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Lower half vertical of one interlaced field creeping up GDM-1952

Discussion in 'Electronic Repair' started by Glynn R., Mar 18, 2005.

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  1. Glynn R.

    Glynn R. Guest

    I'd almost bet this is another dried-up electrolytic, I just can't seem
    to find it! The perpetrator is an HP 98751a, aka Sony GDM-1952, (aka
    Rasterops 19, Supermac etc), and it is a 5-BNC RGB fixed-frequency
    monitor that does 1024x768 INTERLACED only. Given that, it has been a
    great display anyway and I'd hate to throw it out.

    Problem started a few weeks ago at turn-on. The screen seemed to be
    traced fully and normally by one field, while the other scrunched up
    away from the bottom-- so that I had two mouse-pointers spaced about an
    inch apart when at the lower half of the screen, but moving the mouse
    up the screen, the pointer images melded back together. As the monitor
    warmed, the scrunched field stretched back to normal over about 5
    minutes. I operated it "as-is" a couple of weeks, just waiting for it
    to settle out.

    Well, it is worse now-- the whole lowest quarter of the screen is
    affected, and no getting back to normal after even 30 minutes of
    on-time. I downloaded a zip of .pcx files of the GDM-1952 Service
    manual, which has the schemata but they neglected to scan in usual
    boilerplate like Theory of Operation or anything. So I'm guessing what
    I need to look for. But I am not seeing electrolytics in area which
    might affect one field but not the other!

    Has anyone else battled this symptom? Am I making stupid assumptions or
    have I missed something? My current web method of posting does not
    allow me to send a file along, but for those who already have access to
    the schematics, I am suspicious of C1, C2, and C8 (in the
    SyncSep/PulseGen section). My reasons for looking at these are: 1) they
    are electrolytic; and 2) they are roughly around an area that looks
    like it might have something to say about one fields vertical as
    opposed to anothers. But I may be all wet.

    TIA-- Glynn.
  2. Franc Zabkar

    Franc Zabkar Guest

    I saw problems like this in fixed frequency 19"/20" RGB Sony monitors
    attached to IBM 5080/5085 CADCAM workstations. The problem was dried
    out caps on a little daughter PCB hanging off the middle of one of the
    main PCBs. Sorry, I can't be any more specific than that. BTW, when
    was your monitor manufactured?

    - Franc Zabkar
  3. Glynn R.

    Glynn R. Guest

    I saw problems like this in fixed frequency 19"/20" RGB Sony monitors
    was your monitor manufactured?

    Feb 1990. I would imagine your IBM workstations were probably in the
    same timeframe-- we may be talking about the same chassis.

    There are several "daughterboards" on this mainboard. They are all
    soldered on heavy wires-- for robustness I suppose. You said in the
    middle-- that does narrow it down and I think I could get to that one's
    caps without having to unsolder the whole board-- well maybe not!!

    That board, about 3x3.5", is likely to be what the schematics show as
    Board "Dc", subtitled "T/B correction". That makes sense, doesn't it?

    I think I'll see if I can pull that board without destroying anything--
    thanks much, Franc, for the tip!!
  4. Franc Zabkar

    Franc Zabkar Guest

    I think so.
    The size sounds right. IIRC, the PCB had a few aluminium electrolytics
    and a few tantalums. I changed them all, but I suspect the electros
    were the guilty ones.

    I didn't have a circuit diagram at the time (1991?), but I saw the
    vertical ghosting on a CRO. IIRC, the image was a jittery parabola.

    - Franc Zabkar
  5. Franc Zabkar

    Franc Zabkar Guest

    I desoldered the PCB to gain access to the caps. I can't remember if
    really needed to, though.

    - Franc Zabkar
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