Connect with us

Toshiba CX2780A Vertical Deflection problem

Discussion in 'Electronic Repair' started by vict, Mar 9, 2006.

Scroll to continue with content
  1. vict

    vict Guest

    Hi,

    Id like to get some advice to repair this set. The problem is: bright
    horizontal line in the center of the screen and then picture above it,
    which occupies about 25% of the screen. Picture compressed at the
    center and almost normal at the top of the visible rectangle. So
    picture expands gradually in the 25% height of the screen.
    I'm familiar with electronic repairs, but not TVs though. I have
    digital multimeter and reading different groups already changed all
    electrolythics with numbers 3xx and IC301 LA7837, - no difference
    whatsoever.
    Numbers of capacitors are not typical on this set.
    I would throw it away unless this repair was for person who cannot
    afford professional repair and to buy a new TV. Also to my desire to
    continue investing time in this TV set adds that this person has
    another old Toshiba CF27??? TV (same size, one year difference) with
    exactly the same problem, so after figuring out this one I might as
    well easily fix another for her.
    I also checked/replaced one diode in pump up circuit (between pins 8
    and 13) - again - no difference. Nothing looks wrong on PCB, also
    checked without taking them out several resistors, mostly big ones
    around IC301 - all seem good.

    Schematic diagram would help, but I missed one recently sold at eBay
    for CX2780A & CX29801 in the same book.

    I'm observing 9V on pin 1 of IC301 and 0.9V on pin 12. Some other TVs
    have 9V, but typical is 12V on pin 1 and on pin 12 I think should be
    around half of supply voltage, which is 30V.
    (Several other toshibas i found have 27V on schematic diagram).

    Signal to IC301 comes from IC501 TA8801N and I'm wondering is there any
    sense in replaciing this one or maybe some non-electrolythics.
    Reading groups about same fault I see that some people fixed it with
    capacitor changes while others with similar issues didn't get any
    answers beyond to change el. capacitors.

    Any help will be very appreciated

    thanks,
    Vict
     
  2. Guest

    You missed at least one capacitor. Go through the set again and look
    for either a couple of 1uf or 2.2uf capacitors that use to be red in
    color.
     
  3. vict

    vict Guest

    Thanks for helping. I seriously doubt that I missed any electrolythic
    capacitor. All of them are black hi-temp, only some dark-brown. There
    is one red capacitor immediately very close to IC301, but it is not
    radial electrolythic, looks like something about 0.1 mF in value.
    Should I change this one?
    thanks!
     
  4. vict

    vict Guest

    Thanks for helping. I seriously doubt that I missed any electrolythic
    capacitor. All of them are black hi-temp, only some dark-brown. There
    is one red capacitor immediately very close to IC301, but it is not
    radial electrolythic, looks like something about 0.1 mF in value.
    Should I change this one?
    thanks!
     
  5. Guest

    C305 and C342 are the typical culprits in addition to C308 in that
    chassis.
    One is next to the jungle ic.

    Full repair typically takes only a few capacitors in the power supply,
    and the vertical circuit.
    The semiconductors in that series of Toshiba rarely if ever fail with
    the exception of the vertical output due to people continuing to watch
    the set with distorted vertical deflection.
     
  6. From my own experience! If this does not do it , change the IC.


    ---------------------------------------------------------------
    Text pasted from WinSTIPS 2.0
    http://www.ServiceSoftware.com
    ---------------------------------------------------------------

    TOSHIBA CX2780A

    Vertical Stretched

    Jeff Stielau : 6/21/03 : Shoreline Electronics Repair

    Change C305 & C341 In The Vertical Amp Circuit.
    Change C308 For Good Measure.....220uf 35v

    ---------------------------------------------------------------



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

    ========================
     
  7. vict

    vict Guest

    Thanks for all help, but picture stays exactly the same. Here is what
    I've replaced so far:

    IC301 LA7837
    C308 220.0 mF 35V 105C
    C305 1.0 mF 50V 105C
    C306 2200.0mF 25V 105C - this one has the same suspicious voltage as
    output pin 12 = 0.9V instead of about 14V
    C310 1000.0mf 35V 85C - sits away from IC301;
    C311 100.0mF 35V 105C;
    C330 10.0mf 16V 85C non-polar - without plus or minus signs
    C342 1.0mf 50V 105C;
    C344 2.2mf 50V 105C,- this is the one near IC501 TA8801N;
    C345 100.0mf 35V 105C;
    Changed D301 too between pins 8 and 13.

    I don't see no more 3xx electrolythics to change and I can not locate
    any(not electrolythic or electrolythic) C341. Seems that feedback
    circuit might severely reduce pin 12 DC, because not only it low on pin
    12 0.9V instead of 14V; on pin 7 IC301 where feedback returns one of
    the (other toshiba TV) schematics I have indicates 5.0V and I have
    0.2V. Maybe some resistor or non electrolythic capacitor shortened?

    thanks
     
  8. OK....check the value of the fusible resistor that supplies B+ to the LA7837

    --

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

    =================================
     
  9. vict

    vict Guest

    I don't know how fusible resistor looks like and what is B+, sorry.
    There are 2 bigger resistors close to IC301.
    One in sequence Pin12 - L301 - R336. Took out, measured 270 Ohm, same
    as label on resistor.
    Another big resistor with one end grounded is R305. Measured value 1.6
    Ohm.

    Supply pin 8 of IC301 has about 30V and pin13 is 29.3.
    I already disconnected all parts from pin 12, still output DC voltage
    0.9V way lower than expected.

    thanks a LOT for helping!
     
  10. Jason D.

    Jason D. Guest

    Missed C301, also red by the end of IC301. Again another 1uF 50V.
    This is part of ramp generator circuit inside that IC301.

    Cheers, Wizard
     
  11. vict

    vict Guest

    Wizard,

    thanks for the advice, unfortunately I cannot find C301. Do you have
    schematics or just speak from experience? The problem is that CX2780
    has somewhat different numbers than other sets. Also there are no red
    electrolythic capacitors in this set except one on the opposite end of
    pcb in the power supply block. All electrolythics are black, only 2 or
    3 are blue and I've replaced two dark brown ones. I cannot locate
    anything labeled C301.
    There is still small chance that I could've missed some C3xx capacitor,
    because at least three were quite far from IC301 and overall there are
    maybe 50 of them on PCB, but nothing is nearby IC301 was missed.
    BTW another set with the same problem, which I don't want to open
    without success on the CX2780A, is called CF2768B.
    Any new advice where or what to look for will be highly appreciated!

    thanks!
     
  12. vict

    vict Guest

    Hi, All!

    I have to conclude this weird attempt to repair the set. After getting
    the schematics for CF2768B similar TV SET and researching it I figured,
    that eventually I've run out of ideas. So, I decided to assemble non
    working TV and put it somewhere in garage till I get oscilloscope
    (maybe never)
    I have to mention that I've disconnected output circuit from pin 12 to
    research voltage dependency on non loaded circuit, and some
    electrolythics I've replaced were soldered on reverse side of PCB. So,
    I restored output circuit and moved some capacitors from reverse side
    to where they supposed to be. In the process I put back original C306
    2200 mf, because new one was slightly bigger than needed.
    After TV was restored to "before repair" state with some new capacitors
    and IC301 out of the blue it started to work again. All voltages on
    IC301 are about the same except pins 7 and 12 of course. So it seems
    output-nad feedback circuit might have been a problem, but what was
    fixed I don't know and don't understand why it didn't work after my
    replacements immediately.

    Thanks to All willing and helped!!!
    V.
    ............
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-