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Homebrew TVT I picked up

One thing that is confusing to me -- if you look at the photos I posted of the 'video card', there are several chips marked with '7430'. But looking at the underlying part numbers they don't match. Anyone know what's going on there?

I'm trying to figure out which chip is the shift register to see if that's doing its job properly. I'm thinking it's the 74165, which I've already replaced.
 
Strange that it's stamped exactly the same way on multiple chips from different vendors. But seems logical.
 
I've got quite a few like that myself. That's the only explanation that I've come up with. If there's a secret code there, I haven't been able to figure out what it would be.
 
I made a bit of a discovery - that top board on the far right, the one with the EPROM marked 3.. if that board is disconnected, that little 'cursor' line vanishes, and only returns when the board is reinserted. So I'm thinking that may well be some kind of cursor control board.

Unfortunately, if that EPROM is junk, I'm just spinning my wheels.

I'm continuing to swap ICs.. the only one I don't have is this lone signetics 8808.

What I'm aiming for right now is just to get well formed, if nonsense characters. From there I will trace out the keyboard circuit, see if anything is happening on any of the boards when keys are pressed. Also will find out where the serial output is (I mean, I'm assuming it has that).. and then see if anything is happening there. I feel like I'm close.

Thing that bothers me is, if it's a TVT or terminal, why isn't there a clear screen/home key somewhere?
 
See this is why I'm wondering about the '7430' stampings. That chip was stamped 7430 also. Kinda coincidental if it is a direct replacement for a 7430 and they stamped 7430 on it?

Made another discovery... the EPROM that is on a small board by itself that the keyboard feeds into - none of the data output pins were live. I discovered that VCC pin 12 wasn't getting +5v. Socket is a bit loose. Put some wire in there, now when I type keys the data lines briefly change depending on what I type. No effect on the screen, but I'm guessing that's a sign that that 1702 is doing something. It's interesting, because when you hit a key on the keyboard while probing the keyboard pins directly, the output changes and stays that way until you hit another key. But the 1702's outputs only change briefly and then go back to where they were. I guess it's doing some kinda strobe function.
 
Tried that... doesn't change anything - just distorts it worse.

I've swapped nearly every IC.. checked all the RAM. I actually pulled the RAM entirely and fired up, and got (slightly distorted) boxes. Only ICs I don't have handy to try swapping are 7493s. The problem is I'm sure there is some rust in the sockets. Would that deoxit stuff help?
 
I wonder if it's even generating horizontal sync.

DeOxit sure won't hurt. I like to use an ohmmeter to check the resistance from each chip's pin to the bottom of the socket.
 
I have been checking resistance from pin to socket. That was how I caught an issue with that EPROM. The problem is there's hundreds of pins, and this thing is so intricate and fragile that I'm not able to twist the board over and really probe a lot of the innermost ICs. Without any sort of schematic (although I have been following Chuck's advice and looking for circuits using similar ICs/arrangements), I'm terrified of breaking a wire loose.
 
I'd strongly recommend carefully reviewing Don's TVT Cookbook. He discusses things present on this kit, such as using 1702s to re-map characters, cassette interfaces, etc. While not an exact rendition of what you have, I believe that Don's book was used as source material.
 
I have that very book and have been eagerly thumbing through it. I'm starting to wonder about the horizontal sync also and am trying out this scope at targeting that question.
 
Found the keyboard strobe.. broken wire to the keyboard connector. Still no action onscreen though. I think we are dealing with corrosion issues for sure... gotta get some deoxit and at least try that against it.
 
Here's a suggestion that might work for you. Do you happen to have a 24-pin male DIP header such as this example?

It appears that at least a couple of your 1702s serve the purpose of "translating" one 8 bit code to another. So, if you wire up a DIP header such as the one above, with A0->D0, A1->D1....A7->D7, you can get a substitute for the 1702 that creates a "straight across" mapping. The purpose is to identify any faulty 1702s. If you use this plug in a suspect 1702 position and things are wrong, but look better, you have the culprit.

As far as the keyboard goes, I'd check the output of the UART to see if there's any action there when you press a key.
 
Remove the chips and carefully use a brass brush in the sockets with the DeOxit.

Next I would see what the video output looks like on an oscilloscope.
 
Is the output of the UART SO pin 25? I don't see anything happening there. I see a 74123 nearby getting triggered as I type... but I see nothing on the SO pin.

I don't have a 24 pin header but I do have some 24 pin sockets.. couldn't probably achieve the same outcome. I think the first EPROM is operating.. I can see the output pins changing as I type.. unless they would do that regardless.

I traced that 9 pin molex a little bit... it does seem to conform somewhat to SWTPC standard... the first of the three pins is ground, the two clock pins are tied together, pin 4 is out.. and I think pin 5 and 6 are going somewhere into the serial board.
 
falter said:
What do you make of stabillant 22? I've had that suggested also.
Click to expand...

This is the first I've heard of it. The company isn't very good at explaining why I'd want to use their product. My best guess is that this is something I'd use after the connections are clean. But I'm not certain.
 
This is what someone on Facebook said about stabilant:

Corrosion and sockets - you need Stabilant 22. One at a time remove chips, put stabilant 22 in the socket, and put the chip in. It has conductivity for a few microns but infinite resistance over that. Fixes socket issues quick.
 
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