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Thread: Need some help getting a DEC VT100 working

  1. #1
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    Default Need some help getting a DEC VT100 working

    Hi lucky lucky i got a vt100 in a flea market, cost me like 20 usd dollars i was unable to say no to this beauty !.
    And was shrink wrapped, looking great getting tanned in the scorching sun of summer 38C.
    Took this baby home removed the shrink wrap was in great external condition, the keyboard rubber feet was melted and stuck to the top of the terminal i have to cut them using a knife.

    names that start with a and end in y

    Now the problem, first the fuse was blown and the terminal was set to 110v this is a 220v country, change the setting replace the fuse and get it powering up.


    Disassembled it, was quite easy. But but but but like Jeff Goldblum could say theres no display in the terminal. Hooked an external tv using the composite output and can see some corrupted text fonts on the TV. No video on the internal display.

    The keyboard light online turn on but no beeps or blips from the speaker.

    The cathode in the picture tube light up and got a bright line and dot when powering it up.

    So i believe i have two problems one related to the analog board on the monitor itslef (no picture) and another related to the digital side of things (garbled up charters on the external composite output).

    Using the multi meter checked the digital board i have a steady +12 and +5 lines on the power rails.

    Maybe someone can help me in getting this to work i'm quite lost!.

  2. #2

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    When there is no beep at startup my experience is that the low ROM is bad. If you have the AVO this ROM can be replaced with a ordinary 2716. Another option is to use a MCM68766 to replace all the ROMs on the main board. Now, MCM68766 is not that common so one may need to make an adapter. I did this based on a design on OSHpark. Here is a link to my page: http://www.datormuseum.se/peripherals/terminals/vt100

    Since you get something on the screen the HV might work. But I once had a flyback fail in these.

    Good luck!

  3. #3
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    Thanks for the information and your webpage will help me a lot .
    Do you know the video input on the back of the terminal if you plug a composite signal it should display on the internal monitor ?.

  4. #4
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    got the screen fixed


    Now the hard part i have checked the links on how to replace the roms using the flash chip and don't look very easy.

  5. #5
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    Now replaced the digital board an it beeeps! and works can get onto the menus setup do stuff great!.
    Then checked the monitor board that have 3 diodes in terrible shape replaced them and everything seem great well for like 15 minutes.
    Monitor board:





    When the terminal is turn on the screen is centered but will slowly crawl to the left side and then start rollovering a little then will stay like this for the rest of the time is running, if you turn it off and let it cool down the same will hapen centered first and then slowly will crawl to the left side like this pics.

    Someone that knows why can this happen ?.



  6. #6
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    First check your 12Vdc supply if that is stable.

    Than check if it is really the center or width problem.
    Width than both sites left and right will move.

    Then look at the Center circuit A103. It is a DC off-set circuit.
    So think one of the capacitors are the trouble makers.

    A little trick is also to use some Heat apply ant and/or Freezespray.
    That way you can find easy your bad component by localy heat and cold on specific component.

    It could be a bad resistor, seldom, but with the trick you are able to find it.

    VT100-sch-part-center.jpg

  7. #7
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    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.

    VT100-sch-part2-CR409-Blown.jpg

    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

    VT100-sch-part2-Center.jpg

  8. #8
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    Going to check all your suggestions thanks for all this information i was a little lost on what to do next .
    Last edited by soviet9922; July 31st, 2019 at 07:09 AM.

  9. #9

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    It looks like the picture is wider in both horisontal and vertical direction. This can be caused by lower HV voltage. Lower HV can be caused by low 12V input voltage. But then usualyy the deflection is affected so there is less deflection current.

    It is a good advice to check the 12V input to the board. 12 is regulated by a secondary side regulator so it might be someting wrong there. (since the +5V appear working just fine)

  10. #10
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    Quote Originally Posted by MattisLind View Post
    [...] one may need to make an adapter [for the ROMs]. I did this based on a design on OSHpark. Here is a link to my page: http://www.datormuseum.se/peripherals/terminals/vt100
    Oh very nice writeup. I've linked to that from the terminals wiki page for the VT100.

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