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Thread: TV Typewriter replica project back on track

  1. #641
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    From what I gather from the article, the I/O to the TVT is done via separate 8-bit I/O ports, not via the LEDs.

  2. #642
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    So at last I have, I think, 8V zener diodes. I got a bunch of 1N4738As. I'm not sure if these are 8.2 or 8.5V ... mouser says 8.2v but other sites say 8.5v. Or if it even matters. Basically I need one to adapt the ASCII encoder I built from 10V power input to 12V. I'm hoping if they're 8.5V that doesn't go too far in the wrong direction.

    So I'm going to be feeding +12V (I think) from the TVT to the encoder.

    The only part I'm having trouble wrapping my head around is how the encoder will 'speak' to the TVT. By default, if no key is pressed, the encoder sets the outputs to low (actually, I think it's off completely). When you hit a key, it then brings the relevant bits high. On the TVT, all the ASCII code inputs (A1-A7) are high by default, and code selection is made by making certain bits low. So I'm not sure if this is going to work the way I'm hoping, or if i have to adapt.

    Another thing I have to wrap my head around is how keypressed works. I believe on the encoder it goes high but on the TVT it must be brought low when key is pressed.

    I'm assuming I can do no harm by connecting all the inputs and keypressed as things are (assuming the voltage is ok from the TVT after the zener is in)?

  3. #643
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    Okay so I got the ASCII encoder all wired up to the TVT annnnnnd...

    Nada.

    Not a darn thing.

    It's getting power, but now when I hit keys we're not switching from a low to high state on any of the A1-A7 bits. I *am* hearing a change in tone and a brief flicker on the logic probe when a key is pressed, but that is it.

    I probably should have verified the encoder was still working as it was before I changed diodes to the 1N4738A. I don't think I did anything wrong there - the magazine article for the encoder says if you want to adapt to 10V, just change to an 8V zener. But it's like there's not enough power or something. Something weird anyway. I was running it at 9V before with the 1N4736...

  4. #644
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    Sorry to hear about it. Unfortunately, I've lost the link to your encoder article. Care to supply it again? I recall that I didn't much like it.

  5. #645
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    Yeah you don't like this one.

    http://www.swtpc.com/mholley/RadioEl...RE_Apr1973.htm

    Unfortunately it is what was used in the prototype.. and that is what I am replicating.

    I removed everything but power connections and still not working right. Might change diodes again and try from 9v again.

  6. #646

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    I was quite interested in this thread but .... might take me a while to read all 65 pages! Is there a summary of where you got to and the issue you fixed so far? (I am just being a bit. lazy and trying to avoid 65 pages).

  7. #647
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    Okay, here's where I'd start. Power the keyboard decoder up (you don't need the TVT for this). Check the voltage between pins 4 and 11 on each of the ICs. That is, check the voltage at the IC pins, not at the solder pads at the socket. You want to flush out any possibility of bad sockets. The voltage should read (if Don's operating these ICs within specifications) somewhere between 3 and 4 volts. RTL is pretty forgiving, but voltage below 3V should ring alarm bells.

    Next, look at the voltages on the input pins of the hex inverters. They should be static, with the exception of the "key pressed" signal at IC3; i.e. they should change only as you press and release keys--they do not pulse.

    Now, check the outputs of the inverters--they should track the inputs. That is, if an input is at 2-3 V, the output should be near 0--and vice-versa.

    Finally, the only signal that should pulse will be the "key pressed" signal on IC3--you should get a nice brief pulse every time you depress a key.

  8. #648
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    Thanks Chuck. I will have to run it off the 12V from the TVT (I've disconnected the address lines and all that for now). I'll check the voltages. I should have checked all this before hooking up, but the 9V adapter I was using blew up. I'm also trying to remember now, as it has been a year or so, why I have the + line connected to CTRL...

    Anyway I'll check the voltages and get back to you.

  9. #649
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    Quote Originally Posted by Chuck(G) View Post
    Okay, here's where I'd start. Power the keyboard decoder up (you don't need the TVT for this). Check the voltage between pins 4 and 11 on each of the ICs. That is, check the voltage at the IC pins, not at the solder pads at the socket. You want to flush out any possibility of bad sockets. The voltage should read (if Don's operating these ICs within specifications) somewhere between 3 and 4 volts. RTL is pretty forgiving, but voltage below 3V should ring alarm bells.
    Nope.. voltage between pins 4 and 11 is 9.1V on all of them. That's with +12V connected from the TVT for power.

    Next, look at the voltages on the input pins of the hex inverters. They should be static, with the exception of the "key pressed" signal at IC3; i.e. they should change only as you press and release keys--they do not pulse.
    It's a bit tricky with only two hands to check, but I checked a couple of the outputs on IC3 and IC2. It seems like they are at 0.110V and do not change at all with any keypress.

  10. #650
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    Okay, 9.1V is pretty high for an RTL device, but it shouldn't be fatal. But there's something wrong.

    Measure the voltage across D27. You'd stated that it was an 8V zener diode, so you should read exactly that. The voltage therefore, across R20 should be 12V - 8V = 4V.

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