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Thread: Repairing Oliveti M24 monitor

  1. #1

    Default Repairing Oliveti M24 monitor

    Hi,

    I am trying to fix my monitor. It seems like the horizontal sync is not OK. Things seem to be at the correct height.
    The monitor is Olivetti CDU 1200 D12CS63, Toshiba , feb 1986 from Olivetti M24 computer. The video card is OK.

    Here are some photos and a video: https://1drv.ms/u/s!AsoOXKPKfQ6FgsJa...rJMBw?e=9tczyo

    Can I do something without changing ICs? There are some white things at the back. Is there a way to adjust the horizontal sync with them?

  2. #2
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    Default

    First check if the HSync and VSync signal are Ok on the video cable, so verify that there is connection between the graphics card plug and the other end of the video cable inside the monitor. Maybe it's that simple.

    Otherwise you may need circuit diagram and oscilloscope. Please be carefull because of high voltage inside that monitor.

  3. #3

    Default

    Hi,

    I am looking again at my Olivetti M24 monitor.
    The video card is OK, because I got a signal on a VGA monitor, but the VGA monitor gives a big error message in the center (perturbing the view). So I am trying again on the original monitor.

    Settings:

    1) It has the following adjustable settings at the back when opened (small white gauges):

    - SUB.BRIGHT
    - SUB.CONT.1
    - SUB.CONT.2
    - V.HEIGHT
    - SUB.HEIGHT
    - V.HOLD
    - H.HOLD
    - H.SHIFT

    2) Two white bigger gauges:

    - FOCUS
    - SCREEN

    3) And then closer to the screen it has different type - bigger black screw driver adjustable:

    - V.POSITION R353
    - H.POSITION R453
    There is also H.WIDTH, but it looks strange - hardly accessible and it does not look adjustable. For sure it is different from all the others.


    Here is some explanation of the settings: http://newlifegames.net/nlg/index.php?topic=309.0, but not all.

    What I need is the "horizontal sync" and maybe "vertical sync" too. I think I should try these:
    - V.HOLD
    - H.HOLD
    Last edited by tonata; January 19th, 2021 at 01:39 AM.

  4. #4

    Default

    So I managed to adjust:

    - V.HOLD
    - H.HOLD

    and it got much better, but it still does not work. The H.SHIFT acts as an additional adjustment of H.HOLD, but the value does not stay. The image keeps going slowly left to right or right to left. The moment I center it, it is fine for max 4 seconds and then again it starts going slowly to the left or to the right.

    Question: Are these gauges resistances? Can they be the problem? Can I replace them with something that will hold its value?
    Or maybe it is another component?

    After restart it gets worse, but it is always this H.SHIFT that must be adjusted.

  5. #5

    Default

    Ok, I have no idea of how this really works, but this is how I would do it if I had to invent it:
    The sync signal is made of pulses, so you can't have direct adjustments, you feed it in a Monostable Multivibrator and use it to charge a small capacitor so that you have a series of small tension ramps triggered by your sync pulses, now you can use a potentiometer to set an adjustable trigger level and a comparator to digitize it for the rest of the system. This way you can introduce an adjustable delay regardless of how long the original sync signal was, you may skip the Monostable if the signal is wide enough in the beginning.
    IMG_20210120_130601.jpg

    Of course, this is all my fantasy, it may work in a very different way, I know nothing of CRTs. But if it works as I envisioned you may want to check the ramp capacitor.

  6. #6
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    Maybe the potentiometers are not good anymore. If they are quite open to see the contact, they might be dirty, dust, oxydized, maybe you can try to clean them. I did that several times successfully with contact spray and turing left right several times and then readjust. In another case I had to replace such a potentiometer by a new one of the same value and pinout to fix the issue.

  7. #7
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    Default

    That was my first thought--spray some contact cleaner into the pot, give the shaft a few turns. I was working with a monitor recently with similar issues and I had to disassemble a pot to get to the wiping surface. (It was one of those kit designs where you selected the resistive element, and then the shaft assembly and screwed them together.) But once cleaned, it was fine.

    Most conventional potentiometers consist of a wiping contact over a carbon layer deposited on a phenolic substrate. Contacts corrode with age, the carbon flakes, dirt gets into the assembly and you get erratic operation.

    Are there any Hantarex prints on this monitor? (I think that Hantarex made most of the 6300 displays).

  8. #8

    Default

    Quote Originally Posted by Chuck(G) View Post
    Are there any Hantarex prints on this monitor? (I think that Hantarex made most of the 6300 displays).
    Not the colour display of the M24, that one was made in Japan (i think by Mitsubishi). I am looking for the schematics as well. It should be part# DSM 2421C.

  9. #9

    Default

    No it says Toshiba CDU 1200.

    Thank for your hints! I will start with the pot because searching for a bad capacitor will be more complicated.
    I have attached two photos of my pot. I am not sure how to disassemble it or I just need to spray it from the back? Have you encountered this type?

    photo1.png photo2.png

    I see some people use DeOxit D5S, bit it is something imported from USA and price goes to 40-50 dollars. But wd-40 contact cleaner is very cheap. The other popular one on our Amazon is: Facom 006064.
    Last edited by tonata; January 23rd, 2021 at 08:51 AM.

  10. #10
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    I use plain old CRC QD 2130 contact cleaner, but even any automotive contact cleaner should work. In the old days, the cleaner of choice would have been carbon tetrachloride, then Freon TF. Now, it's mostly a mixture of plastic-safe solvents, as nearly as I can tell. Alcohol probably wouldn't be optimal in this application because it's lousy at cutting grease and attracts water.

    You want to get the stuff between the "knob" and the body of the potentiometer; that is, the side facing the knob. Given that the ones pictured aren't sealed units, I have no doubt that dirt plays a part in your situation.

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