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Toshiba RPTV 50H72 - blue screen only

Discussion in 'Electronic Repair' started by bharat, May 1, 2004.

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  1. bharat

    bharat Guest

    Now I am happy!!! And I agree I have a lot to learn about TV repair.
    But with the help from the 'net, I should be able to get this one
    working. Hopefully, I won't need to repair another one for a while.

    Thanks to everyone who posted help.

    I replaced the bad signal board with one from the functioning set.
    The TV worked fine. Based on the hints from David and Ken, I think
    the TV is going into Blue screen mute mode (the picture looks fine
    whenever I could see it). I will check it based on responses from Ken
    and David and see if I can get it working this weekend.

    Sorry I lost my mind there earlier. When David posted that note saying
    I do not have any business doing repairs on electronic systems, it got
    my professional "ego" hurt (I guess) (specially because he did not
    provide any help in that posting. In retrospect, he WAS proving help
    by making sure I do not electrocute myself - without knowing my
    background. But at the time, it didn't sound nice). I also
    understand from Ken's posting why my statements would not make much
    sense. I should have just stayed away from making any comments and
    just asked for help. It would have saved a lot of bandwidth! Live
    and learn!

    Another reason for my temper-tantrum (I guess) was my old habits. I
    used newsgroups extensively when I was a grad student. Then, we did
    not post anything that was not useful (internet was not
    available...only a few people had access to newsgroups. Bandwidth was
    low). I always believed the postings were not for money or fame, but
    for the satisfaction of helping others (same is true today. If any of
    you who helped are waiting for a check from me, you will be waiting a
    long time:)). When I saw David's post, it just irked me that he was
    not telling me anything useful (I repeatedly stated in my postings
    that if you can't help, stay away...I guess old habits die hard!).

    Anyway, I like this newsgroup. I will start reading it on a regular
    basis and see if I can provide help. I may not be a good
    trouble-shooter, but I am a good designer. If you need help in the
    design area, do drop a line.

  2. David

    David Guest

    You old hat knowledge forgets that modern tv sets have something called blue
    screen mute when there is no signal. The fact that there are no retrace
    lines on the blue and a normal picture for a second whenever changing
    sources 99% rules out a problem with the cathode drive on the blue tube.
    There is a possibility that a problem with the AKB circuit could be
    overdriving the blue, but an image would still be visible, but very blue.

    You are correct in leave the yoke out of it.

    The sync however does so much more on a modern tv than causing the picture
    to roll or tear horizontally if those are a problem. Without horizontal or
    vertical sync pulses to the system control, the tv will shut down to protect
    the picture tubes. Without the sync pulses to the jungle ic, odds are no on
    screen display will be visible. The signal sync coming off the luminance is
    required for the micro to detect the closed captioning information and if
    missing will likely result in a blue screen mute.

    bharat: another tip here. If you turn on closed captioning in the user menu
    since the menu was working, and you select a tv channel that you know is
    sending closed captioning (use another tv with it turned on to be sure), do
    you get the closed captioning on screen or just a blank box? If closed
    captioning is not working, the micro definitely is not seeing the sync
    pulses off the incoming signal.

  3. Does the set still mute the video on ALL sources? 1080i? 480p? 480i? Do
    you see any OSD, any PIP/POP?

  4. bharat

    bharat Guest

    None of the sources I checked worked. The screen stays blue. No PIP
    either. Video is certainly (as far as I can tell) in blue screen mute
    mode (never heard of this before...but then I am not an expert on
    these matters!!!:)). I haven't checked CC input as per David. I
    will check it tonight. RGB outputs on jungle IC are not what the
    manual says they should be (staying HIGH most of the time as David

    Last night, I checked the voltages on the sync inputs on micro. The
    H-Sync node on the micro is supposed to have 3.0V according to the
    manual and it was showing 2.5 (my kids came back before I could check
    it on had to give up). SCL1 and SDA1 (don't know
    what these signals are...if you can post something about their
    function, I will appreciate it) also are supposed to be 4.6V but were
    showing around 4.0V. All these measurements are DC. Rest of signals
    were showing expected (as per the schematic) voltages.

    Can a 0.5V drop on H-sync cause blue_screen mute? I will have to
    trace that one to see why that is low. Also, I will check all sync
    signals on oscilloscope to check for proper waveforms. Is micro the
    only circuit that will cause a blue-screen mute (BSM)? David had
    listed a lot of reasons for BSM but only micro as the controller for
    BSM. Another questions I have is how does the menu screen work if
    sync signals are messed up? I thought all screen functions will be
    needing sync signals and menu is certainly screen-based. Does this
    mean sync are OK but something else is causing BSM or are menu sync
    signals different from normal sync signals?

    Gotta run....I gave my final and kids are lining up to know their
    grades and I haven't graded the exams yet!!

    Why does my posting take a lot longer (at least it seems to me that
    way) than everyone else's posting? It seems David posted twice before
    mine even showed up!

  5. John Miller

    John Miller Guest

    You're posting via Google; David is posting via
  6. Ken Weitzel

    Ken Weitzel Guest


    I believe you'll find that the law calls for ungraded
    papers to all recieve the default A+, or 4, or 100%;
    whichever is best in your country. :)

    I have one more tiny suggestion if I may? Keep in
    mind it comes from an "old hat" as David so generously
    called it :)

    I, too, had never seen a blue screen (except for
    Windows) so played around a little. Big 5 bedroom
    house for one old man and one little girl, but lots
    of TV's...

    All connected to cable boxes. Tried them.
    Turn the cable box off via remote, and the TV
    mutes audio, the raster goes very, very
    dim for about 5 seconds; then the tv automagically
    shuts off. (seperate ac feeds)

    One in the basement belongs to my cottage
    neighbor, waiting for transport to the cottage
    for fishing season opening tomorrow.
    Plugged it in - snow and hiss. Stuck a
    resistor lead into the rf input, poor picture
    and sound or snow and hiss on all channels.

    Wondered why the difference, worried over it for
    a while, and finally found the difference.

    User menu - choose "cable" or "tv". The sets
    upstairs set to cable; the one in the basement
    set to tv.

    So - is it possible that the set you're working
    on in the cable position with no cable box?
    (switching modules would have also switched
    eproms, might have been different)

    Worth a shot, anyway.

    Good luck.

  7. r c

    r c Guest

    I have een some great suggestions.
    Here is mine.
    start over.
    use your 5 senses.
    give it an extreme visual inspection,
    smell the unit, .
    feel the unit, etc
    Then, check your power supplies, both main and local, ( in any
    suspected areas)
    Get the manual.
    Use sympton analyisis diagnosis to narrow your troubleshooting.
    If you have a PIP board, suspect that. (and or component therein)
    Tuner possibly, and whatever video switching
    devices/circuits would be suspect as well.
    Looks like a possible "crap"-acitor hunt

    those may play a role
    good luck
  8. Cable mode allows the set to recieve cable frequencies instead of the UHF
    that the air or tv mode allows. It has nothing to do with whether the set
    isconnected to a cable box or not on most sets.

  9. PIP and capacitor problems have not been common issues with this set. This
    guy needs to be using a scope at this point.

  10. Ken Weitzel

    Ken Weitzel Guest

    Hi Leonard...

    Sounds like you might be in the same "old hat" category
    as me. Give up, you can't win :) :)

    That's the way I remember it too. First uhf which
    of course was a whole different thing.

    Then a mechanical switch which allowed the tuner
    to go up to channel 99 or thereabouts in cable

    I expected now the same, with the tuner just going
    even higher. Playing with the "connected to
    nothing" set downstairs in cable position still
    only allows channels 2 through 13. Same in both

    Dunno. I'm waiting for my sons-in-law to help
    me load it into the car for the trip to the
    cabin - and the walleyes are waiting for our
    ceremonial 12:01 am casts off the end of the
    dock - grand daughter first, of course :)

    Walleye filets for breakfast... have a good
    weekend everyone.

    Take care.

  11. David

    David Guest

    You need an oscilloscope and isolation transformer to diagnose the set. You
    also need to understand what HOT ground and Signal ground are and where they
    are located in the tv set for you scope ground (use the tuner shield).

    H-sync will be at horizontal sweep frequency pulse, 15kHz or 32Khz
    approximate, may be positive or negative going.
    V-sync will be at vertical sweep rate of usually 60Hz.

    The signal sync will be the combined vertical and horizontal sync with the
    video information removed. This is the one you really need to look at.
    Somewhere between the signal circuits and the system control.
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