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How to eliminate horizonal overscan when no service adjustment?

Discussion in 'Electronic Repair' started by Peabody, Jun 7, 2007.

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  1. Peabody

    Peabody Guest

    It's a JVC AV-27920 television, circa 1995 or so.

    I understand that manufacturers deliberately build in overscan so
    that any shrinkage resulting from CRT aging won't produce visible
    black borders. But I would like see all the picture.

    The service menu has adjustments for vertical size and position, so
    I assume I can use those to get rid of all, or at least most, of the
    vertical overscan.

    But there is no width or horizontal size adjustment, only horizontal
    position.

    So I would like to think about making changes to the circuit that
    would give me this adjustment.

    In theory, I guess I don't want to change the horizontal frequency,
    just the relationship between the yoke and the guns on each line.
    So maybe instead of an actual width adjustment, it would work well
    enough to change where the scan begins, or ends, and then use the
    position adjustment to center it again.

    And I wonder if I could do this by adding additional inductance in
    series with the horizontal deflection yoke, with the idea that more
    inductance would slow down the rate of change in the yoke, so that
    the beams don't progress horizontally as fast as before, and
    therefore finish the line before reaching the edge of the screen.

    Is this all crazy? It doesn't seem like it should be. I mean,
    width/size adjustments are not unknown in the modern world, so it
    should be possible to insert one. I just don't know enough about
    televisions to know how to do it.

    I would appreciate any suggestions anyone might have.

    Oh, and I do have the schematic. Sams 4080.
     
  2. James Sweet

    James Sweet Guest



    Usually that's precisely how it's done. Look for an inductor in the
    horizontal section with an adjustable slug, it may have enough range to do
    what you want without even modifying anything.
     
  3. Peabody

    Peabody Guest

    I don't see anything adjustable. There's a big IC that
    produces an "HD" output, which goes to the base of the
    Horizontal Drive transistor (NPN), the collector of which
    connects to the Horizontal Output transistor by way of a
    transformer. But there's nothing on the schematic
    suggesting anything is adjustable.

    Any idea what kind of coil I would need to add? How many
    turns? What gauge wire?
     
  4. James Sweet

    James Sweet Guest

    It's usually a coil about 1/2" diameter and an inch tall wound with what
    looks to be around #14 Litz wire in series with the yoke. If you can find an
    older junk TV you could probably salvage something.
     
  5. Guest

     
  6. Arfa Daily

    Arfa Daily Guest

    Is there not a set HT pot in the power supply ? Usually, turning this down
    by a few volts will shrink the picture all round. You can then just correct
    the vertical via the menu. I would not personally try adding inductance into
    the H-OP stage or scan coil path. This is a very stressed and very carefully
    designed bit of circuitry, and some extra L that it doesn't want, could lead
    to all sorts of unwanted effects such as ringing, poor linearity, or even
    overheating and ultimate failure of the H-OP transistor. If you were to
    start adding L, then you would probably also have to alter the value of the
    S correction cap in series with the scan coils, as this forms a tuned
    circuit with the scan coils to deliberately make them ring in order to
    achieve the velocity modulation of the beam required to get a linear sweep
    on a flat(ish) tube face. I'm actually surprised that you feel that you have
    that degree of overscan that it's affecting your viewing. If you really are
    missing actual picture, then it must be significantly more than the 'normal'
    7 - 10% raster overscan. Remember that the broadcast picture is not as wide
    as the scanned raster, and a lot of the relationships that we used to use to
    judge picture width, have now gone out of the window, with all the different
    'widescreen' formats that they keep broadcasting now, and which many older
    TVs just don't look good on. Have you actually looked at the picture on a
    proper 4:3 test pattern ? It's the only way really to make a valid judgement
    on the picture geometry. You may of course have an actual fault. It's not at
    all uncommon for the values of the components in the set HT circuit to
    drift, resulting in a slowly 'growing' scan.

    Arfa
     
  7. Peabody

    Peabody Guest

    says...
    I'm not really sure of anything. :)

    The IC that provides the signal to the Horizontal Drive
    transistor is a TA1242N, 56 pins. It's pin 32 of that chip,
    labeled HD. The waveform shown on the schematic shows it
    going high (3.5V) for about 40% of the time, then back low
    for the remaining 60%. I don't know if that's what you mean
    by "horizontal scan waveform." It really looks pretty much
    like a square wave. By the time you get to the Horizontal
    Output transitor, the waveform looks completely different.

    The only datasheet I've found for the TA1242 is in Japanese,
    but there is a logical diagram of the chip in the schematic.
    There are inputs that it appears my have some effect on that
    pin 32 output, including 32fH, HP, XRAY, and a number of
    others that aren't labeled.

    I just don't have enough information to answer your
    question. There doesn't appear to be anything adjustable in
    this part of the circuit, although there may well be parts
    whose value could be changed to affect the waveform. I just
    don't know.
     
  8. Arfa Daily

    Arfa Daily Guest

    Square wave drive is correct. It's integrated into a sawtooth scan waveform,
    by the inductance of the scan coils.

    Arfa
     
  9. Peabody

    Peabody Guest

    Arfa Daily says...
    I have a fair amount of experience with digital circuits,
    but not analog, and particularly not television. So I know
    you'll understand when I ask:

    Is HT the flyback transformer? If so, are you talking about
    the source that supplies the primary of that transformer as
    well as the deflection coil? It's 134V on the schematic.

    If so, that is controlled by a STR30134 regulator. There
    are no pots there, but there is a pair of fixed resistors
    as a divider that appear to control the output voltage, and
    I'm sure I could insert an adjustment there if that would do
    it.
    It has been this way since it was new, so it's not an aging
    problem. I've always had the sense that I was missing part
    of the picture, and when I got a DVD player that also lets
    me zoom OUT, the difference became apparent. It may
    not be more than 10%, and perhaps others would not object to
    it, but I would just like to see what's actually in the
    picture, rather than a cropped version.
    No, I don't have any service equipment beyond a multimeter.
    The problem shows up more with the DVD player than on
    broadcast, particularly when playing things like mpeg1
    videos that I've downloaded. Probably because in that
    situation, the full raster is used.
    I don't think so, and I don't have the sense that it's
    getting worse. I think it's just the design of this set.
    But if it's possible to fix it, I would like to.

    Thanks very much for your help.
     
  10. Arfa Daily

    Arfa Daily Guest

    "Set HT" refers to the 134v. Some manufacturers call this the 'HT' and the
    output of the flyback transformer 'EHT' or 'HV'. Others call the output of
    the flyback simply 'HT' so there's no 'definitive' answer to that question,
    but it all boils down to the same thing in the end, and this variation in
    terms is just accepted. Altering the supply voltage to the H-OP stage - ie
    making the 134v something different - will affect the overall raster size,
    as well as affecting the high voltage output from the flyback Tx. Tv sets
    normally have an overvolts protection circuit, to stop the flyback voltage
    becoming excessive, as this could result in the CRT emitting x-rays beyond
    the maximum allowed limit. I think that before getting complicated with
    adding L and so on in the deflection current path, I would first try making
    the 134v slightly adjustable, and just seeing whether that does it for you.

    Arfa
     
  11. Peabody

    Peabody Guest

    Arfa Daily says...
    And there are other supply voltages provided by the
    secondary of the flyback. In my case a 197V and a 25V, plus
    some more that are regulated. So I assume I'm not going to
    be able to change it much before things really start to go
    wacky. But then, just a little may be enough.

    Well, I can certainly make the 134V adjustable. I'll do
    that and see what happens.

    Thanks again for your help.
     
  12. Peabody

    Peabody Guest

    Peabody says...
    Best laid plans.....

    I found the data sheet for the regulator, and it turns out
    it isn't adjustable. Hence the name STR30134 for 134V. The
    voltage divider I found that feeds pin 2 just provides a
    source to drive the darlington base, but it is further
    clipped internally. I would have to reduce the voltage
    there below the clipped value to affect the output voltage,
    but then I would probably lose all regulation.

    There is already a fixed 1500 ohm, 3W, resistor between the
    134V supply and the deflection coil. What about making that
    a higher value?

    Or, inserting one or more forward bias diodes in series at
    that point to drop the voltage about .6V each. Maybe even a
    1N400x would work ok at this frequency.
     
  13. James Sweet

    James Sweet Guest


    I still think adding inductance to the horizontal yoke circuit is the
    easiest way to go. I first saw this method described for converting a B&W TV
    to a computer monitor in a book from the late 1970s, monitors designed for
    use in arcade games use the same method. If you go changing the voltage
    around it will affect other things.
     
  14. Franc Zabkar

    Franc Zabkar Guest

    Assuming your TV is faulty, I would check the tuning capacitor
    attached to the collector of the horizontal output transistor.

    - Franc Zabkar
     
  15. Peabody

    Peabody Guest

    James Sweet says...
    Below is the circuit for the horizontal deflection. (You
    would have to view it using a non-proportional font.)

    When Q522 turns on, it grounds everything - the yoke circuit
    and the flyback primary in parallel, both fed by the 134V
    supply.

    Does it matter where I insert the coil? I had thought maybe
    immediately before or after R526, but I'm not sure if it
    matters which one.

    I assume the value of L521 should not be messed with.

    Placing it immediately before or after the yoke would
    require cutting one of the wires going to the yoke, which I
    would rather not do.
     
  16. Guest

    Thats not going to work. HT voltage affects both scan voltage/current
    and EHT voltage in equal measure, so it has no effect on picture size.
    This is why old TVs with no HT regulation stayed at more or less the
    same picture size as the voltage wandered all over the shop. The only
    thing that altered pic size on those was loading of the EHT, which
    depended on total picture brightness.

    If you've played with tv psus you'll be aware of how much one can 'get
    away with' when it comes to HT lines.

    Boosting HT was formerly used as a last ditch method to get more
    emission from dying tubes. It works, but is not recommendable.

    If you want to adjust HT V to tweak scan, you'd need to just adjust
    the voltage to the horiz scan output stage. That could be done with a
    string of diodes, each of which will drop around a volt IRL. Due to
    lack of cct details, wise to add an ultrafast diode pointing the other
    way across your diode string, and maybe an RC too.


    NT
     
  17. Arfa Daily

    Arfa Daily Guest

    What you say, is of course, fundamentally true, but I think that the key
    here is your "more or less" statement. I've never known the relationship
    between scan size and HT setting to be so tight as to maintain the picture
    size constant. With most sets that have adjustable HT, altering the setting
    will have some effect on the picture size, and it was just something that
    the OP could have tried as perhaps a 'simple' fix for what appears to be a
    fairly minor amount of overscan that he's trying to correct. As it happens,
    the HT is not adjustable on his set, so it's a moot point anyway.

    I think that a couple of diodes in the HT feed to the OP stage, as you
    suggest, might be a simple way of giving it a try.

    Arfa
     
  18. Franc Zabkar

    Franc Zabkar Guest

    That doesn't look right. The LH end of the yoke normally goes to
    ground via another capacitor (not the tuning cap, C524).

    - Franc Zabkar
     
  19. Peabody

    Peabody Guest

    Franc Zabkar says...
    I left out one cap (C526) back near the 134V source. So it
    should look like this:

    |---------------FLYBACK---------|
    | |
    | |
    | |
    134V--|--R526-|----L521------|--YOKE--|---Q522---|--GND
    1.5K | | | H.OUT |
    | | | |
    | | | |
    |--R530--C527--| |---C524---|
    |
    |
    C526
    |
    |
    GND


    But somehow I don't think that's what you had in mind. The
    "bottoms" of both the flyback primary and the horizontal
    deflection coil are tied together as shown. They go to
    ground through the low-impedance path to ground when Q522
    turns on, and through C524. at other times.
     
  20. Peabody

    Peabody Guest

    Arfa Daily says...
    And where would you put these diodes so that nothing "tuned"
    would be screwed up? With reference to the diagram:



    |----------------FLYBACK---------|
    | |
    | |
    | |
    134V--|---R526-|----L521------|--YOKE--|---Q522---|--GND
    1.5K | | | H.OUT |
    | | | |
    | | | |
    |--R530--C527--| |---C524---|
    |
    |
    C526
    |
    |
    GND

    It would seem that putting one or more diodes immediately to
    the left, as shown, of R526 would drop the voltage only
    through the deflection coil circuit, not the flyback.

    Then the question would be what kind of diodes to use for
    the voltage drop, and what to use as a reverse protection
    diode, if any. Where I live now, I'm pretty much limited to
    Radio Shack, my junque box, or mail order. RS has signal
    diodes like the 1N914, Zeners, rectifier diodes like the
    1N400x (1 amp) and 1N540x (3 amp),and a 1KV, 2.5 amp PTC205,
    whatever that is. I assume only the signal or zener diodes
    would be at all fast.

    It's not clear to me that there will be any spiking at that
    point that needs to be protected against. By the way, R526
    is a 3W resistor. I don't know enough about TV's to guess
    how much current goes through the horizontal deflection
    coil, but all the parts in this part of the circuit have
    pretty hefty ratings.
     
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