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Thread: Looking for VT102 repair info/advice

  1. #1

    Default Looking for VT102 repair info/advice

    I recently got my hands on a VT102, but it doesn't seem to be quite working. All the characters seem to be corrupted:
    IMG_2555-1.jpgIMG_2558-1.jpg

    Having had no prior experience with this kind of hardware, I'm wondering if anyone has seen this before and could tell me if it's something I could fix myself. Something wrong with the character generator ROM, maybe?

    I also received a VT240 (powers up but displays some kind of error about the main board), a VR201 monitor, and an LA50-RA printer, none of which I'm as interested in. So they're available for anyone interested who can pick up from central Massachusetts.

  2. #2
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    Are you sure that you have all your communication settings correct?
    Also, was the VR201 monitor compatible with the VAXstation 2000 or was it limited ot the DECmate/Rainbow systems? (which I assume use their own proprietary monochrome video signal)
    = Excellent space heater

  3. #3

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    The VT102 isn't connected to anything yet. Those two pictures are of the setup screens, and if I put it into offline mode and start typing, the characters are all messed up in the same way. So I don't think it's a communication settings issue unless it somehow has to be connected to display characters properly...

    I have no idea about the compatibility of the VR201. Is there some identifying feature or label I should look for? If it helps, I think it's a VR201-A (it's white).

  4. #4
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    Quote Originally Posted by iprefermuffins View Post
    I recently got my hands on a VT102, but it doesn't seem to be quite working. All the characters seem to be corrupted:
    IMG_2555-1.jpgIMG_2558-1.jpg

    Having had no prior experience with this kind of hardware, I'm wondering if anyone has seen this before and could tell me if it's something I could fix myself. Something wrong with the character generator ROM, maybe?
    Possible, but without consulting the schematic it could probably also be the video RAM or buffers; interesting that spaces in text show as $$s (bit3 high?) but the rest of the screen is blank.

    First thing to try: move any socketed chips back and forth a bit in their sockets if possible, or remove and (carefully) replace; assuming multiple socketed video RAM chips, try exchanging them to see if symptoms change.

  5. #5
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    Yes, the third bit from the least significant end is stuck. It could be the chargen rom or some other latch/buffer. I have not seen a VT102 FMPS (print set) on bitsavers or elsewhere. The VT102 is a cost-reduced VT100, but probably close enough to figure this out with only VT100 prints and the VT102 board. I have repaired a VT100 with a silimar problem once, but it took a few hours to get through.

    ISTR that the VT102 had very few (if any) sockets. I have one, but it's deep in the back of a closet.

    VR201 works with many machines. VT240, decmate II, decmate III, Pro series, Rainbow. The LA50 will make a nice local printer for either the VT102 or VT240.

    Lou
    Last edited by Lou - N2MIY; August 23rd, 2010 at 02:19 AM.

  6. #6
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    I believe the VT100 was a rebadged Televideo 925.

    If so, I gave Bitsavers a Televideo 950 Maintenance Manual (59MB file) a while back, and it has troubleshooting info for the 910, 825 & 950 in it.
    It might be worth pulling that down from Bitsavers.

    I have a couple TV910 boards, and a TV950 board (some working parts and some not working), so if you need a part, I might have it.

    Also see this post titled Wrapped Video - at the end is some info on who might have parts/knowledge for repairs.


    http://www.vintage-computer.com/vcfo...=Wrapped+video


    EDIT: Whoops - forget the comment above about a VT100 being a rebadged Televideo - I was thinking of a Stratus V101 which is a rebadged Televideo.
    In any case, the guy with the parts/repair knowledge (in the link to the other thread) also has DEC stuff.
    Last edited by Lorne; August 23rd, 2010 at 08:51 AM.

  7. #7
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    The VT100 technical manual is on bitsavers and is extremely detailed and well written. I am sure that most of the theory also applies to the VT102. I wish, however, that at least the print set for the VT102 would turn up.

    Lou

  8. #8

    Default

    Thanks for the input, everyone. I studied the VT102 board and the VT100 FMPS, and the affected area (char gen rom and the latches and buffers) seems to be the same or very similar. I'm at a loss for how to actually identify what's going wrong though. I have a feeling that a logic analyzer or oscilloscope would be useful here, but I don't have access to one at the moment, unfortunately.

    Lou, you mentioned you had repaired a VT100 with a similar problem before. Can you describe what you did to diagnose and fix it?

    Here's an interesting symptom: When I try to switch to the alternate chargen set (which there isn't a rom for), typed characters mostly show up as white rectangles as expected. However, the first few characters on each line appear to be cycling rapidly over a sequence of assorted symbols. I wonder if this indicates a problem with latch timing or something similar.

  9. #9
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    If the VT-102 ROMs are socketed I could potentially post the contents of mine. I won't have time to pop it open this weekend, besides, my workbench is already full. But I'll put this on the task list board and try to get to it soon.

    If you've already reseated any socketed ICs, look at the pins on the board for a bad joint. Also make sure boards are seated well and the connectors look good.

    Yes, an o-scope helps a lot with problems of this sort. A logic probe could see if a line is getting toggled, too.
    Mark, W8BIT http://saundby.com/

  10. #10
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    I pulled out my notebooks and can tell you about my VT100 repair. There was a stuck bit on the output of the DC011 video timing IC. Specifically LBA6.

    You will see that on page 4-62 and 4-63 0f the VT100 rech manual (my hardcopy one here is ek-vt100-tm-003) there are timing diagrams for the line buffer outputs. I watched them with a scope, looking for transitions, not correctly timed transitions, but any transitions, and LBA6 was stuck. With my logic analyzer, I could have looked for properly timed transitions, but it didn't come to that.

    From the timing diagrams, I was able to fabricate the LBA6 timing from LBA4 and LBA5 with a 7402 and a 7473A. The DC011 chip is unobtanium and the fun of this hobby for me is fixing everything (sound like Tony Duell?) See this photo of the "repair": http://www.vintage-computer.com/vcfo...achmentid=2421

    Your problem is slightly different. My problem produced screens of garbage, where yours the problem is further "downstream". If you have the VT100 print set, your problem is on sheet 5 (BV5). You need to look at the data lines on the line buffer ram outputs, tri state buffer, char gen latch, and if they're all good, I might suspect a bad input on the chargen itself. Remember, not only must the signal coming to an input be good, the input itself must be functioning. If I did not have my scope, I would build a crude logic probe with an LED and maybe a 74LS00 as an input buffer. If the LED flickers, you've got transitions. If not, it's stuck.

    Kind of off topic, but if anyone is intersted in examining the "raw" contents of the VT100
    chargen rom, I desoldered one, dumped it, and manipulated it in excel to draw the characters by turning cells red or white. Send a PM with a proper e-mail address and I'll send the spreadsheet.

    Lou

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