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Thread: CRT with image that jumps all over the place

  1. #1

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    CRT with image that jumps all over the place

    Hi Guys,

    I'm having troubles sorting a monitor issue out, so thought I would open it up to suggestions/thoughts from the experts. Some of this was documented in my project thread, but I'll put as much info here to help explain the problem.

    I have taken a short movie of the monitor in action to show the issue: http://www.newhighscore.net/images/h...derboyjump.mp4

    When I first start the cab up, the screen is about 99% ok - maybe some slight movement at the top of the screen. After about 5 - 10 minutes, this movie is the result.

    Now to explain the set up a little.

    I'm running an original SEGA Wonderboy PCB, hooked up to a JAMMA harness (via converter). The CGA plug from the harness connects up to one of these:

    b_38e100fa86913af0c5fdf2b.jpg

    I am running a 19" Dell CRT monitor, which also plugs into the CGA to VGA convertor board. The video converter board is powered via a 5V wire and ground wire, connected to the switcher.

    So my first thoughts were the monitor is stuffed. To confirm that I plugged it into my laptop, but found it worked fine. I was able to get my desktop on screen and leave it running for 20+ minutes without issue. So I decided to test different resolutions from the convertor board (while running the game), the same issue is present in all.

    My next thought is the converter board is at fault. To test this I hooked up an LCD monitor from my PC - but found the picture displayed correctly. Wonderboy looked great (well as good as a classic can look on an LCD screen ).

    I rechecked the voltage output from the switcher to the converter board, along with how much power was getting the PCB. Everything checked out OK. I also went over the wiring, and it seems fine too.

    Another thought I had was the PCB and monitor interfering with each other - so I took the PCB out and ran the game with the board outside the monitor. Same flickering issue.

    So I'm at a bit of a loss. If the monitor is faulty, why does it work on the laptop? If the converter board is faulty (or even the switcher or jamma harness), why does the LCD display the image fine.

    Any thoughts or suggestions?

  2. #2

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    I'll start the ball rolling..

    No idea really, but are you using the same VGA cable throughout your testing process..? That jumping looks like a dodgy connection in the cable perhaps..

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    Cheers Dylan!

    The cables are different - the cable for the CRT is connected to the monitor, so no way to disconnect it sadly.

    A few things that make me think the cable is ok
    1) CRT works fine connected to my laptop (no image issues)
    2) When the cab first boots, the image is pretty good - very very minimal flicker. The flicker gets worse over a period of 5 - 10 minutes.

    I tested the CRT connected to my laptop for 20+ minutes without any drop in visual quality.

    One thing I will note is that Womble rewired the ground, 12V and 5V wires from my harness the other day to get it working better with the older style PCBs - the 5V wire was way too thin, which was causing sprite flickering. I wouldn't mind trying to locate another jamma harness to see if the harness is the cause of the fault, although the image is fine when I use my LCD - so it makes me think my current harness is fine.

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    I think you may need a new monitor.

    What I think is happening: The converter outputs a vertical frequency that's marginally good for your monitor. The monitor can only keep up with the sync pulses for a period of time, until it begins jumping clock cycles and the result is what you see. When it's flickering like that, if you turn the game off, then turn it back on near immediately, does it do the jumping right away or only after a few minutes again?

    The flicker would not happen on an LCD because it uses a completely different sync circuit compared to the analog signal processing of crts. I would suggest trying it with a different CRT monitor so you can draw some conclusions.
    Last edited by Rattanee; 11th November 2011 at 07:41 AM.

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    ^^ You talking refresh rates..? Good thinking 99.

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    Quote Originally Posted by Rattanee View Post
    I think you may need a new monitor.

    What I think is happening: The converter outputs a vertical frequency that's marginally good for your monitor. The monitor can only keep up with the sync pulses for a period of time, until it begins jumping clock cycles and the result is what you see. When it's flickering like that, if you turn the game off, then turn it back on near immediately, does it do the jumping right away or only after a few minutes again?

    The flicker would not happen on an LCD because it uses a completely different sync circuit compared to the analog signal processing of crts. I would suggest trying it with a different CRT monitor so you can draw some conclusions.
    This is something I was begining to suspect and indeed being able to test it out on another CRT would be great - I just cant locate one close by at the moment I bid on one on ebay earlier in the week - it sold for over $15!!!!! I gave up bidding in the end.

    I will double check tonight - but im 90% certain that the flickering WILL remain if the game is switched off for <5 minutes.

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    Tried the auto adjust button on the converter. Might need adjustment settings changed

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    Thats definitely a monitor issue, your harness is fine. Something is very borderline in the monitor path, either the monitor itself, or the signal converter, your best bet would be to ditch the converter and get a TV tube and a jomac chassis.
    Sic transit gloria Atari!

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    Yeah Ive run through various options with the converter - but no luck.

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    When you connect the monitor to your laptop are you running it at 640x480? Most video cards wont go that low these days, the monitor may be only barely able to deal with such a low resolution image and refresh rate.
    Sic transit gloria Atari!

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