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Thread: VT420 troubleshooting

  1. #11

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    Yep. That is the schematic I scanned. I posted it to cctalk last year, but forgot to cc Al Kossow. So it didn't end up bitsavers apparently. Got to fix that. It was an ex DEC field engineer that gave it to me. http://storage.datormuseum.se/u/9693...g-drawings.pdf

    My experience is that it is very unlikely that transformer goes bad. With the exception of one FBT I had no transformer going bad at all. When the PSU ticks it is very likely that it tries to start up but discover some kind of situation that causes it to turn off immediately. Over current situation is in my experience most common source. That could be the load itself or faulty output rectifiers.

    Check all the output rectifiers. Disconnect the load and use a proper dummy load. Some PSU don't like to have uneven load. Depending on how the regulation is done uneven load could cause crowbar circuit to trap and then there is over current situation as a result. In this case I cannot see any crowbar in the circuit. It seems to regulate on the sum of the 41V and 5V.

    Could be an electrolytic cap that is bad, but have yet to see an electrolytic go short circuit, more often I have seen them go open circuit or low capacitance. Tantalum capacitors on the other hand is a completely different story.

  2. #12
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  3. #13

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    I had a VT420 that was ticking like this and I spent a lot of time looking at the secondary side but couldn't find anything wrong. The problem turned out to be the transformer but not in the way that you might expect. There was a tiny break in one of the windings where is goes from the post on the PCB into the coil. It was the one on the primary side that supplies power to the UC3842 control circuitry. Enough power was being supplied by the startup resistor R804 to get the PSU going but of course not enough to keep it running. VCC on the UC3842 would quickly drop, the PSU would shutdown, VCC would come back up and the cycle repeated. I managed to resolder the winding back on to the post. I don't think this is a common problem but it's something to watch out for.

  4. #14
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    Don’t like the looks of that, maybe one of the diodes had shorted on the secondary side and somehow it did not shut down and burnt up the transformer? If anything went bad on the primary side like the FET failing or shorted diodes or what ever on that side would have thought the fuse would blow. Maybe that was just a transformer built on a Monday morning or Friday afternoon? You very rarely see a transformer fail in a switching supply without something causing it. I have never seen one fail without an external cause like a short on the secondary side.
    Don’t know if T802 is different for the 220 volt version from the US version that runs at 120 volts. Lot more voltage available to the transformer on the export version and if they used the same transformer maybe the EU versions are more prone to insulation break down? Today we take it for granted that most switching power supplies will work from 100 to 250 volts without issue but back when they were built don’t know if the inputs were that robust. They did go with a higher working voltage filter capacitor and maybe a couple other small changes on the 220 volt version verse the 120 volt version.

  5. #15
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    Taken the flyback out of the replacement board and put it in the original and the display is now filling the screen. The brightness is only just ok even with every external and internal pot wound up to the max but it works

    The other board is available for just postage if it would be of use ? The digital side seems to work ok, it just wouldn't fill the screen even no matter how i tweaked the vertical size etc (it also has no flyback either).
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  6. #16
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    Thanks for the info, I doubt I'll get time to work on this over the weekend now. My real job has taken over @Gary C, Glad you have made progress. I may take you up on the offer if I find my transformer is faulty as I imagine one of these will be hard to come by. I will send you a PM. Thanks.

  7. #17
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    @DAve G4UGM, I checked out groups.IO and found that I was already a member..doh!!. I've just replied to a post from earlier this year about a PDP-11 with VT320's.

  8. #18
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    Well, mine is working but its a bit dim.

    The heater voltage is spot on at 12V, the Screen and Brightness supplies to grids G1 & G2 are fine and at max you can see the raster,. The contrast circuit seems to be working correctly and wound up to max with about 55V at the cathode Pk-Pk, so it looks as if the tube phosphor is on its last legs.

    I do remember there were some 'life extenders' you used to be able to get that you put between the tube connection and the tube PCB, but dont think they are available now.

    A new tube is not really on the cards, shame really.
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  9. #19
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    Those old tube extenders or brighteners were a transformer that boosted the filament voltage a volt or two and got a bit more emission from the tube that way, don’t think that would work with a 12 volt DC filament.
    Are there things burnt into the screen? Usually you see that before the terminal displays die from lack of emission.

  10. #20
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    Thinking about the lack of screen burn, maybe this tube has some life. I can't be sure what part of the system this screen was used for but worth having another look.

    Putting the scope on the Cathode, the voltage between black and white screens is confusing me. Black screen appears to have a constant high voltage between the blanking periods,
    Black screen
    Black screen.jpg
    and the white has a signal that falls towards the 0V (E)
    White screen
    White screen dim.jpg

    However, looking at the circuit, the video signals drive a push pull amp with the contrast setting the gain from a 5V signal. Interestingly, there is a 5V adjustment on the board.

    Tweaked up the 5V and the screens contrast has increased

    Looking at the signal at the cathode, the voltage has risen about 10V.

    White screen dim.jpg White screen.jpg

    With all the messing about with the yoke, the linearity is a bit off and might be over driving things a bit, but hey it works.
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