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Thread: Repairing Oliveti M24 monitor

  1. #11

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    Thanks. I made a picture just to verify I was able to understand in English.

    photo1_pos.png

  2. #12
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    Yes, that looks right.

  3. #13

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    It is unlikely the pots problem. It is more likely that the sync signal is no getting to the multivibrator. The fact that you are able to get the adjustment to bring the horizontal to the correct value means that that part is working. Without the sync, it is expected to drift with small temperature changes.
    Dwight

  4. #14

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    Interesting: mine is marked as DSM 2421 C on the outside and XC-1200U inside, but bought boards, the CRT etc. have Mitsubishi markings.
    PC110007.JPG

  5. #15

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    Quote Originally Posted by Gianfilippo View Post
    Interesting: mine is marked as DSM 2421 C on the outside and XC-1200U inside, but bought boards, the CRT etc. have Mitsubishi markings.
    PC110007.JPG
    Do you see where the multivibrator and/or the ramp capacitor are most likely located? When I disassemble it I will try to focus on this area.

  6. #16
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    Markings on the CRT have little to do with the manufacturer of the electronics. In the USA, for example, "Clinton" was a very common name to see on CRT labels.

  7. #17

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    Quote Originally Posted by Chuck(G) View Post
    Markings on the CRT have little to do with the manufacturer of the electronics. In the USA, for example, "Clinton" was a very common name to see on CRT labels.
    But the PCBs have Mitsubishi logos too (see picture), I could try asking Toshiba if they recognize the model number.
    PC110007.JPGIMG_20201209_234559 (2).jpg

  8. #18

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    I agree with Dwight.

    What is happening when you adjust the H hold is that the picture can be made to appear normally, but it is drifting left or right and falling out of lock because it is not sync locked.

    This means that the H sync pulse is not reaching the H oscillator circuit's AFC area in the monitor or if it is, the circuit that processes this pulse is defective.

    Basically the incoming H sync pulse is compared with the H scan oscillator (sometimes a multivibrator) frequency in a type of comparator circuit, this creates a error voltage that is used to adjust the H scan osc, so it falls into lock with the incoming H sync pulse. Once locked, if you adjust the H hold control, the picture will appear to move left and right. And you will get the same effect if you adjust the H phase control(H position), this merely alters the phase delay of one of the H pulses prior to the comparator, so when sync locked it will alter the horizontal position of the image on the screen.

    In any case, this kind of fault requires a scope to trace the incoming H sync pulse, up to the circuit where it is compared with the H scan oscillator. Often this is a circuit using a pair of diodes (a type of comparator) called an AFC circuit (automatic frequency control) or "flywheel circuit" in vintage TV nomenclature, it produces a variable DC voltage to control the H scan oscillator frequency. It is a form of a PLL (phased locked loop) which is a negative feedback loop of sorts.

    This kind of fault is pretty easy to sort out, that is if you have the schematic and a scope. Without the schematic it is "awkward" but still possible if you draw out the schematic from what you can see on the pcb.

  9. #19

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    I have attached a block diagram which might help make sense of my post on the topic.

    In the early days of TV's and video monitors, the phase comparator was simply made from two diodes and it acted a lot like a sample hold circuit.

    Later of course in TV's and modern CRT computer VDU's that circuitry got incorporated into a horizontal processor IC, which generally has in it the phase comparator, the H scan osc, the H driver circuits to drive the Horizontal scan output transistor (HOT). Connected to that IC are the components, resistors, capacitors that determine the filtering of the DC control voltage and the H oscillator's free running frequency. So you may find quite a lot of variation in the circuitry, depending on if it was built with discrete components, or done with IC's.

    So if you have a VDU where it won't H lock, then you need to ensure it is being fed with the H sync pulses (of the correct amplitude, polarity and frequency) at its input connector and that the pulse is making it all the way to the H processor IC's sync input pin.
    Attached Images Attached Images

  10. #20

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    So it was not the pot indeed. I sprayed and I do not know how much time one needs to wait but it is the same as before.

    Thank you for all the insights. Even disassembling seems difficult and I need to discharge it first.

    I was thinking ... I can access the pot and the sync signal without disassembling ... so is it possible to build an external circuit that uses the sync signal and the pot signal to control the pot (electronically instead of manually)? Just an idea.

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