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Homebrew TVT I picked up

falter

falter

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Joined
Jan 22, 2011
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Vancouver, BC
At least, I assume it's a TVT:

https://www.ebay.com/itm/Vintage-Co...KyNi5tVPJo%3D&orig_cvip=true&nordt=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.
 
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.
 
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/folders/1rf7FAdmtQD1nhOnoEbiij-juC7CpdmOt?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.
 
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.
 
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. :)
 
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.
 
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.
 
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.
 
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
 
falter said:
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.
Click to expand...

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.
 
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.
 
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.
 
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.
 
falter said:
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.
Click to expand...

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
 
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.
 
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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