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Thread: TV Typewriter replica project back on track

  1. #601
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    Another interesting thing I learned..

    If I connect the same clocks I'm told to connect to B1 to B6 instead, I get the proper output. For example, if I connect clock R to B6 instead of B1, I get half @s and half blanks on the sreen. Q gives me two stripes of @s... etc.. but this is the result I should be getting if connected to B1. The H clock, which is supposed to produce two vertical stripes of @s does that but then gives me one column of @s at the far right.

  2. #602
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    At the prompting of Don I decided to add the 2524v ahift registers to see what happened. They do change the behavior of the machine a bit.. now if I hit Home the screen changes from @s to a randomish assortment of characters... usually a column of Hs or a whole page of 7s. I also notice if I switch from Page A to B it changes to @s again and then if I switch back to A it redisplays whatever characters the Home button produced. So I think it the memory itself is sort of working.. storing info. I have tried grounding say A2 while hitting Home.. no change in behavior.

    I think the cursor board needs major debugging though. I cannot control the cursor at all, and when the cursor switch is on I still have two cursors. These disappear if I switch from Subtract to Add. Very strange.

  3. #603
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    I jumpered A1-A5 to ground and did a Home test. Voila.. blank screen! I do appear to have one bad 2524 in there.. when I was firing it up initially I would get a field of Ps instead of @s.. changed that out.. got the @s.. now I press Home and we have a nice clear screen.

    Don thinks my double cursor has something to do with IC7 on the timing board. I've tested extensively but unless ALL my 8288s are bad, everything looks ok. There's no shorts, the connections from IC pins to socket is good... yeah. I've ordered some spares anyway just in case though.

  4. #604
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    Toldja. Your SR IC problem sounds more to me like a bad connection issue.

    Did you get your USB scope going?

  5. #605
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    I'm still figuring it out. I cannot get it to show any waveforms. I think I've set it up correctly but still wading through the (awful) manual.

    Meanwhile I have ordered this: https://rover.ebay.com/rover/0/0/0?m...2F302132526962

    Is there a way of simulating a keypress while I await the part I need to connect my ascii encoder? I tried grounding a few address lines and then toggling the keypressed (pin 22) by grounding that.. no dice though.

  6. #606
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    You could have gotten a decent standalone for not much more. My problem with the USB scopes is that they'll work as long as the host software (PC) allows them to work. Somewhere here, I have the analogs to the USB scope--EPROM programmers and logic analyzers that used the parallel printer port.

    Of course, once Windows NT-based OSes came in, these were history, since it got a lot more difficult (but not impossible) to access the printer port (if it existed--check around on modern desktops and see how many have printer ports). Also, since these were "old" products by then, there was no interest on the part of the vendor to update the software.

    On the other hand, a 1960s tube-type scope still works fine.

    Please forgive my ranting.

    As far as the keyboard simulation, let me check. It could well be that the keypressed strobe is of a different polarity.

  7. #607
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    You should be able to strobe keys into memory by grounding "C" on the keyboard edge connector. Note in the errata, the missing labels are called out. If that doesn't work, check that C is high with nothing connected--and check to see that you don't have "protect" enabled.

  8. #608
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    I think Pin 22 connects to C.. I'll check that.

    Also I just had a thought.. I don't know if the diode I picked for the encoder is correct. The original encoder parts list is here: http://www.swtpc.com/mholley/RadioEl..._1973_pg58.jpg -- it calls for a 1n4002 rectifier diode, which is what I have right now for 10V operation. It says use with 12v can be had with an 8.2v zener diode.. but I didn't think about wattage.. I got a 1W... is that too much/little?

  9. #609
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    You've got me struggling here. I don't see any 1N4002 diode in the parts list of the keyboard encoder. I do see a 1N4002 in the teletype adapter circuit (for current loop)--but that's not a zener.

  10. #610
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    Yeah I screwed up. I was looking at the parts list for the teletype circuit. Doh.

    The actual diode I installed that handles power is a 1N4736. 1 watt. I ordered the right thing, but regrettably it all literally comes on the slow boat from China. Because Canada Post takes 10 weeks to even touch anything originating there it'll be a few months probably before I can test with keyboard. Wish I'd thought to deal with that earlier.

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