CRT TV picture problem

Discussion in 'General Electronics Chat' started by tpny, Sep 17, 2014.

  1. tpny

    Thread Starter Member

    May 6, 2012
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    Hi my crt tv's picture went black. Sound is ok. On screen display channel numbers and volumn bars are scrunched together in a line midway down the screen. Is it a vertical deflection problem?.. Please see attached picture. Any advice, comment is most appreciated.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Loss of vertical deflection, find the vert output transistors for a primary culprit.
    Max.
     
  3. #12

    Expert

    Nov 30, 2010
    16,345
    6,831
    My TV had bad solder joints on the vertical deflection chip.
     
    bwilliams60 likes this.
  4. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    You could have lost the "boost HT" which is generated by an overwind on the horizontal output Transformer.
    It usually looks like about +160v.

    If it is "a bit less" (around 100v) when you look at it with a DMM,it is still can be a faulty filter cap on that supply.

    I had this problem with my Mother-in-law's TV,& didn't have an Oscilloscope.
    The DMM (Fluke 77) saw the unfiltered 15kHz halfwaves as DC of a lower voltage.
     
  5. absf

    Senior Member

    Dec 29, 2010
    1,493
    372
    It can also be a bad/open vertical height preset.
    Or a bad service switch OR it is in the wrong position.

    Allen
     
  6. tpny

    Thread Starter Member

    May 6, 2012
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    This is my TV's schematic, any pointers on where these components are.. Thanks!
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,055
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    What schematic?
     
  8. tom_s

    Member

    Jun 27, 2014
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    my money's on a dry/cold joint

    if you tap the tv on the side (NOT hard), does it come back up?

    edit: yes, the tv has to be on and looking at the screen while manipulating :)
     
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  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    As with the majority of problems in any circuit, a schematic is required and LOGICAL fault finding using a meter and/or 'scope.
    Anything else is pure speculation!
    Max.
     
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  10. tpny

    Thread Starter Member

    May 6, 2012
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    please see attached manual, thanks
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I would start by taking voltages around the TA8427 IC vert output, the output side is 14.5dc with a 55v peak pulse to drive the vertical scan coils, if no 14.5 check the vert fuse.
    It also helps if you have a scope, look for leakage in the electrolytic caps in that stage.
    Also in the Troubleshooting it shows primary points to test for vert problems.
    Max.
     
    Last edited: Sep 18, 2014
  12. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    I'd have a look at D301 - it is after all; a flyback output - and I think that's what D301 does.

    Switching speeds and voltages are a bit milder than in the horizontal flyback, but I'd go for something a bit quicker than a 1N4005. Frame rate is only 50/60Hz but the retrace is faster than that.

    Once or twice I've been caught out by V-out diodes - they can check OK and still not work. Test by substitution is best.

    Many V-out chips have a high voltage headroom for the sawtooth generator to improve linearity, usually 30V or more - that means probably a high value resistor lurking somewhere just waiting to go high or open.
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
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    D301 appears part of the vertical phase detection signal sent to the base QA14 in the video section.
    Max.
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
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    Nope - it goes from the fuse on the 28V supply, pretty much where I'd expect a V-flyback diode to be.

    The 220uF cap on the cathode side is probably the bootstrap for the O/P stage bias chain - that can cause trouble too, but probably not down to a fuzzy line.
     
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