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Thread: Homebrew TVT I picked up

  1. #201
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    Here is a video of it operating more or less as it should:

    https://youtu.be/oOEAWxSb1PQ

    I found a bad 74193 in the video board that wasn't counting across columns properly. I also (re)learned that you can clear screen on a CT-1024 compliant terminal from SWTBUG by issuing the C command. I did that, and it works.

    The only weird thing is the way the horizontal changes. When the unit first powers up, I have to dial in the monitor so that everything is way over to the left before it's readable. When I (C)lear, it suddenly moves everything to the center and with a bit of adjustment it almost perfect. When I (P)unch, suddenly it wants me back over the to left again. I'm hoping better documentation of the circuit will explain which parts of the unit are controlling this.

    I've also ordered replacement caps for the PSU.

    Otherwise, I'm hoping a socket changeout and such will render this thing fully usable. I think I'm gonna test out the tape input output to see what that does, if anything.

  2. #202
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    It actually kind of seems like the more stuff you have onscreen, the more likely the horizontal sync changes. Hrmm.

  3. #203

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    Wow, I saw S records.
    The worst is, it may be something close to the socket. Hopefully it is the socket.
    Dwight

  4. #204

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    I still would like to understand what the EPROMs are for. Once you have things stable, I think we can do some scope work and resolve what their purpose was. It is still funny that they are there. Having two makes some sense but three doesn't. Finding out what they do is also a good idea because they would need to be backed up.
    It seems it was still build close to the original TVT design from the description in the book. It is quite a bit different than the one in Radio-Electronics magazine, though.
    Dwight

  5. #205
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    For sure.

    I'd like to find out how the tape encoding system works. I'm curious how it is initiated/stopped. On an AC-30, you have to toggle a switch on and off to record what it coming off the terminal/computer. There's no switch here, so is it encoding all the time? Gonna hook up a tape recorder when I get a sec and see if any signal is there.

  6. #206
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    Maybe I should just go ahead and build one of those 1702 EPROM reader/burners suggested to me here and elsewhere. Might be an interesting project, and useful from time to time.

  7. #207

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    Quote Originally Posted by falter View Post
    For sure.

    I'd like to find out how the tape encoding system works. I'm curious how it is initiated/stopped. On an AC-30, you have to toggle a switch on and off to record what it coming off the terminal/computer. There's no switch here, so is it encoding all the time? Gonna hook up a tape recorder when I get a sec and see if any signal is there.
    Most all micro-prototyping boards that use cassette use Kansas City format or Byte format or modification of it.
    Again, yet another reason to lock your logic probe in the desk and start looking at levels and wave forms on your scope.
    Dwight

  8. #208

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    Quote Originally Posted by falter View Post
    Maybe I should just go ahead and build one of those 1702 EPROM reader/burners suggested to me here and elsewhere. Might be an interesting project, and useful from time to time.

    I suspect the keyboard make ASCII parallel. I recall from before that when you hit a key, you said that it held the value. You should be able to probe the address in and the data out of the keyboard EPROM, even with your probe.
    Just a few letters, like A, B, C will tell us what is up with that EPROM. Just a simple table will give us some clues. Also, for the other two EPROMs, checking A0 for changes and when should be interesting.
    Although I now have a way to read 1702s, just some simple switch and LEDs are enough to handle 256 bytes. That is what I did when I got my SIM4-01 that had the code in 1702s to read and write 1702s.
    Dwight

  9. #209

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    As for the horizontal problem, it is again something for you to look at with your scope.
    Dwight

  10. #210
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    I haven't much time tonight but I hooked up the recorder just to see what would happen. If you aren't punching anything and just working with the computer, it produces a continuous leader tone. When you tell the 6800 to punch tape, it changes and emits a series of noises, similar to what you'd expect in a segment of tape where there's data. However, the sound it produces seems to be repetitive. I tried a quick punch of all memory contents, running the tape on record for about a minute. You could hear essentially the same pattern over and over again, sort of like 'whaaaa-donk-whaaaa-donk'-whaaa-donk'. I tried actually inputting data in a short series from $0000 to 00A0 and recording that, and it did produce sounds for a shorter time period, indicating the circuitry knows when to stop, but when I attempted to read it back, nada.

    Anyway, that'll be a job for the scope when I get done with a work project. I'm impressed it does anything at all!

    Kinda cool to see it doing something after who knows how many years tucked in a shed. It's sad to think the builder is gone, but his work is still very much alive.

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