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Thread: Homebrew TVT I picked up

  1. #1
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    Default Homebrew TVT I picked up

    At least, I assume it's a TVT:

    https://www.ebay.com/itm/Vintage-Com...rdt=true&rt=nc

    I don't see any microprocessor and I don't think there's enough there to be a TTL-based computer. Seems like the big board is basically the timing and video, the middle is obviously serial communication, but the board on the right.. not sure. I think there is a third small board underneath there.

    Not sure what the function of the batteries would be -- offline data?

    Going to need one heck of a clean, and hopefully it doesn't burn my place down.

  2. #2
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    I don't know. Dates point to a construction year around 1978, which is pretty late for most of the stuff. There are, in addition to the Moto character generator ROM, 3 1702A EPROMs, but only 2x2102 (1Kx8) memories. And yes, there's definitely another layer of perfboard under what's visible. Strange setup, but who knows...

    I wouldn't be surprised to see a microprocessor hidden below-deck--it might explain the batteries.

  3. #3
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    I posted this on the Facebook group but will post it here. I finally got this 'TVT' in. Unfortunately the seller was not too careful about packing (sigh), but I don't think it took any critical hits, other than a broken slot which for now I was able to push back together. Here are some up close pics. The keyboard PCB, being thin, took some breaks at the mount points.

    https://drive.google.com/drive/folde...Ot?usp=sharing

    I am thinking this may indeed be a TVT of sorts, perhaps intended for use with a SWTPC system. Only reason I guess the latter is the presence of the Molex header, which is set up almost identically to the way the way it would be for use with a SWTPC MP-C or MP-S card. I would have guessed it was just a CT1024 in wirewrap form, except for the use of a 6571 instead of a 2513 as character generator, the presence of EPROMs, etc.

    I did pull the top right card to see what was underneath, but there's no microprocessor there. So if this thing is a computer, its brain is implemented in TTL somehow. I don't know.

    Really makes me wonder about the uses of the three 1702A EPROMs. My best guess is this is some kind of smart terminal that had the ability to load data in from tape. I note it has what appear to be connectors for a tape recorder. There's also a voltage regulator with two removable wires that appear to attach to it but are not attached. Not sure what purpose it serves.

    Thoughts are welcome. I've no idea how to proceed here -- I don't want to plug in and risk frying the 1702s without knowing what is on them, but I don't think I have a way to dump them either, unless maybe my Challenger II could be used for that purpose. Can't remember if it has spare 1702 sockets.

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    You do like challenges, don't you...

    Looks fairly well built though, FWIW.

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    No microprocessor here, for certain. We see a Moto 8K character generator ROM, which requires a Vbb of -3V; hence the battery holder.

    The 1702s are a mystery. Why anyone would be using them after 1978 is a bigger mystery.

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    I've had some speculation of it being an RTTY terminal on Facebook. But that connector doesn't make sense to me.

    I suppose another possibility is that this is just a piece of a system someone put together. Maybe the other bits got lost.

    I'm tempted to just plug it in and see what it does. That huge cap on the PSU kinda scares me tho.

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    Start by powering up the supply without a load. It's a linear one and so should sit up and work. The big electrolytic is 50-50 probable dry.

    If you connect up the thing, make sure to add two good AA cells,or you might damage the character generator ROM.

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    Well, you're on the money about the battery function Chuck - I did a continuity test between one of the poles of the battery holder and pin 15 on each of the eproms (VBB) and sure enough, they link up. I guess there wasn't a way to generate -3V easily with the existing PSU?

    I *really* wish I had a way to read those EPROMs before attempting to power this up.. just in case I screw up somewhere.

  9. #9
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    Is it possible the 1702As are essentially behaving as low-rent PALs in this circuit? I know that was a thing people used to do with PROMs back in the day.

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    Well, they might be used in sequencers also. But I can't figure out why they're needed.

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