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Thread: Homebrew TVT I picked up

  1. #11
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    I read the little plug wires more closely. They clearly say 'RCDR' on them, so I assume it did some mind of recording or reading to tapes

  2. #12
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    Quote Originally Posted by falter View Post
    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.
    There is--the Vbb needs are very low current--it's basically a substrate bias. A resistor and a zener off of any of the -V supplies should have done it nicely, though there is a power sequencing issue on these old MOS ICs, that I seem to recall. In any case, the batteries would have lasted their shelf life.

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    Thanks Chuck. So you would be alright to just leave the batteries supplying -3v with the thing turned off?

    I'm going to do some testing today, hopefully not set myself on fire. Side note: I have an OSI 500 board from 1978 that uses 1702s.. so they did persist a while I guess.

  4. #14
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    If it hasn't been mentioned already you should probably put new opaque tape over the windows on those EPROMS the original has fallen off of.

  5. #15
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    Yes, you can leave the batteries in, as they supply a bias current; they don't actually take any part in operation and the current draw is in microamperes.

    Are you old enough to remember "C" bias batteries?

  6. #16
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    Never heard of C bias batteries. I'm 43.. so I don't know if that's too young or just ignorance.

    I've been working on cleaning this thing up... I'm trying to figure out the 7805 voltage regulator that has disconnected leads. The way it is set up, it has what appears to be a ground wire screwed and soldered into the top part that normally screws into a heatsink. There are two loose wires nearby, one which heads to the perfboard (presumably the +5V line) and then one that runs to the PSU (presumably the supply voltage). But there appears to be nothing attached to the ground leg. Are they just simply grounding it off the top here? I'm assuming that's what they're doing.

  7. #17
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    Is there continuity between the PSU ground/common and the heat sink? If so, you have your answer.

  8. #18

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    Quote Originally Posted by falter View Post
    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.
    I'm a little confused. The Vbb pin of the 1702s are only use during programming. During normal reading, it is suppose to be tied to +5V and not -3V. The character ROM is something else. As Chuck says, it needs the -3V bias voltage.
    I wouldn't power up the 1702s with the bias lead connected to -3V!!
    This sounds a little funny to me. I'm wondering if the 1702s were just stuffed into the sockets for looks??
    If you look at the operational specs on the 1702s, you'll see that they have no ground pin. These are PMOS devices. Unlike NMOS, they have relatively strong pull up but weak pull down. When connected to TTL, it will pull the TTL input slightly below ground but since the current is so small, it isn't and issue for the TTL device.
    The 1702s require -9V on the VDD pin. I'd remove them all before powering up. It makes me wonder why -3V would be wired to the 1702s???
    Dwight

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    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.

  10. #20
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    Good point, Dwight. I was a little murky on that one also.

    Unless and until we can actually see how at least one 1702A is connected to the surrounding circuitry, I'm really suspicious about their presence. As I mentioned,, the contraption looks like it was constructed in 1979, which would be very late for using the 1702s. Unless the builder had a bucket of them, it would make no sense. 1702s are quirky; require rather extreme voltages for programming (something like -48 and -35 for Vpp and Vgg) and quite slow.

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