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Thread: TV Typewriter replica project back on track

  1. #621
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    Hmph. So it seems like the circuit that handles KEYPRESSED is ok. Everything seems to feed through to the input clock line for IC2. And yet nothing is entered and the cursor doesn't seem to advance.

  2. #622
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    Made a tiny bit of progress. I began to be suspicious of these 74197 counters (which are all dated 1969 or 1970) on the cursor board. So I started swapping them. To my surprise, I went from having two winking cursors onscreen to eight! I kept swapping around trying to figure out what the reason was but no luck. Anyway, on a whim I switched to Subtract mode.. voila! Suddenly I had a normal, single, blinking cursor. Triggering keypress moved it a step backwards. But going back to 'Add' left me with the same problem as before.. multiple cursors all pegged to the left side of the screen and winking erratically.

    I suppose its possible these 74197s are just bad.. but I have 10 of them and it'd be surprising to me that they'd all be bad.

    Here's video of it:

    https://www.youtube.com/watch?v=yFkNMSjDO5Q

    I'm suspecting maybe something's screwy with bus lines 27 and 30, which control direction.
    Last edited by falter; Yesterday at 11:02 AM.

  3. #623
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    Hmmm.. I wonder if I'm the victim of a typo here. Check out the schematic: http://www.swtpc.com/mholley/TV_Typewriter/page_16.pdf

    It shows C3 as 620pf. But in the parts list here: http://www.swtpc.com/mholley/TV_Typewriter/page_06.pdf

    It shows as 6200pf, and that's what I installed... seems odd that the C3 would be 10x the capacity of C4..

  4. #624
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    I see on the schematic and parts list C3 is 620 pF and C4 is .0062 uF or 6200 pF. (US conventions didn't use nF back in those days). Like I said, the cursor board circuit is very strange and I don't like it one bit. Apparently, neither did Don because he dropped the whole scheme on the TVT II.

    At any rate, if you've got your scope working, check pin 2 of IC1 for a brief pulse when you insert a character.

  5. #625
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    Yeah I was off on that one.

    I've made another discovery. What I'm interpreting as 'multiple' cursors is actually one cursor continuously moving down row by row and over and over again in a loop. If I switch to Add, then switch to Subtract at different intervals, I can stop the cursor at different points. It's like it's endlessly linefeeding.

  6. #626
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    Remember that any control character will cause a linefeed. Maybe it's time you should look at the detection circuitry for that.

  7. #627
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    Well the interesting thing is Line 27 and 30 are both supposed to be open for one direction and shorted for the other. However, 27 is always open no matter what.

  8. #628
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    Don had me connect the self test to board connector pin 21 as well as pin 15 of IC10 on the memory board. I sent this back to him.

    https://drive.google.com/file/d/10N1...ew?usp=sharing

    Not sure what it tells me yet, but hopefully Don knows.

  9. #629
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    I am really hoping it doesn't come to this, but I may have to redo a couple of these boards.

    The TVT boards are supposed to be interchangeable - that is they can be stacked any way you want in no particular order. But I am finding I'm having inexplicable issues when they are assembled different ways. The fault, I think is partly the bus design itself -- the pins were not well aligned in the original artwork, and the design itself is clunky. But it may also be issues in my manufacturing process, especially places where the drill bit too hard or some copper lifted slightly. I'm going to persist with it a bit longer, but I think it's coming down to a round 3.

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