picture material is lost. Also, you have to
    increase the vertical scan a bit to get truly circular images - thus,
    the TV has a built in "analog zoom" of about 10% (guessed) and, of
    course, a small loss of effective resolution (and NTSC is low
    already!)

    While there is an easily accessible height control (no need to even
    open the case), I cannot find a width control. Does anyone know:
    1. If there is one
    2. If so, where is it?
    3. If not, is there a "work around" to cut the overscan just a bit
    (short of rebuilding the horizontal scan circuitry!)
    I do not have a schematic.

    Thanks for all replies. Perhaps you could also copy them to
    "analogdino 'at' rogers 'dot' com" (formatted correctly) so I am
    reminded to check back here!
    Cheers,
    Roger
     
  2. NSM

    NSM Guest

    Usually not. This is typical.
    Some sets used a square copper foil shunt under the yoke to control width.
    Others used a variable series inductor.

    N
     
  3. Engineer

    Engineer Guest

    Interesting... I guess they all overscan. Our main TV, a 32 inch
    Toshiba, also overscans and has no width control, either.

    I presume the inductance alluded to attenuates the 15.75 KHz line
    drive current amplitude. But that's a triangular waveform, so I'm a
    bit concerned about the frequency response of a purely inductive
    attenuator - might affect both horizontal linearity and flyback time.
    Would a resistor in series with the scan coil be better?
    Are there other ways?
    Cheers,
    Roger
     
  4. Ol' Duffer

    Ol' Duffer Guest

    Your thinking is exactly backward.
    V = L * dI/dT.
    The waveform is a ramp because the yoke is predominately inductive.
    Adding resistance would affect linearity and "flyback time" by
    reducing the circuit Q.
     
  5. Ol' Duffer

    Ol' Duffer Guest

    You could reduce the supply voltage to the sweep circuit,
    but this is usually also the high voltage generator circuit,
    so this will affect brightness and focus.
     
  6. Engineer

    Engineer Guest

    For a small change in HV this might be OK as there are both focus and
    brightness pots, neither at full scale either way. But if the HV went
    down with a sweep voltage reduction, the height and width would both
    go up! Which would dominate? Not so sure this is a solution.

    The answer must lie in the width scan current. I presume the maximum
    horizontal deflection angle(width) is directly proportional to the
    max. yoke magnetic field which, in turn, is given by the max.
    horizontal deflection coil current. Thus, we need horizontal yoke
    current control to incrementally reduce the line scan current without
    distorting the waveform - and then find somewhere in the circuit to
    put it in without damaging the TV. Perhaps a variable series inductor
    is the way to go.

    There's also the matter of picture position. Years ago, I worked on a
    TV with "shuffle plates" (was that the name?) that allowed one to
    position the picture - nothing like this on this RCA set. They give
    you a height adjustment, but not width and no position!

    I'm stuck for now! But someone must have fixed the endemic width
    overscan on domestic quality TV's over the last 15 years.
    Cheers,
    Roger
    PS. How different from PC monitor CRTs - all parameters adjustable
    from the front, no less!
     
  7. Jerry G.

    Jerry G. Guest

    You should look at getting a schematic to have an idea of how a width
    control is designed. If you can invest in a copy of the service manual for
    your set, there may be a setup procedure for the width control.

    In many of the sets from that era there were some soldered jumpers for
    setting the width. The adjustment was course, but there is nothing to stop
    someone from changing some of the component values as necessary.

    In most of these sets, they did not put any resistance directly in series
    with the deflection yoke. If you put a resistor in series with the windings
    of the yoke with many of the designs, there would be some linearity change.

    What you want to do is to linearly control the amplitude of the drive pulses
    to the yoke without effecting their shape or timing. In sets that use a scan
    amplifier rather than drive the yoke directly from a flyback, the horizontal
    drive pulse amplitude is controlled mainly with a variable resistor, or set
    of taps, as mentioned above.

    In regular consumer type TV sets, the yoke is driven almost directly from
    the flyback. They use an RC type combination of components to have a linear
    effect on the drive pulses to the yoke. Many of these designs used a width
    coil. By inserting a ferrite core in to the coil, the scan was reduced,
    because the reactance of the coil increased. This is in the case where the
    width coil was sort of in series with the yoke. There were some caps, and
    resistors used in the configuration to try to keep the drive pulses as
    linear as possible, and with minimum timing shift through the designed range
    of adjustability. The precise range was not very wide, but it did the job.

    In the case of a colour set, this type of width change, will usually effect
    the convergence, especially at the corners. I used to find that when I did a
    change of more than a few percent, I usually had to touch up the convergence
    towards the corners and sides.

    Home receivers were usually overscanned by about 5 to 7%. This is to prevent
    any visibility of any errors in the corners and sides on the part of the
    broadcaster. Today's sets can be made tighter to specs, because the
    broadcast equipment of today is a lot better than that of 20 years ago. Many
    of today's sets have about a 2 to 3% overscan. Some of the new higher end
    CRT sets, are set to within 1% of overscan.

    The LCD and Plasma sets are displaying the picture exactly to the edge.
    These have no scanning. They are pixel addressed.

    Back in the late 80's many of the broadcasters were still using equipment
    from the late 60's and early 70's if it was still working, and able to be
    maintained.

    --

    Jerry G.
    ======


    Our den TV, an RCA 20 inch Colortrack, series CTC158/159, manufactured
    in August 1989, has more horizontal overscan than I would like, i.e.
    some extreme L & R picture material is lost. Also, you have to
    increase the vertical scan a bit to get truly circular images - thus,
    the TV has a built in "analog zoom" of about 10% (guessed) and, of
    course, a small loss of effective resolution (and NTSC is low
    already!)

    While there is an easily accessible height control (no need to even
    open the case), I cannot find a width control. Does anyone know:
    1. If there is one
    2. If so, where is it?
    3. If not, is there a "work around" to cut the overscan just a bit
    (short of rebuilding the horizontal scan circuitry!)
    I do not have a schematic.

    Thanks for all replies. Perhaps you could also copy them to
    "analogdino 'at' rogers 'dot' com" (formatted correctly) so I am
    reminded to check back here!
    Cheers,
    Roger
     
  8. Phil Bowser

    Phil Bowser Guest

    Interesting... I guess they all overscan. Our main TV, a 32 inch

    Are you sure? Most sets 27" and larger offer pincushion correction
    circuitry that allows for pincushion amplitude (bowing) and DC (width)
    adjustment. It may be done by software via the I2C bus on most newer sets
    (in a service menu) but on sets smaller than 27" it is standard practice for
    manufacturers to simply use a "pincushion-corrected yoke" which explains
    why you have no adjustment ability on your 20" RCA set. A lot of these
    older sets (especially Zenith, and some RCAs) had a "lot" of overscan
    designed into them, since tolerances were wide, and a consumer is far more
    likely to complain if they see "black" at the top, bottom, or sides than
    they would be to complain of "excessive" overscan. If you think there is an
    abnormal amount of it, I would check the horiz. B+ voltage to see if it is
    clean and within spec. - if it droops, the picture will grow due to
    insufficient high voltage, and may even "breath" more than normal with
    bright / dark scenes.
     
  9. Engineer

    Engineer Guest

    (snip)

    Thanks for all replies.
    Cheers,
    Roger
     
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