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Thread: Monochrome monitor picture "bounces" intermittently

  1. #1

    Default Monochrome monitor picture "bounces" intermittently

    So I recently purchased a monochrome green monitor at VCFMW, to go along with another purchase, an Apple ][. It seems to work well enough, but the picture seems to, every once in a while, bob up and down.
    I can't figure out why it's happening, and I'm not able to find anything that seems to correlate with the image bobbing. I know it's not the Apple ][ since the entire raster bobs.
    I've never seen this in a CRT, but I have relatively not much experience with these. Is this a problem anyone has seen before?

    The behavior can be observed at around 22 seconds and 32 seconds into this video.
    Current favorites: IBM 5160 (EGA+Hercules, 4 floppies, ST-225, XT-IDE), Compaq Portable ii (replaced PSU and keyboard, EMS card), ASR 33 Teletype (needs work), Apple ][e
    Wishlist: Toshiba Libretto docking station, IBM 5161 (expansion chassis), IBM Professional Graphics Controller (PGC), IBM 3270 PC keyboard card, Tandy disk-video interface (for Model 100-series), DG Nova IO controller, and the meaning of life.

  2. #2
    Join Date
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    Do you mean the horizon line dropping down on the left half of the screen at 31 secs?
    That is a result of the vertical resolution of the monitor

  3. #3

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    The key question is whether the entire raster moves vertically (bobs) or the picture information in the raster bobs. Its not possible to see this in your video as the brightness is not high enough to see the raster, only the video information/image.

    In the first case, if is a raster disturbance, there is a problem in the monitor where a small shift in the vertical scanning current is occurring in the vertical deflection amplifier altering the raster position or amplitude.The overall vertical scanning amplitude is set by the Height control, so rotate that a few times in case the potentiometer has gone noisy. A power supply glitch/noise/error would cause a disturbance, or a failing component. The vertical scanning current is fairly high at its peak and resistance in plug connections to the vertical yoke coils on oxidized connectors can cause problems, intermittent or poor solder joins etc. If that was happening the cause could be fairly easily found with a scope.

    In the second case if it is the video information bobbing up an down in the scanning raster, it is a relative vertical sync timing issue (the signal you are feeding it) with respect to the vertical scan oscillator frequency in the monitor. So either of these has an issue. If you feed the video signal into another test monitor (or check it on a scope) and the bobbing is not there, then it is a timing (intermittent vertical scan oscillator frequency shift) issue in the monitor, probably.

    The vertical scan frequency in the monitor is controlled by a preset potentiometer called "Vertical Hold". It could possibly be a noisy potentiometer so rotate it a few times. However it could be a circuit component or power supply issue.It would be fairly easy to track it down with a scope.

    When the vertical hold is set properly, the free running frequency of the oscillator is set just a little slower than the vertical sync rate, and in the moments just before vertical lock, the image on the screen should have a black blanking bar, rolling upwards a little, before it appears to roll out of view at the top and vertical lock is achieved.

    Find another video source known to be normal, feed that to the monitor to confirm the problem resides in the monitor and not the signal you are feeding it with.

    Post another video with higher brightness and reduced Height, so the top and bottom (beginning and end of the vertical component of the scanning raster) is visible to confirm whether it is the raster or the image in it, that is moving. In a nutshell, initially rotate the vertical height and hold controls to exercise them and see if there is any improvement with the problem.
    Last edited by Hugo Holden; September 18th, 2019 at 09:48 AM.

  4. #4
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    Quote Originally Posted by Hugo Holden View Post
    Post another video with higher brightness and reduced Height,
    And DON'T HAND-HOLD THE CAMERA

  5. #5

    Default

    Quote Originally Posted by Hugo Holden View Post
    The key question is whether the entire raster moves vertically (bobs) or the picture information in the raster bobs. Its not possible to see this in your video as the brightness is not high enough to see the raster, only the video information/image.

    In the first case, if is a raster disturbance, there is a problem in the monitor where a small shift in the vertical scanning current is occurring in the vertical deflection amplifier altering the raster position or amplitude.The overall vertical scanning amplitude is set by the Height control, so rotate that a few times in case the potentiometer has gone noisy. A power supply glitch/noise/error would cause a disturbance, or a failing component. The vertical scanning current is fairly high at its peak and resistance in plug connections to the vertical yoke coils on oxidized connectors can cause problems, intermittent or poor solder joins etc. If that was happening the cause could be fairly easily found with a scope.

    In the second case if it is the video information bobbing up an down in the scanning raster, it is a relative vertical sync timing issue (the signal you are feeding it) with respect to the vertical scan oscillator frequency in the monitor. So either of these has an issue. If you feed the video signal into another test monitor (or check it on a scope) and the bobbing is not there, then it is a timing (intermittent vertical scan oscillator frequency shift) issue in the monitor, probably.

    The vertical scan frequency in the monitor is controlled by a preset potentiometer called "Vertical Hold". It could possibly be a noisy potentiometer so rotate it a few times. However it could be a circuit component or power supply issue.It would be fairly easy to track it down with a scope.

    When the vertical hold is set properly, the free running frequency of the oscillator is set just a little slower than the vertical sync rate, and in the moments just before vertical lock, the image on the screen should have a black blanking bar, rolling upwards a little, before it appears to roll out of view at the top and vertical lock is achieved.

    Find another video source known to be normal, feed that to the monitor to confirm the problem resides in the monitor and not the signal you are feeding it with.

    Post another video with higher brightness and reduced Height, so the top and bottom (beginning and end of the vertical component of the scanning raster) is visible to confirm whether it is the raster or the image in it, that is moving. In a nutshell, initially rotate the vertical height and hold controls to exercise them and see if there is any improvement with the problem.
    As I said, it's the entire raster. A second video showing this is .
    I've tried fiddling with the vertical hold, vertical size, and another control that read "V(ertical?) LINE" which didn't seem to do much, nor did hooking it up to a known-good composite source. The bobbing behavior is still present. What really gets me is how infrequent and how apparently arbitrary it is. Tapping, thumping, etc do nothing, and sometimes the bobs are seconds apart, sometimes minutes.
    Current favorites: IBM 5160 (EGA+Hercules, 4 floppies, ST-225, XT-IDE), Compaq Portable ii (replaced PSU and keyboard, EMS card), ASR 33 Teletype (needs work), Apple ][e
    Wishlist: Toshiba Libretto docking station, IBM 5161 (expansion chassis), IBM Professional Graphics Controller (PGC), IBM 3270 PC keyboard card, Tandy disk-video interface (for Model 100-series), DG Nova IO controller, and the meaning of life.

  6. #6

    Default

    Excellent video.

    What you are seeing is an intermittent offset in the vertical yoke current. There are a number of ways this could come about. An intermittent component in the vertical deflection amplifier could do this. Or possibly a faulty electrolytic capacitor that couples the vertical deflection amplifier's output to the yoke and acts as a DC blocking device.

    In theory at least, if this is the case, you should be able to see the raster bob up and down with no video signal being fed to the monitor, and just the raster alone visible, is that the case?

    (The infrequency and arbitrary thing can happen with a faulty resistor or capacitor or transistor or solder join in the vertical scan circuits).

    Likely the power supply is normal as the scan width is unaffected.

    A scope would help to locate the problem in the vertical amplifier.

    Initially for this fault, without test equipment, I would replace the electrolytic coupling capacitor from the vertical output stage's output connection that feeds the vertical coils.

    Usually a vertical scan output stage is very similar in design to its power audio output stage counterpart designed with a few or more discrete transistors. Sometimes the output stage is based on an IC and has a coupling capacitor to isolate the DC from the yoke.

    Because the disturbance is fairly brisk, it could still be coupled from the output of the amplifier, via a normal coupling capacitor, to the vertical yoke coils and the coupling capacitor may not be at fault.

    Do you have or can get a schematic ?

  7. #7

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    That's a rather fancy version of a monitor which I have several more plain models of. They are rather nice monitors which seem to be Asian (don't recall if they were Taiwanese or Korean) TVs without the tuner.

    I've never had any trouble with these monitors. But I've had one other monitor which exhibited this symptom, except that the jump was very regular (every twenty seconds or so) and was the result of a week power supply filter.

  8. #8

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    Quote Originally Posted by KC9UDX View Post
    That's a rather fancy version of a monitor which I have several more plain models of. They are rather nice monitors which seem to be Asian (don't recall if they were Taiwanese or Korean) TVs without the tuner.

    I've never had any trouble with these monitors. But I've had one other monitor which exhibited this symptom, except that the jump was very regular (every twenty seconds or so) and was the result of a week power supply filter.
    Yes it is possible that a glitch in the power supply could be getting onto the vertical amplifiers' circuitry and just occasionally reaching the threshold to cause it. That of course would be obvious with a scope on the power supply rail to the vertical amplifier.
    With the relatively random nature of it, it could be a noisy component like a faulty transistor in the vertical amplifier's input circuitry, producing a transient that disturbs the scan. Sometimes transistors can produce very low frequency noise pulses or offsets in their collector currents that are infrequent.

    And it does look like one of those monitors that was intended to be a TV and had the tuner left out. Interestingly Atari used a very similar monitor to this in their Arcade Pong machine that was in fact a TV with the Tuner and RF circuits not used.

  9. #9
    Join Date
    Mar 2012
    Location
    Augusta, Georgia, USA
    Posts
    662

    Default

    Drying cap(s) or maybe cold solder joint... Probably the former.

  10. #10

    Default

    Quote Originally Posted by KC9UDX View Post
    That's a rather fancy version of a monitor which I have several more plain models of. They are rather nice monitors which seem to be Asian (don't recall if they were Taiwanese or Korean) TVs without the tuner.

    I've never had any trouble with these monitors. But I've had one other monitor which exhibited this symptom, except that the jump was very regular (every twenty seconds or so) and was the result of a week power supply filter.
    Do you have the schematics? I haven't been able to find any for my version -- a BM-12au.
    Interestingly enough, LGR actually found and restored one of these.
    .
    Unfortunately for me, and fortunately for him, he didn't have to deal with any malfunctions.

    Initially for this fault, without test equipment, I would replace the electrolytic coupling capacitor from the vertical output stage's output connection that feeds the vertical coils.
    So... How do I tell which this is, without a schematic?
    I've taken pictures of the circuit board, front and back. See below. The "vertical" section is clearly delineated, but there's a whole cluster of electrolytic caps there. Which one should I replace?

    P1130727.jpg
    P1130730.jpg
    Current favorites: IBM 5160 (EGA+Hercules, 4 floppies, ST-225, XT-IDE), Compaq Portable ii (replaced PSU and keyboard, EMS card), ASR 33 Teletype (needs work), Apple ][e
    Wishlist: Toshiba Libretto docking station, IBM 5161 (expansion chassis), IBM Professional Graphics Controller (PGC), IBM 3270 PC keyboard card, Tandy disk-video interface (for Model 100-series), DG Nova IO controller, and the meaning of life.

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