In my last post, I finally got my TV Typewriter to actually display something onscreen! It wasn’t much, but seeing 16 rows of 32 well formed boxes on my screen made me feel like I’d reached low earth orbit, and was now on my way to the moon.
According to the TVT construction guide, the next step in our journey is to add the fabled 2513 character generator. This is exciting because this is the point where the TV Typewriter actually starts putting recognizable stuff onscreen! The instructions advise strongly to check all the power pins. These ICs were rather exotic in their day (and pricey), and frying one in 1973 did not mean a quick call to Mouser.
Applying what I’ve learned so far, I check all the voltages and they’re all good. I also apply extra caution and check all around for shorts caused by bad soldering or solder flux. Where I see shiny flux where it shouldn’t be, I use a tiny jeweller’s flatblade screwdriver to scrape it out. Not the ideal way but effective.
The construction guide advises the next step is to plug the board into the other two TVT boards, connect TV or Monitor, and fire it all up. If all is ok, I should have a whole screen of @ symbols (this is the default binary code the address lines produce if nothing is trying to change them). And we do!
Woohoo! Now, I do see one little problem – there are some pixels activated that shouldn’t be. I’m reassured by more experienced hands that this is likely just a short somewhere.
Still, the amazement isn’t wearing off. This is a fairly complicated electronic device and I’ve got it working! So many TVT builders before me did not even get past the planning stage, let alone get (mostly) working video output!
Ignoring the extra pixels for now, I now craft a ‘330 ohm jumper wire’ by attaching a 330 jumper to one end of a solid core #22 wire. The goal here is to attach one end of the jumper to one of the +5V ‘ports’ on the bus, and then briefly touch the other end to the ‘signal end’ of each resistor tied to the 2513’s address lines. The result should change the screen full of @s to As, Ds, Hs, Ps or blanks. At least, according to the manual. I try this out, and it sort f goes as I’m expecting, except when I connect R49, I get a B instead of P. About 30 min of fooling around ensues. I’m sending off what I’m getting to my vcfed friend Chuck, and he’s suggesting I’ve got something backwards, or that possibly the documentation is in error. Eventually we figure out that B is what should be produced, not P, and that I’m suffering from a bit of PCB dyslexia and mixing up which pins are which on the 2513. Once we account for that, we’re all good!
The next step according to the manual is to add the 2518 shift register. This another rare IC, especially one dated to 1973, and I’m a bit nervous about plugging it in. But after checking and rechecking, I go for it. Now comes the real test – jumpering in several ‘clock’ wires from marked points on the timing board to bus ‘ports’ B1-B7, to try and get the TVT to produce a full screen of all available characters. This should be interesting!
Having successfully gotten (I think) a tune into the TV using the TVT’s sketchy RF modulator, the next stage is to put some actual integrated circuits to work and see if we can get this this puppy to do something interesting.
The construction guide advises that I need to completely build up the Timing board and part of Memory Board A (Page A), just enough on the latter to try and get something onscreen. Here’s the Timing board all dolled up and ready for action!
At this stage of construction, we are not yet adding the character generator, just IC 10 (a PISO generator) and IC 11 (an open collector NAND gate). First, the TVT should establish 32 columns and 16 rows of boxes as the field where text would appear. Then, with everything sort of floating open, it should set each of those boxes to be fully on, meaning we’ll have 512 of them on screen together if it all works.
After a few very careful checks for solder bridges, I stack the two boards into the motherboard, and go for it. And….
Nada.
No boxes, nothing onscreen. I check pin 20 on the system bus (the composite video out pin) for voltage, and it has it. I then decide to connect a composite monitor directly and bypass the TV for now, just to be a bit more certain that what I’m not seeing isn’t due to the RF modulator playing games. But there’s nothing there.
On the advice of Chuck from VCFED, I start digging into the timing board circuitry. First, we want to make sure there’s an actual clock signal happening, otherwise nothing at all will function. We’re looking at the MC4024 ‘dual astable’ chip used in conjunction with the crystal to create our system clock. Based on pinouts and the TVT schematic, I should be seeing something on the XTAL (crystal) pins of that chip, pins 3 and 4. But all is silent. My logic probe indicates no pulse activity at all.
Mystified, I start probing around the chip with the logic probe. I’m really scratching my head. Is it a bad chip? Maybe.. these things are 40+ years old and failure is always an option. I swap one of my precious spares in. Same thing. I swap the other spare in. Nope. So I’m really scratching my head here trying to figure out what the heck is going on. Chuck’s concerned I might have gotten the wrong 4024 – there is a CD4024 made by TI and Fairchild – similar part number, but very different chip. I verify mine are Motorola MC4024s. Did I get a bad bunch? Certainly possible. But not likely. I get back to basics and look for voltage at pins 1 and 14. Without adequate power (+5V), nothing much is gonna happen. I find 5V at pin 14 but *not* at pin 1. Hrmm.
It turns out after an hour of puzzlement that part of the answer is at hand. As I look more closely, I realize that I have goofed! I failed to solder in one end of a jumper (dang blasted jumper wires!) that should have connected 5V to the trace leading to that pin. Soldering it in, I now have 5V.. but still nothing on screen when I power up. Argh! Thought that would do it.
More headscratching and investigation ensues, before it dawns on me that my IC socket might be a problem. Thus far, I have been conducting my voltage test from the socket pins on the backside of the board. Now it dawns on me to check from topside. Sure enough, although voltage is getting to the socket pin, it is *not* getting to the actual pin 1 of the MC4024. Aha! After testing things a bit, I realize something is broken in the socket. This was a hazard of using vintage sockets of dubious provenance. To test things out, I connect pin 1 and 14 via a jumper, and now I have something on both my monitor and TV:
Yay! Sort of. What’s onscreen *kind of* looks like boxes. But not nice, bright white boxes. It’s the same on composite… so it’s not an RF modulator issue. I’m going to get rid of this socket and replace it obviously since something’s borked on pin 1. I’m advancing, an inch at a time.
I keep probing around and find more problems. Pin 1 of IC9 (a 7402) on the timing board isn’t connected properly. I resolder that and now I have a signal there I didn’t, but still no change on screen.
Next I notice pins 3 and 4 on IC9 aren’t showing anything. The soldering looks ok and socket itself seems ok, so I swap to another 7402. Now i’ve got activity on those pins, but still no change onscreen. Agh! So frustrating!
Chuck suggests that I need to check pin 45, the character clock pin. I check it, but there’s no pulse there, so that indicates no dots are getting shifted out to the screen.
His next suggestion is to really check out those Signetics N8288s. These are very old divide by 12 counters that were a Signetics-only part and went extinct not long after their introduction. Chuck warns that the manufacturing processes used on these were not quite dialed in back in the early 70s, and it’s quite possible these have just degraded and died over the course of 40 something years.
I probe these ICs carefully following clock signals around. The probe’s speaker changes tone and the pulse speeds up or slows down depending on where I’m probing, as the clock is being divided progressively further and further. I’m scratching my head again – it looks like the 8288s are working just fine. However that proves to be incorrect. One of the 8288s, in IC2, is getting an inconsistent, flaky signal at pin 5 and 6. I notice if I press on the pin itself with my logic probe with a little bit of pressure, I get signal. As soon as I let the pressure off, nothing. I decide to (again) replace a socket, hoping the replacement works, as my stock of vintage sockets is dwindling fast.
That doesn’t solve the problem although now those pins are getting a much better signal. I keep digging around and eventually find IC9 is still not working correctly. I pull the board out and shine a bright light from on side so I can see the traces clearly on the other. It turns out I’ve accidentally bridged pins 2 and 3!
I make the correction, inspect the board one more time, and then do a literal Plug’n’Pray hoping I’ve found it. To my amazement and delight, this happens!
The RF modulator has dialed itself out a bit again but it’s clear from what I can see that we’ve finally got something to light up on screen. Connecting to the much clearer composite, it’s confirmed! I now have my boxes!
Wow. That was epic!
So I’ve learned a few things. One, if you’ve built it yourself, assume ‘user error’ is at fault before assuming your ICs or components are the issue. With traces, IC pads and the like packed in so tightly together, it is very easy, especially for a novice like me, to accidentally connect things that aren’t meant to be connected with solder, or not even solder them at all! Double, triple and quadruple check your work. Employ some form of illumination under your boards to really verify that you’ve got everything in order.
Further, be wary of sockets – especially vintage sockets. Even if unused, the quality of these varies and they do degrade over time, depending on how they are stored. Never assume because everything looks okay that it is. Check each and every pin on your IC with the pins on your socket and make sure they are actually connected!
And of course if you’re lost, it’s good to have the ‘phone a friend’ option. The vintage computing forums and email lists are fantastic for this. Everyone is so helpful and kind (and understanding!). Chuck’s expertise was invaluable here as it told me what to look for. It’s all a learning process.
I’m so impressed this thing is actually doing something! I honestly didn’t think I’d get this far. Okay, on to the next step!
After successfully testing the power supply on my TVT’s mainboard, I now need to move on to actually making the TVT do something. The next instructions from the guide have me building the RF modulator, which is the component that gives the TV Typewriter its name. As mentioned before, I have had advice not to bother with this and just go with straight composite video. However, I felt doing this, while easier, would sidestep the main achievement of the device, which was that it used something everyone had in their home (a TV) already.
Anyway, building the RF modulator is straightforward enough – the trickiest bit is winding the 14ga tinned bus wire into a coil that helps generate a signal for the TV to pick up. You can see the result behind the trimmer capacitor. The wire is very thick and does not want to twist into a coil easily. It requires the use of a 3/8″ drill bit and very strong hands. There is a point on the second coil from the right where a tiny piece of wire ‘taps’ (is soldered) into it to draw the signal.
After that, I install all the required pieces, including my 300ohm twinlead with ‘gimmick attenuator’. If you’re of a certain age, you’ll remember that twinlead is basically a flat cable with two wires embedded in either side. In the days before TVs had coax inputs, you had screw terminals on the back for VHF and UHF reception. These twinlead cables went between there and the antenna attached to your house. The ‘gimmick’ attenuator is nothing more than a short piece of twinlead that is cut and electrical taped a few inches up from where the twinlead is soldered into the mainframe board. This provides a kind of capacitor that further hones the signal going to the TV, so as not to overpower it.
With everything installed, we are ready to do a test! The goal of this part of construction is to find a channel between 2 and 5 we can tune the device to. At this stage, we are trying to tune it so that the screen goes completely blank, and any static noise is minimized or eliminated. Ultimately I found the best channel to tune to was 5. Here is a video of my dialing it in (please ignore my messy shop!):
You can see how the screen goes to a solid blue. Offscreen, I am using a small screwdriver to achieve that screen by adjusting the 33pf trimmer capacitor next to the coil. Anyway, there you have it! Another exciting first step towards a working TVT!
