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Thread: VT420 troubleshooting

  1. #1
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    Default VT420 troubleshooting

    Does anybody have experience troubleshooting a VT420? I have one that does not display on screen when powered up but makes a continuous clicking sound (switch mode startup?). I have the service manual but I have not been able to find a circuit diagram. I cannot see the heaters glowing on the CRT and no beep or LED's on keyboard. Thanks.

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    Quote Originally Posted by PG31 View Post
    Does anybody have experience troubleshooting a VT420? I have one that does not display on screen when powered up but makes a continuous clicking sound (switch mode startup?). I have the service manual but I have not been able to find a circuit diagram. I cannot see the heaters glowing on the CRT and no beep or LED's on keyboard. Thanks.
    Noot got any info, but does sound like the PSU. Might be wirth asking on the DEC Legacy list..
    Dave
    G4UGM

    Looking for Analog Computers, Drum Plotters, and Graphics Terminals

  3. #3
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    Circuit diagram for the 420 is not available on line (i've looked)

    I am trying to get mine working, but its very dim and all squashed up. Need a HV probe for my Fluke to see if the flyback is putting out the correct voltage (or if the tube is on its way out), and annoyingly I cannot find mine anywhere.

    May have some spares from my messing about. Will let you know.
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    Thanks both, I have also asked on a couple of DEC Facebook groups. No joy yet. @Gary, you will find in the service guide a description of each potentiometer function on the circuit board. You may be able to get it a little brighter with the subcontrast pot. I've always used non conductive tools when adjusting any pots or inductors on a monitor/crt board.

  5. #5
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    It’s completely antidotal and may be way off track but two things I know from some time spent fixing both TV sets and later switching supplies. From what you describe it sounds like the switching supply is coming up and instantly shutting down like its supposed to do in the event of a failure anywhere else in the terminal.
    From the back of the set on the right hand side is the switching supply. Do not think you have issues with the hot side of the supply because of what you say so I would look at the output of the supply just past the switching transformer and check for short across one of the two DC outputs, maybe something like a shorted capacitor right in that power supply block. Another thing to look at is to check the Horizontal output transistor located over on the heat sink by the flyback and the damper diode next to the flyback. Also use to see a lot of failures on small displays of thinks like the yoke on the tube where you were able to pull the four pin plug for the yoke and the set will come back to life if its bad but beware there will be no deflection and you can quickly damage the tube this way.
    Beware!! That display has a section of the PC card that’s at line potential. Be extremely careful where you touch or ground to and do not connect any test equipment to the HOT part of the chassis. This can kill you! Also remember that the display develops around 15 kV from the flyback up thru that red wire to the tube so once you get it working don’t mess around with the red wire or the flyback. Unlike the hot or primary side of the switching supply it won’t kill you but can easily knock you on your ass.

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    Disclaimer: I am not telling you to do this unless you have an understanding of working with high voltage analog circuits, and all of the relevant safety protocols and procedures involved. The display has lethal voltages involved and once the cover is removed they are exposed. The protective shielding will not prevent you or protect you from coming in contact with lethal voltages that are involved.

    Looked at the schematic and also one of my 420 and see where if you look by the +5 volt regulated pot you will see three small capacitors, C908 is the +12 volt feed, C910 is the +5 volt feed and C913 is the -12 volt feed.
    With the display turned off check the DC resistance to ground on each power bus. It should be at least 50 or so Ohms to ground on the +12 and +5 volt bus with around 500 Ohms on the -12 volt bus. You can use the ground strap on the tube for a meter ground.
    There are a handful of electrolytic around the power supply and if any short that will shut down the power supply after a couple cycles.
    If you can get the board out check the resistance to ground across C619, that’s the + 41 volt source for the Horizontal output transistor. If there is a short or low reading you may have a blown horizontal output transistor.
    If all that’s good you may want to look at C804 the big electrolytic on the hot side of the power supply along with C805 and C808 being if they are open they can keep the power supply from starting. Just remember they are on the HOT side of the chassis and all connections to the AC mains must be removed before doing any of this.

    I do apologies if you do have a background and training in this in terms of overstating the safety aspects but this is right up there with disassembling and working on switching supplies and if you have never received any training on this it is a dangerous thing.

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    Thanks wupf and Qbus, great we now have a diagram. I have some experience fixing monitors (Zentec, Redifon, Newbury Data and Wyse mainly), albeit it 25 years ago. I will in this instance make all the "cold" component checks first. I have a capacitance meter, multimeter and scope.

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    Quote Originally Posted by PG31 View Post
    Thanks wupf and Qbus, great we now have a diagram. I have some experience fixing monitors (Zentec, Redifon, Newbury Data and Wyse mainly), albeit it 25 years ago. I will in this instance make all the "cold" component checks first. I have a capacitance meter, multimeter and scope.
    Sorry I don't have a name. I am close and think I have a working VT420 so let me know if you fail.

    Dec Legacy is NOT a facebook group its a groups.io list. Most members avoid facebook like the plague but do exhibit their systems in Windermere from time to time.


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    Dave
    G4UGM

    Looking for Analog Computers, Drum Plotters, and Graphics Terminals

  10. #10
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    Hi Dave, I'm Paul. I've removed and cleaned the circuit board. I found the top of transformer T802 inside the terminal. Not 100% sure if this transformer has blew. I'll remove it from PCB at the weekend and check for open circuits and shorts. I took some photos but I'm having problems uploading them due to size being too large.

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