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Thread: Tandy 3 in uncertain condition

  1. #61
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    And I just realized I had left it turned on on the other side of the office. according to the time on it, it's been on for over an hour with no video problems.
    I turned it off and back on, it's viewable but has the 2 pixel vertical jitter going on.

    When it goes to the failure mode I will take a picture and post it.

    The image is static, just overlapping itself 3 times, one which starts barely in the visible area behind the trim.

    Edit: This refers to the previous post which may be on the previous page of the thread, look back one page to see what I mean!
    Thanks
    Last edited by TheDrip; December 3rd, 2020 at 10:43 AM.

  2. #62
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    From the pictures you posted it is hard to see the problem you described; it looks to me as though the brightness and contrast pots are perhaps set too high, and maybe the scanning focus is a little off (fatter at the center of the screen).

    Have you verified the settings on the video board? In other words, have you turned the VHOLD adjustment in each direction until you have located the center or "sweet spot" of its range? It's conceivable that your new video board was set for its old system and now finds itself facing slightly different values.

    -CH-

  3. #63
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    I tried multiple times to get a a picture of the problem before realizing that I would only ever catch one frame of rendering.

    Sometimes it works great, sometimes it essentially renders each frame offset by 1/3 of the screen. It doesn't "roll", you just see 3 screens overlapping.

    I have not adjusted vhold since the image does work, usually when the board/CRT are cold.

    I reflowed about half of the board last night with no improvement.

    One behavior that has become consistent that MAY point the ginger at a particular device, if the machine is turned on and having video problems and I unplug the plug from the CRT board to the mainboard/pots, then plug it back in, the image comes back very stable.

    Its almost like the issue is in the video signal on the mainboard, but I have no idea what to look for there either. Video/analog is very very out of my wheelhouse.

  4. #64
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    Try to take a video .... setup you camera and let it record for a while ... maybe it will record the problem

  5. #65
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    I caught it pretty well finally

    Edit: pardon the blair witch shaky cam. A YouTuber I am not!

    Last edited by TheDrip; December 4th, 2020 at 01:20 PM.

  6. #66
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    Quote Originally Posted by TheDrip View Post
    I tried multiple times to get a a picture of the problem before realizing that I would only ever catch one frame of rendering.

    Sometimes it works great, sometimes it essentially renders each frame offset by 1/3 of the screen. It doesn't "roll", you just see 3 screens overlapping.

    I have not adjusted vhold since the image does work, usually when the board/CRT are cold.

    I reflowed about half of the board last night with no improvement.

    One behavior that has become consistent that MAY point the ginger at a particular device, if the machine is turned on and having video problems and I unplug the plug from the CRT board to the mainboard/pots, then plug it back in, the image comes back very stable.

    Its almost like the issue is in the video signal on the mainboard, but I have no idea what to look for there either. Video/analog is very very out of my wheelhouse.
    On a Model III video output is memory mapped; there is a section of memory 1024 bytes wide beginning at the upper end of the first 16k (most of which is ROM BASIC). (Sorry I don't have the exact location at hand). Have you run a memory check - perhaps included among your floppies?

    Watching the video the characters are well-formed, rather than the random garbage that results from corrupted memory. One thought is that the problem is a timing issue: A simple experiment would be to gently pry up each memory IC and push it back down into place. Sometimes oxides build up on the pins and increase resistance.

    -CH-
    Last edited by clh333; December 4th, 2020 at 01:46 PM.

  7. #67
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    I have run memtest a number of times while the screen was fine and while malfunctioning, it's good.

    The behavior doesn't seem to be in the pixel data, but rather the vertical hold/sync

    It doesnt show well in the video but on top of showing the 2 or 3 overlays, the entire screen is jumping up and down by a ppixel dimension or two as if the v sync is jittering.

  8. #68
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    Once work (from home!) ends, I will get in there with some de-oxit on the vhold adjustment and do my best DJ impression, then re-set it.
    We'll see how that goes.

  9. #69
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    Quote Originally Posted by TheDrip View Post
    One behavior that has become consistent that MAY point the ginger at a particular device, if the machine is turned on and having video problems and I unplug the plug from the CRT board to the mainboard/pots, then plug it back in, the image comes back very stable.
    That does look suspiciously like an issue with the vertical sync signal. On the bright side if it recovers by unplugging/plugging the CRT to mainboard wire it's probably just something like an oxidized connection or dry solder joint somewhere.
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  10. #70
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    Code:
    VIDEO SYNC
    
    If there is no vertical sync, check the waveform at pin 12 of
    Flip/Flop IC (U23) on the System board.  If the waveform is
    present check the voltages and components associated
    with the Sync Amp Transistor (Q601) on the Video Monitor
    board.  If the waveform is missing at pin 12 of IC U23, check
    the waveform at pin 10 of IC U23.  If the waveform is good at
    pin 10 of IC U23 check Electrolytic C58, Resistor R14, and IC
    U23.  If the waveform is missing at pin 10 of IC U23, check
    the waveforms at pins 12 and 13 of IC U39.  If the waveforms
    check good at pins 12 and 13 of IC U39 check IC U39 by
    substitution.  If the waveform is missing at pin 12 of IC U39,
    check Flip/Flop IC (U1), IC U2, Counter IC (U20), and IC U21.
    If the waveform is missing at pin 13 of IC U39 check IC U22.
    
    
    If there is no horizontal sync, check the waveform at pin 13
    of Flip/Flop IC (U23).  If the waveform is good, check the
    voltages and components associated with the Horizontal
    Sync Transistor (Q701) and check Diodes CR501, CR502, and
    associated components on the Video Monitor board.  If the
    waveform  is  missing at pin 13 of IC U23, check the waveform
    at pin 2 of IC U23.  If the waveform is good at pin 2
    check Capacitor C59, Resistor R10, and IC U23.  If the wave-
    form is missing at pin 2 of IC U23 check the waveforms at
    pins 1 and 2 of IC U39.  If the waveforms at pins 1 and 2 are
    good, check IC U39.  If the waveform is missing at pin 1 of
    IC U39 check IC U21 and Counter IC (U56).  If the waveform
    is missing at pin 2 of IC U39 check IC U38.
    Larry

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