• Please review our updated Terms and Rules here

TV Typewriter replica project back on track

Yeah I see those in a picture of the original encoder. I'm thinking I might just order those. I bought this multipack as a convenience for fixing things.. then thought I'd use it for the project.. but I'm not liking how they're all the same color.
 
More stuff came in!

Wasn't happy with the way my plain beige vinyl looked, so I ebayed around and settled on 'Desert Gold Distressed Faux Leather'. Here's a pic of it on the left, compared to a photo of the original under decent light to the right:

20160720_112549.jpg TV_Typewriter_CHM.jpg

I don't know.. I'm feeling pretty good about it. It's got the bleached out areas like Don's. Color is tough because photos present it differently, but I looked at an average of several of them. I think we're in the zone, or as close to it as we'll ever get. And I'm sure Don's has changed in appearance from what it was new.

Also got my 4700uf caps: 20160720_113049.jpg

So here's another authenticity thing -- obviously these do not look vintage -- the original 5000uf caps were big tall metal cans. I'm kicking myself for not saving the outer can part from my TVT2 when we replaced it.. I've heard of people hiding modern stuff under them. I've looked and looked but I don't see anyone making a) 5000uf caps or b) equivalent in solid cans like that. I've also assumed that I wouldn't want 'vintage' NOS caps even if I could find them, since they would have long ago dried out by now?

And anyway, if it were an original, by now the caps would be dried out and probably replaced anyway, right?

Finally here's some zener diodes:

20160720_113324.jpg

Unfortunately I've not seen any good pics where these are used to compare to what original ones would look like.

And I have a line on getting etched metal labels, to replicate the 'tv typewriter' label and switch labelling Don did.

We're on track!
 
You can still get screw-terminal 5000 uF caps, but they may be of a higher working voltage than the originals (not a deal-breaker). Surplus Sales of Nebraska has a lot of nice NOS caps, which are probably just fine for your purposes.

1973-era zeners looked pretty much like the ones you have. Older (1960s) ones looked like top-hat rectifiers.
 
In the only pictures I've seen showing the big cap, they look kind like this:

https://en.wikipedia.org/wiki/TV_Typewriter#/media/File:TV_Typewriter_Kit_1.jpg -- tall, silver can.

I note that one appears to be axial? I don't think the one on Don's prototype was axial.. think it was radial... in pics there's nothing coming out of the top. I think it was probably exactly the same thing my CT1024 had.

I note in Brent's there, he used a 15VDC vs 10VDC. So maybe I'm limiting myself by trying to match the exact voltages called for? I still don't understand how that all works. Is it like amperage where you can be higher, just not lower?

I also just got in touch with a Canadian fellow who built an original TVT in 1974 from the SWTPC boards. He sent me pics.. but his went off script a bit on the caps -- he had some kind of custom case going. I'm going to ask him for permission to share the pics. Pics of original TVTs seem to be exceedingly rare.
 
In the event, I found a 5000uf 20V Mallory silver can cap. Supposedly tested ok. It doesn't have the wire terminals or snaps.. more like what's on the end of a dryer plug.. but I think that's how it was on my CT1024. Anyway, $10... I can afford that and if need be I'll hollow it out and just hide a 4700uf under there. Not sure what the 'safe' procedure for hollowing out is though.
 
falter said:
In the event, I found a 5000uf 20V Mallory silver can cap. Supposedly tested ok. It doesn't have the wire terminals or snaps.. more like what's on the end of a dryer plug.. but I think that's how it was on my CT1024. Anyway, $10... I can afford that and if need be I'll hollow it out and just hide a 4700uf under there. Not sure what the 'safe' procedure for hollowing out is though.
Click to expand...

Wear gloves and eye protection, you can use a Dremel to cut the old capacitor open, sometimes you can remove the end cap. The innards of the capacitor should be considered chemical waste as they can contain stuff like borium-acid. There are some tips to be found on the web about this topic, an example:

http://www.nmr.mgh.harvard.edu/~reese/electrolytics/
 
Got the little molex connectors I need in today. Impressed they're still out there.

20160722_172134_resized.jpg

So having seen them up close, it looks like essentially on the boards above the motherboard, I will be soldering these in, and then soldering pins on the board above that plug into them. In the manual they call for 'pins that fit into molex connectors'. I'm wondering what the exact name would be to find them on, say, Mouser. I'm sure there is a specific type of pin that would solder in and be the right length, etc.
 
gertk said:
Wear gloves and eye protection, you can use a Dremel to cut the old capacitor open, sometimes you can remove the end cap. The innards of the capacitor should be considered chemical waste as they can contain stuff like borium-acid.
Click to expand...

Do you mean Boric acid ( H3BO3 )? It's not considered to be a hazardous material here. In fact, it's used in eyewashes. It's considered to be one of the safest materials to be used to kill roaches and ants. Most old capacitors use an electrolyte of borax (sodium perborate, which is used as a water softener and laundry additive here) and ethylene glycol (auto antifreeze). Neither of which is particularly nasty.

Old oil capacitors, as seen in motor starting capacitors, can be nasty because the oild generally contains PCBs (polychlorinated biphenyls), which are considered to be hazardous materials.
 
Got my parts list done for the non-IC stuff.

Annnnd already the crystal is tripping me up. I don't know why this is so hard to find. I've had to search for three different crystals for 3 different computers with specific frequencies and not *once* have I found one. It's bizarre.

The TVT calls for a 920khz 'crystal resonator'. I'm assuming they mean oscillator, or that the two are one and the same, since they mention oscillator later on. Either way, nobody has one. Hmm.
 
No, they mean crystal or resonator, not oscillator. 920KHz ceramic resonators are made, but not common. Here are some.

What's the application? If it's for baud rate generation, 1.8432 MHz crystals (2x) are quite common.
 
Last edited:
Oh boy. I have much to learn.

So there is a crystal that doesn't oscillate?

They specifically call for a '4561 series-resonant crystal'. The paets placement diagram looks like the crystal oscillators I have in my DG machine.. unless I'm mistaking what an oscillator is..
 
Just like an RC or LC network, a crystal resonates--it's a timing element, but does nothing by itself. You need to include gain/excitation in the circuit to oscillate. So an oscillator always includes an active element to put power into the system.

Put it this way--an empty beer bottle is a resonator, but without someone to blow into it, you won't hear anything (active excitation=oscillation).

Unless, of course, you're into perpetual-motion machines. :)
 
Okay.. that makes sense. But for search purposes.. am I just looking for a '920khz crystal'? Or what is commonly referred to as a '920khz crystal oscillator'? I read that 'series-resonant' refers to the type of oscillation it does.
 
"Series resonant" pertains more to the application (circuit) that it's used in. But yes, you want a crystal or a resonator (ceramic resonators are similar to crystals, but use a molded ceramic element rather than a milled quartz crystal as the timing basis).

If this device is what I think it is, 921 KHz or 922 KHz might be closer to the target. 1.8432MHz was used for a baud-rate timebase on UARTs quite often. So 921.6 KHz would be the ideal.

920KHz used to be used in some PAL TV circuits, so it was easier to find back in the day. If my supposition is correct, the 920KHz is feeding a divide-by-two circuit to develop the baud clock. So use of a 1.8432MHz crystal would just give you double the baud/bit rate. So, say, 2400 instead of 1200 bps.
 
Cool. I emailed Don just to double check if a substitute would work. We'll see if he responds.

From what I've read, the crystal handles the timing of pretty much everything in the device, so I'm not sure increasing it massively would work. From the manual:

"The timing board contains a crystal divider and TTL... countdown chain that generates all of the needed signals to run the typewriter in the proper sequence"

I'm just surprised these things are proving so difficult to find. I feel like I'm doing something wrong when I'm searching. I've found every other thing in multiples, even though many were well over 40 years old. Even those shift registers! But I can't find a crystal?! :)
 
Not at all unusual--vendors cut and sell crystals for common frequencies--and what was common yesterday may not be common today. Witness 1.8432 MHz and 18.432 MHz crystals and oscillators supplanting the 921KHz ones.

In my young years, I can remember using valve-grinding compound and a glass plate to grind FT243-style crystals upwards to hit a frequency. If you needed to lower the frequency of a crystal, you could often just use an ordinary no. 2 pencil to add a little mass to the crystal. Of course, FT243 crystals could be disassembled with only a screwdriver; not so with today's crystals.

294212.jpg
 
You must log in or register to reply here.
Back
Top