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Homebrew TVT I picked up

Here is a video of it operating more or less as it should:

https://youtu.be/oOEAWxSb1PQ

I found a bad 74193 in the video board that wasn't counting across columns properly. I also (re)learned that you can clear screen on a CT-1024 compliant terminal from SWTBUG by issuing the C command. I did that, and it works.

The only weird thing is the way the horizontal changes. When the unit first powers up, I have to dial in the monitor so that everything is way over to the left before it's readable. When I (C)lear, it suddenly moves everything to the center and with a bit of adjustment it almost perfect. When I (P)unch, suddenly it wants me back over the to left again. I'm hoping better documentation of the circuit will explain which parts of the unit are controlling this.

I've also ordered replacement caps for the PSU.

Otherwise, I'm hoping a socket changeout and such will render this thing fully usable. I think I'm gonna test out the tape input output to see what that does, if anything.
 
It actually kind of seems like the more stuff you have onscreen, the more likely the horizontal sync changes. Hrmm.
 
I still would like to understand what the EPROMs are for. Once you have things stable, I think we can do some scope work and resolve what their purpose was. It is still funny that they are there. Having two makes some sense but three doesn't. Finding out what they do is also a good idea because they would need to be backed up.
It seems it was still build close to the original TVT design from the description in the book. It is quite a bit different than the one in Radio-Electronics magazine, though.
Dwight
 
For sure.

I'd like to find out how the tape encoding system works. I'm curious how it is initiated/stopped. On an AC-30, you have to toggle a switch on and off to record what it coming off the terminal/computer. There's no switch here, so is it encoding all the time? Gonna hook up a tape recorder when I get a sec and see if any signal is there.
 
Maybe I should just go ahead and build one of those 1702 EPROM reader/burners suggested to me here and elsewhere. Might be an interesting project, and useful from time to time.
 
falter said:
For sure.

I'd like to find out how the tape encoding system works. I'm curious how it is initiated/stopped. On an AC-30, you have to toggle a switch on and off to record what it coming off the terminal/computer. There's no switch here, so is it encoding all the time? Gonna hook up a tape recorder when I get a sec and see if any signal is there.
Click to expand...

Most all micro-prototyping boards that use cassette use Kansas City format or Byte format or modification of it.
Again, yet another reason to lock your logic probe in the desk and start looking at levels and wave forms on your scope.
Dwight
 
falter said:
Maybe I should just go ahead and build one of those 1702 EPROM reader/burners suggested to me here and elsewhere. Might be an interesting project, and useful from time to time.
Click to expand...


I suspect the keyboard make ASCII parallel. I recall from before that when you hit a key, you said that it held the value. You should be able to probe the address in and the data out of the keyboard EPROM, even with your probe.
Just a few letters, like A, B, C will tell us what is up with that EPROM. Just a simple table will give us some clues. Also, for the other two EPROMs, checking A0 for changes and when should be interesting.
Although I now have a way to read 1702s, just some simple switch and LEDs are enough to handle 256 bytes. That is what I did when I got my SIM4-01 that had the code in 1702s to read and write 1702s.
Dwight
 
I haven't much time tonight but I hooked up the recorder just to see what would happen. If you aren't punching anything and just working with the computer, it produces a continuous leader tone. When you tell the 6800 to punch tape, it changes and emits a series of noises, similar to what you'd expect in a segment of tape where there's data. However, the sound it produces seems to be repetitive. I tried a quick punch of all memory contents, running the tape on record for about a minute. You could hear essentially the same pattern over and over again, sort of like 'whaaaa-donk-whaaaa-donk'-whaaa-donk'. I tried actually inputting data in a short series from $0000 to 00A0 and recording that, and it did produce sounds for a shorter time period, indicating the circuitry knows when to stop, but when I attempted to read it back, nada.

Anyway, that'll be a job for the scope when I get done with a work project. I'm impressed it does anything at all!

Kinda cool to see it doing something after who knows how many years tucked in a shed. It's sad to think the builder is gone, but his work is still very much alive.
 
So I've revived this project now that I think I have a replacement card slot. I showed it to Adrian from Adrian's Digital Basement and he pointed out that the power supply had no fuse?? Is that something I should be properly terrified of? I was using it as is before. I'm assuming having it on a fused power strip probably isn't enough? If I were to fuse it, how would I figure out what amperage I need?

Many thanks!
 
It depends whether you are talking about the mains side or the low voltage side of course.

You 100% need a fuse in the mains side. Ideally (if you are playing around with vintage computer equipment (especially with the lid open)) the mains power should go through an earth leakage or residual current circuit breaker. That way, you get to live and your wife won’t have to dispose of your vast collection!

A mains fuse is not designed to protect the low voltage side of the power supply. This would only happen by ‘accident’ in the case where there was a serious overcurrent.

In order to calculate the fuse size, you need to look at the worst case current consumption for each of the boards you have in the system, sum them together, and allow for any potential expansion and a margin of error. Either that, or use a fuse close to the rated value of the power supply unit.

For example, if the power supply is rated at +5V 40A, then that gives you a clue as to the fuse rating - the next lowest fuse below 40A.

However, if the cards you have in the system work out to consume 20A (and you can measure this with a clamp on ammeter attachment to your multimeter), then a 25A fuse would be fine - unless you were going to expand the system (in which case I would start off at 25A and then re-evaluate that when I wanted to expand the machine in the future when I got there).

Dave
 
Thanks Dave! Yeah there is no fuse on the mains side.. and the leads for the mains are open and exposed. Whoever designed this thing had zero regard for safety, although apparently both of us have been able to operate it without incident thus far.

I've no idea how much amperage the boards draw. I would assume not very much.. most of the mains fuses I see have small amperage like 1 or 1.5a?
 
Okay quick question.

I had the TVT up and running yesterday - or at least, I had the power on just checking DC voltages. Today as it had been moved around a lot, I thought I would check the resistance between the hot and neutral pins on the mains AC plug. I am only getting 0.17ohm resistance. Does that make any sense? Or have i developed a short somewhere?
 
Actually I think I answered my own question.

I forgot this PSU has no power switch. It's just plug in and go. I went to my SWTPC machine, pressed power and resistance there dropped to almost nothing also. So I think that's normal.
 
If you have a linear power supply (i.e. a transformer) these things have two (2) components to limit the mains current; resistance and inductance. The resistance will affect the dc reading (which is what you are measuring with your multimeter) whereas the inductance will be the dominant effect when it is connected to an ac mains supply.

You actually need to measure the impedance of the transformer to know whether it is working or not. The resistance will tell you whether a winding is open circuit or not - that's all.

You do need a mains fuse. 3A would probably be adequate. In the UK all plugtops have to be fused - so the only choice is people putting the incorrect fuse in...

Dave
 
Yeah I'm going to figure out how to wire a fuse in. They didn't leave much space on the PSU board. Also going to get a new filter cap for it. It does have earth ground at least. I can't believe they ran it like this. Is a 3A fused power bar good enough as a temporary solution?
 
I've repaired the broken 44 pin cinch slot/connector, so that's all good now. I have to put my monitor in high impedance mode to get the characters to look properly. Previously, I had been able, for a short duration, to get the terminal to behave as it should with my 6800 computer attached. However now, when the computer is powered up you see cursor movement, some random characters and carriage returns, but it is not responding to keypresses or evidently sending anything out. I'll have to probe the UART board and see if anything is coming out of there or if something else failed in the last few years.
 
Ah.. the ultra thin PCB radio shack PCB that the keyboard is on took some damage during shipping years ago, and it looks like more cracks have developed interrupting traces. Fixed a few last night but found more on the front side. That might explain why it's not typing anything.

Also have to figure out why it's printing out wrong characters when it receives data from the computer.

Can't believe the guy didn't put a fuse in. What is it about the 70s and not caring about safety? I've seen so many things like that.
 
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