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Thread: Need some help getting a DEC VT100 working

  1. #11
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    Quote Originally Posted by MauriceH View Post
    Ah I see you have a different board.

    Ok CR409 BLOWN, THen check C486 , Remark a current through a 100k resister will seldom lead to blow a Diode.
    C486 is at the bottom at ground so think it is leaky.

    Attachment 54943

    That could be also the trouble maker for center problem caused by current draw in T402.

    Otherwise check arround the NE555 IC. A new circuit to start the line at the left.
    Again check the 12V must be stable.
    Or check that small capacitor 4,7uF C437

    Attachment 54944
    i have been looking at the shematic but must be for a diferent board because the component numbers don't match up C486 for example don't exist on the pcb. Also theres no 555 ic on the monitor board.

    Anyway like you say will replace the caps but will start by just replacing all electrostatics there's not to many of them.
    This one silver one in the picture puzzles my never seen one like this how the value is read is a normal electrolytic ?.

  2. #12
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    Yes, it's a real electrolytic. I have a couple just like that, Sprague brand and all.

    "Don't be vague--ask for Sprague" was the old advertising slogan.

  3. #13
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    Sounds like a great slogan xD.
    But i don't what is the value means to get a replacement.
    reads:
    300
    75-6 DC

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  5. #15
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    Don't let be mislead bij old caps, specially in this kind of really good expensive(way back) engineered products.
    They put good stuff in them.

    Also don't put the very cheap new ones in it, otherwise in 5years your back in replacing caps.

    Best is to check the old ones with a good capacitor measure equipment like a "Peak esr70".

    Check the riplle on the DC supply rail, that will indicate a bad cap in the supply.
    Also only focus now on this problem of shifting.
    Or
    is it both sides? Left and right that go out side screen.
    Only left indicates that the starting writing point is not right timing.

    The screen writes lines left to right.
    The screen will want to know the starting point at the left, that is done by a Horizontal sync pulse.
    The time to go back right to left the CRT is blanked out with also a pulse.

    That writing is done with that coil around the neck of the CRT.
    So the sync pulse must be told to start at the left of the CRT and to correct this there is a correction to be made.
    If it starts good and than slowly shifts there must be a DC-voltage shifting.

    If the line gets to long , so both sides, then there is a shift in the length of the writing line.

    If whole picture is growing in all directions it is a High Voltage problem.
    The High voltage pulls the electrons to the screen.
    That is time to travel back to front.

    So low High Voltage the time increase and the writing with the coils on the neck wil be to high according to that.
    That way the whole picture will grow big.
    If High voltage increase the picture become small because the coil writing is to slow.

    The horizontal writing line is also done through that high voltage transformer.
    Follow the horizontal drive and you will end up in that HV-transformer.

    Follow the vertical drive and recon that is is a separate amplifier to drive that vertical coil.

  6. #16
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    I rechecked everything, and replace every cap for the ones i can source locally not sure how good the are.

    The problem continued the only cap i didn't replaces was a electrolytic 75 nano, i can't source one of this so i replaced to a 100v.
    Now after doing this the image shifting slowly to the right fixed it doesn't do this anymore good news.

    But not so good now the screen is a compress in the middle not reaching to the left and right borders.
    Then i used a freezing spray on this guy small transistor in the picture.


    That fixed the problem immediately at least for a few seconds while the cold effect last.
    So i removed to test it when then by accident one of the legs broke.


    I can't find this transistor by the same part number maybe someone knows a replacement ?.


    The only thing having a similar name i found is this MPSA05:

    Says is a NPN transistor checked in my spares and got one (KD205 that have similar electrical characteristics but the legs are not in the same order the original was EBC and this is BCE (according to the datasheet not sure if the correct one).
    So i rewired them in the same order:


    Now when i powerup the terminal the digital board starts can hear it beeps and the leds on the keyboard are ok.
    But have no picture on the tube, i can see the cathode glowing in the back of the tube so there's voltage.

    What i can do to replace this transistor ?, or maybe i got it wrong for the replacement ?.
    Not sure still what kind of transistor was the original one

  7. #17
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    I believe it is an MPSU05 (see page 58 of MP00633_VT100_Schematic_Feb82_text).pdf and drives most of the HV in the VT100.

    It is a Power Bipolar Transistor, 2A I(C), 60V V(BR)CEO, 1-Element, NPN, Silicon, Plastic/Epoxy, 3 Pin, HFE min 60


    Jerry

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