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Tip for Toshiba TV verical repair

Discussion in 'Electronic Repair' started by Gene Gardner, May 6, 2004.

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  1. Gene Gardner

    Gene Gardner Guest

    I inherited a Toshiba model CF2750K, chassis# TAC 9011, with the notorious vertical
    problem...compressed to center and upper half...bottom half dark. After reading some
    of the posted hints, I arbitrarily replaced the 1uf-50v capcitor in the verical section.
    It seemed to be the only 1uf in the vertical section, but it was C-305 rather than
    the C-301 or C-342 mentioned by others. One mentioned that the quality of the capacitor
    was important, so I replaced it with a paper-tubular (mylar?) 1uf-200v. But apparently,
    the set may have had two defects, because when I turned it on, it now had no vertical
    deflection at all (bright horizontal white line in middle). I believe the handling of
    the PC-board had caused a lingering problem to appear. I arbitarily re-soldered the
    seven in-line pins on IC-301 which I believe is probably the Vertical power IC. It
    had a large heat-sink.
    The set now worked about normally, except that the vertical linearity was a little
    disappointing...a slight widening of spacing progressing from bottom toward top, with
    and exaggerated widening on the top inch of the screen. I could only find one pot named
    vertical height, but could find nothing for vertical linearity. There were other
    un-named pots in another section but did not to turn them, and they were sealed. They
    did not have the expected 300 series number.
    I did shunt an extra 660uf across an existing 2200uf (300 series) and shunted an
    extra 25uf across an existing 10uf (300 series)...both in the vertical section.
    Adding these made no significant improvement. Incidentally, capacitors of about 3/8"
    diameter can be tack-soldered on the bottom side (just enough space) without removing
    the original (electrolytics almost never short).
    Does anyone have any hints as to improving the vertical linearity on this model?Is it
    possible that the high quality of my 1uf capacitor was TOO high? (one poster suggested
    that the circuit was very critical to the quality of the 1uf capacitor)
     
  2. Jfet

    Jfet Guest

    Sorry, you mentioned that electrolytics almost never short?

    That wouldn't be correct. They certainly do fail, usually in the shorted condition.

    You may have other caps that are leaking, try using a hair dryer and heat near the other
    caps and see if any of the picture has altered. If it gets better, you know you have a
    bad cap. If you freeze it (using cold spray), you may find the faulty cap short out
    completely and may blow the vertical drive IC.

    Typically, the vertical drive IC generates a lot of heat, most of the caps around it fail
    because of the heat.

    If you removed a 1uF electrolytic, most likely it was polarized. Did you purchase a mylar
    cap that was polarized?? A tantalum cap would be of the best quality, and values
    typically don't exceed 1uF. Tantalums are typically polarized.

    I would also change the vert drive IC, I don't know if you mentioned you did (couldn't
    tell from your post). It is possible this is the route cause of the trouble, causing
    other components to fail in a cascading effect.



    Sorry, you mentioned that electrolytics almost never short?

    That wouldn't be correct. They certainly do fail, usually in the shorted condition.

    You may have other caps that are leaking, try using a hair dryer and heat near the other
    caps and see if any of the picture has altered. If it gets better, you know you have a
    bad cap. If you freeze it (using cold spray), you may find the faulty cap short out
    completely and may blow the vertical drive IC.

    Typically, the vertical drive IC generates a lot of heat, most of the caps around it fail
    because of the heat.

    If you removed a 1uF electrolytic, most likely it was polarized. Did you purchase a mylar
    cap that was polarized?? A tantalum cap would be of the best quality, and values
    typically don't exceed 1uF. Tantalums are typically polarized.

    I would also change the vert drive IC, I don't know if you mentioned you did (couldn't
    tell from your post). It is possible this is the route cause of the trouble, causing
    other components to fail in a cascading effect.
     
  3. Sofie

    Sofie Guest

    Gene:
    Replace (remove the old caps, don't shunt them) the remainder of the C3xx
    caps in and around the vertical output chip and look especially for C301, it
    is hiding about 2 inches away from the rest of the C3xx caps and is toward
    the center of the board near a large chip (jungle IC)..... and definitely
    replace it.... usually 1uf @ 50V .....
    Most of the electrolytic caps, with age and heat will suffer from high ESR,
    degraded capacity, leak corrosive electrolyte and many times will become
    electrically leaky or even short...... because of this you need to actually
    remove the caps, don't shunt them. Unless you have an ESR meter to
    properly test them, just replace them all, there are only about 6 to 8 units
    so it is fairly easy and cheap.
    When replacing the electrolytics be certain to observe polarity.
     
  4. Ditto Sofie's comments. Also, clean any electrolyte from the board and
    resolder all connections in the circuit. The 200v cap may have a high ESR
    and may also cause problems. Use the original value electrolytics.

    Leonard
     
  5. RonKZ650

    RonKZ650 Guest

    Like mentioned earlier, look for another 1uf. There are 2 identical 1uf or
    2.2uf caps used (depending on model), one is near the vertical output IC, the
    other is close to the larger osc/video jungle IC. Look at the color and
    size/shape of the original and compare with other C300 series in the set until
    you find it's matching cap.
    Funny thing is these caps normally check OK on both an ESR meter as well as a
    cap checker, but cause linierity problems. You need to replace them both.
    Ron
     
  6. These are the 3 MUST change caps for that model.

    C308 220uf 50v, C305 1 Uf 50v, C301 1 Uf 50v


    As others mentioned , one of them is usually in the center of the board next
    to the large signal processing IC.



    --
    ==========================
    Jeff Stielau
    Shoreline Electronics Repair
    344 East Main Street
    Clinton,CT 06413
    860-399-1861
    860-664-3535 (fax)

    ========================
    "If you push something hard enough it will fall over."
    Fudd's First Law of Opposition - Sir Sidney Fudd
     
  7. Gene Gardner

    Gene Gardner Guest

    (Gene Gardner) writes:


    Thanks everyone...my third problem is fixed! Your instructions were absolutely right.
    The last vertical linearity problem was cured by "finally" locating C-301. And yes,
    both C-305 and C-301 were the 1uf-50v which were "brownish-maroon" in color. This
    79 year old brain is a slow learner: I wasted a lot of time replacing several other
    electrolytics before the last poster (Shoreline Electonics) reinforced the other posts
    and persuaded me that there was a C-301 far away from the vertical section in a sectioned
    off part near a very large IC about the size of a microprocessor. This was so different
    from my experience many years ago. I would be curious as to the theory of how such a
    beautifully linear vertical sweep can be generated, and with no vertical linearity pot
    or vertical centering pot required. And after all this time (mfd Nov 1991) the picture
    display is near perfect. I'm also amazed that the (probably 16 year old Toshiba, which
    had only 48 channels) that I am now replacing, still has a good picture...I could almost
    say very good....and has never had a component replaced. The only repair was a solder
    joint on the PC board where thermal-fatigue at a 2-watt power resistor connection became
    intermittent and tapping or flexing made it easy to find (similar to the problem I mentioned
    in my original post concerning the Vert Out power module).
     
  8. Jason D.

    Jason D. Guest

    Bravo good going!

    That color is right on accounting for color fading. It is usually
    funny red (almost lipstick red) but it can be that dirty red. Very
    stinky caps indeed when soldered. Phew.

    C301 is the ramp generator circuit using large IC's own fixed contant
    current source to make a linear ramp, centering pot still used in that
    chassis. Then comes out as ramp signal to the vertical IC that does
    own few magics along with own retrace circuit built in.

    Resolder that vertical output IC also if you have already done and
    change 100uF 35V cap that has + side pointing to it by a diode that is
    supplied by 26V source. That's retrace cap, vertical retrace kick is
    boosted to appox 50V using that charged cap as temporary battery in
    series with it's own 26V supply.

    Cheers,

    Wizard
     
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