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Thread: Homebrew TVT I picked up

  1. #21

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    Quote Originally Posted by falter View Post
    As usual, I have confused things. Chuck said character generator... I crosswired that in my head to EPROMs. But yeah, when I did the testing I did find good continuity between the battery holder and pin 15 on each of the EPROMs. So I'm not sure what is going on there.
    The character ROM is the MCM6571. Pin 1 should go to the negative lead of the battery. I suspect you saw all the rest of the parts connected across +5V and ground as the current path. The battery should have one lead connected to ground ( measured as a TTL ground pin ) and the other negative lead to pin one of the MCM6571.
    I'm not sure why they created a separate supply for the MCM6571 as they need a -9V supply for the 1702s. The 1702s actually need a 14V supply ( 5V plus the -9V is 14V total ). Measure carefully with a low ohms meter.
    For the MCM6751, they could have used a voltage divider from the -9V and that would have been enough a 10ma divider would supply the 10ua easily. I'd most likely have used a 1K and a 500 ohm to create the divider. The error would be so small as to not worry.
    Dwight

  2. #22

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    I suspect the 1702s are to create a different data for the AY-5-1013. It may be that they'd been using the TVT for something other than an ASCII serial input. I'm not sure what the 1702 on the board at the right does but the two that are close to the AY-5-1013 look to have their data lines connected to the AY-5-1013 ( Falter can you measure these? ). It is possible that the AY-5-1013 has its data line connected to the address lines of the 1702s. Maybe one is used to translate data input while the other is to translate data output. The third one on the separate board is a question still.
    So, check the AY-5-1013 data lines. I suspect one set go to one of the 1702s address and to the other 1702s data lines.
    Dwight

  3. #23
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    EBCDIC<->ASCII perhaps? But what of the third 1702?

  4. #24

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    Quote Originally Posted by Chuck(G) View Post
    EBCDIC<->ASCII perhaps? But what of the third 1702?
    That makes sense. He'd need to trace the address and data line of the third one to try and figure out. As I recall the IBM had some funny special displayed characters. Maybe it is an additional character ROM for the things that are not typical ASCII to be displayed.
    Dwight

  5. #25
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    That's strange. I have EBCDIC character generator ROMs from the early-mid 70s. They weren't rare--and given that this thing was circa 1979, "rolling your own" from a 2716 was a very practical option. Same for keyboard encoding.

    On the other hand, if this thing was EBCDIC, the Moto character generator would be inadequate for the full EBCDIC set, as printable EBCDIC from cent to vertical line (0x4a - 0xfa) is 177; more than the 128 afforded by the Moto character generator.

    Check the wordsize jumpering on the UART; if it's 8 data bits, then EBCDIC makes some sense.
    Last edited by Chuck(G); June 18th, 2019 at 10:44 AM.

  6. #26
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    Ok. Pin 1 to the negative battery terminal resistance is zero. So there does appear to be a link there.

  7. #27
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    Okay, I pulled the 1702s and decided to test the PSU and then reconnect to the computer if nothing went sparky sparky.

    And nothing did.. was a total non-event, PSU seemed fine. So I put it all back together with the 'computer' (boy, that was a chore) and then fired up, still with the 1702s out. So far voltages look okay, but definitely not getting anything useful onscreen.. just these weird diagonal lines:

    https://drive.google.com/file/d/14Gz...ew?usp=sharing

    Maybe the EPROMs come into play somewhere here. I don't know. I'm going to finish testing all the voltages for the EPROM power pins, if they line up, might as well give'er. Good chance they're dead anyway.

    One question - this thing has one of those BNC style connectors.. I'm assuming the pinout on those is the same - ground on the outside and VID signal on the inner pin?

  8. #28
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    Okay so checking voltages on the 1702 sockets:

    Pin 12 - VCC - +5.05V
    Pin 15 - VBB - +5.05V
    Pin 16 - VGG - -8.98V
    Pin 22 - VCC - +4.98V
    Pin 23 - VCC - +5.05V
    Pin 24 - VDD - -9V

    I think that's correct yeah?

  9. #29
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    Yes, that's the way that BNCs are used. Doing it a different way would be nonsense. Those diagonal lines could be straightened out to vertical lines, if you'd adjust the vertical sync. But really, this is a job for a 'scope.

    The voltages on the 1702A look to be correct.

  10. #30

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    The connection from EPROM to the UART may be through buffers. Look up the specs on the UART as this part was not software configured, like the 8251 would be. It is configured by connecting the pins to ground or pullups ( in rare case I/O latches ). You can find out things like baud rate, parity and data width with an ohm meter and the spec sheet.
    Dwight

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