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    Thread: Screen jittering after H-Width adjustment.

    1. #1
      Rowr is offline
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      Screen jittering after H-Width adjustment.

      Hi guys, i recently picked up a LAI Fatboy.

      I've rotated the screen from vertical to horizontal without issue, though after finding the edges of the screen not extending far enough out by a few inches either side some resources online led me to believe adjusting the Horizontal width pot might help.

      I bought the correct plastic tooling and carefully tried to adjust a few turns back or forth. At the time it appeared to not make much difference so I turned it a few rotations back to get it about where i found it and then the screen sort of went from stable to all over the place horizontally. It also developed a bit of a horizontal jitter.

      I've spent a few hours total now adjusting the various horizontal pots to try and get the screen back, and while I can adjust it to be more or less stable the jitter is persistent and the screen is now about a third too far to the left and will rarely hold horizontally for too long.

      I've taken out the core and examined it and it doesn't appear broken at all, I've peeked inside the pot and can't see anything obviously wrong. There is a half circle flat piece of plastic that i assume is in there to hold the thread a bit tighter?

      I'm clutching at straws a bit over what is at fault as this is my first machine and I'm struggling to find much online regarding similar issues. The way the screen went from being stable at first to going jittery makes me think perhaps a loose solder joint or something nearby when I adjusted it has gone off or something. I've checked for loose wires.

      So i'm aware now that there is a jumper I would of been better of trying first to resolve the screen width issue and that I was always going to get minimal adjustment from the H size pot. So yeh feeling pretty deflated that I touched it at all now.

      A few other things prior to this though things were still working ok, is that upon powering up the screen starts blurry and will gradually focus in over 15 minutes (although this might be temperature related as today being a warmer day it was clear from the outset when i powered it on. Also the vertical position pot snapped off when i first went to adjust it, it seems the legs on it were already a bit weakened although in hindsight I should of been a bit gentler.

      Now i've contacted Jomac and plan to drop the chassis in as most threads I found regarding similar issues end with that conclusion and i'm too far over my head to be able to diagnose what is going on. But I was hoping if anyone has some more familiarity with what it could be that might make it a bit easier for me to either attempt to fix it instead of sending it off or at least narrow it down for when I send it in.

      I believe the chassis is a nanao ms8 (26a?)

      Sorry for a bit of a depressing first post, thanks for reading.

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      DSC_0349.JPG

    2. #2
      buttersoft's Avatar
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      Well, it's possible you might be able to track down the fault causing the jitter. That's likely to be in the horizontal deflection area. But the focus issue means the resin around the flyback's core is cracking as it ages, i believe. In some cases this can be a fire risk. Taking 15 mins to come into focus means it's time to replace it. Keep in mind that Joey won't work on chassis if you've worked on it yourself though.

      Welcome to AA, by the way

    3. #3
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      Thank you very much that gives me something to go on and thanks for the welcome

      Hmm the flyback situation doesn't sound great...

      I guess I'll get the chassis out and take a look around those areas, though I'll likely leave it to Joey.

    4. #4
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      Well got the chassis off, i'm not clever enough to know if there is anything obvious here but here are some pics in case someone can see something.

      I mean it looks like someone has done something here with this h width coil with the rubber sleeve at the back of the board but its hard to see if things are as they should be. My guess is some part of the solder has cracked where its supposed bridge across or something.

      The flyback core hardly looks like it's falling apart or anything compared to what I have seen online with people having it arcing etc so I'm not sure what that's all about. Is that where I am looking in regards to the resin? my assumption is that your referring to within the hard casing.

      Any other reasons you can think of in regard to the focus thing? Just a bit odd to me it's sort of gone from being fine one minute to taking a while instead of a gradual worsening. Also the link with warmer weather and the fact it turned on focused out of the gate before.

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      Last edited by Rowr; 12th October 2018 at 07:34 PM.

    5. #5
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      I may not have been very clear. The flyback will have been sealed/potted into its case when it was made. Assuming I'm even right to begin with, and it's damaged or just aging, effectively, the damage could even be inside some of the coils. And it's probably the accumulation of hundreds tiny flaws. You can't get in there, it can't be fixed. It's amazing how often old electronics will go from seeming fine to completely broken as something that must have been aging for years finally gives up. If the problem isn't with the flyback, it might be inside the tube. And that's even more impossible to get into.

      Photos aren't going to be much good for diagnosing the horizontal offset problem either, sadly. A failed or out-of-spec component can't be spotted unless there's physical damage, and a dry joint or crack is going to be difficult as well. You could start by examining each solder joint, and metering for connectivity from place to place. You could also try running the chassis (you need the tube connected, obviously) and perhaps pushing gently to flex the board with a piece of dowell or something else non-conductive. After that. you might need to start desoldering components and testing them, starting in the horizontal section - remembering that Joey won't touch the chassis after you start this step.

      The horizontal deflection circuits are intimately linked to the flyback in any raster CRT, so for all i actually know the whole problem might be related to the flyback. Or a capacitor in the horizontal section. Or something else entirely. Deflection problems are not a great place to start when it comes to learning CRT repair.

      If you do find a fix, or if you send the chassis to Joey, i'd be keen to learn what the problem actually was

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      Ok thanks for the specifics that helps me understand a lot better.

      I think it's off to Joey for sure, as it's probably a bit beyond me in complexity and honestly it looks finicky as hell. I haven't looked at the back of one of these chassis before, but if I had to guess it looks like that H width coil was replaced perhaps and me putting a little tension on it has upset some of the delicate solder work. I mean I really don't know for sure but given it went from fine to screwy in the act of playing with it that seems most likely.

      I'd rather have it in working condition if possible rather than donate it too science in the act of poking around too much

      Otherwise I'm stuck here justifying the cost of an old wooden box to my wife

      I guess as far as the flyback and the focus warm up issue I will have to see how that goes, I can live with it in it's current state. If it gets worse I guess my option would be to look at a replacement and the costs associated.
      In any case it's kind of secondary to having the horizontal positioning issue resolved so i'll wait on that.

      I've got a Sony trinitron flatscreen crt sitting in my shed that I found kerbside a few weeks ago that looks like it will fit the cabinet perfectly when decased. Though my preference is to try and salvage the original stuff if possible. Especially as it was basically working fine until I turned the wrong knob.

      Well I'll drop the chassis in next week and fingers crossed I guess, otherwise I'll be back here asking questions about pulling a trinitron apart
      Last edited by Rowr; 14th October 2018 at 08:57 AM.

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