Oh man.. that was close.
I had been warned in embarking on this project that I would likely have a *very* difficult time finding certain parts, especially in the correct vintage that I wanted (1972-1973). As it turns out, I had gotten a little smug and thought I had it all when in fact, I was missing the one thing there was no substitute for — the shift registers.
One of the things that set the original TV Typewriter apart from later variants was its use of shift register memory. To be frank, I’m not an expert on these things and don’t know the exact differences between shift register memory and the kind of RAM we use now, but suffice to say shift registers went back quite a ways before the TV Typewriter and were already well on their way to being obsolete for TVT purposes when Don’s project came out. This is one of the key reasons why the TV Typewriter II project came out shortly afterwards – easier to acquire parts and far better 2102 static RAMs. In the forums discussing my project with others, I was warned that these had been hard to acquire even in 1973, and now would be almost, if not outright, impossible.
Undeterred and being a newbie to the world of vintage ICs, I looked at the parts list for the TVT and discovered I needed ‘2524 shift registers – Signetics’. Okay. I tossed that into Ebay’s search and came up with these:
Hah! I thought — this is easy! The dates were newer than I wanted, but I had them, right? Perfect! This is going to be a snap! Sure, I won’t have the date codes all correct, but I’ll have what I need to get it up and running anyway and can look for closer parts matches later.
Anyway, I kept on acquiring chips here and there assuming my most difficult find was over. But one day on the forums I made a horrifying discovery — that little G after the S in SG2524 actually meant something. A poster had been incredulous that I’d found the elusive shift registers, so to prove it I posted a photo. Turns out, despite the similar part number these were entirely different chips! And looking at the artwork for the Memory board confirmed it — the shift registers I needed were 8 pin ICs.. these were 16! Augh! Turns out, these were ‘signal modulators’. I needed shift registers, and the part number for those was 2524V.
A desperate search commenced. Months went by with.. nothing. Every part site I searched said ‘Nope’. Nothing on ebay. Nothing anywhere. I was apparently looking for crazy rare unobtanium. I was despondent.
Then one day, I got lucky. I found a site that said yes, we have them! 6500 of them, actually! 2524V, 8 pin. Yes!
Sort of.
I don’t know if you’ve ever experienced these ‘surplus parts’ websites. They’re a bit like those ads you see in the paper offering you business opportunities. Make $500 a day like me ! Write for details!. Only the thing they are promising is never quite delivered. Unlike the guy that promises you a life of leisure, many of them don’t even respond to the requests you make of them for quotes. I’d stumbled onto a few sites that purported to be able to locate 2524V chips. They’re all the same — they have a ‘Request Quote’ button you press, and then you put in your particulars and what you’re looking for. And yeah, it’s just goes into a void. Once in a while one of these sites will be bothered enough to respond — usually just a trite four or five word message: Nope.. don’t have it, sorry.
So I was suspicious. Knowing how rare these ICs are, I really doubted anyone had 65 of them lying around, let alone 6500. But like I said, I was lucky. This one not only responded but in the affirmative: yes, we have them. I checked part numbers twice with them, confirmed the pins, every detail matched. The only negative was they were 1976 rather than 1972-1973. But I was okay with that. Something was better than nothing, and mid-70s was definitely better than 80s. I ordered 20; to have enough to build two TVTs (each needs 6) and have spares.
2 weeks and $100US later a dimunitive little package arrived in my post box. And there they were!
I couldn’t believe it! I had the unobtanium in my hands! I wanted to shout it from the rooftops, but they probably would have just locked me up. That’s the problem with this hobby: not enough of us around to appreciate finds like this. Hot damn!
Anyway, the project is a go! And in between waiting for the little 8 pin angels to arrive, I found steadily more and more (correct) ICs online, using what I’d learned about triple checking part numbers.
With ICs almost all in hand, it was time to turn towards the most important part of the whole business — making some boards!