PDA

View Full Version : Sinclair ZX81 in Wood case?



falter
February 1st, 2014, 05:19 PM
http://www.ebay.com/itm/271389971668?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2648

I picked this up. I hadn't yet acquired any Sinclair stuff, so there was that -- but also thought the keyboard and nicely made little box might come in handy for my TV Typewriter project. Anyone ever seen anything like this? The seller was guessing HAM radio.. kind of looks like that. Or maybe just someone trying to make the ZX81 a little easier to work with. Wondering who's keyboard that is.

NeXT
February 1st, 2014, 06:45 PM
It was fairly common to replace the keyboard with ANYTHING other than that awful membrane thing. Someone just took a few extra steps and made a whole new case.

rittwage
February 1st, 2014, 06:45 PM
That is a crazy thing. Someone spent a lot of time on it. Good find at that price!

NF6X
February 1st, 2014, 07:23 PM
I saw that on eBay, too. It looks like somebody went to a LOT of effort to make that ZX81 into a usable machine! The keyboard appears to me to be made with pop-off clear keycaps, and the artist who put it all together must have printed up paper labels with the ZX81 symbology to install under the keycaps. It looks neat!

vwestlife
February 1st, 2014, 07:56 PM
There were companies who made full-stroke keyboard kits for the ZX81 / TS1000 and also cases to put the keyboard and computer in, but the case cost twice as much as the keyboard did, so some people made their own case out of whatever material they had on hand.

billdeg
February 2nd, 2014, 03:49 AM
I have a nearly complete product listing of keyboard replacement products for the t-1000 / ZX81 if you need more info on your particular upgrade, happy to try to help. I agree that someone must have taken a keyboard upgrade kit and then enhanced it with the custom wooden chassis.
Bill

falter
February 2nd, 2014, 09:59 AM
I have a nearly complete product listing of keyboard replacement products for the t-1000 / ZX81 if you need more info on your particular upgrade, happy to try to help. I agree that someone must have taken a keyboard upgrade kit and then enhanced it with the custom wooden chassis.
Bill

Thanks Bill. Yes I did some more digging and found more 'kits' that were sold to make the ZX81 a little more tolerable. I'm not sure if I'll leave it as it is or return it to stock configuration, which to me is more ZX81 like. That box and keyboard I bet could be useful in my TV Typewriter retro build, if the keyboard is suitable.

billdeg
February 2nd, 2014, 10:17 AM
I'd keep as is, there was a huge zx community back then but so few customized units still exist.
Imho anyway...
b

jltursan
February 2nd, 2014, 11:44 PM
Yep, keep it as is. The previous owner has really put a lot of effort into this, it's extremely polished and well done!

Only $25, lucky you :)

Btw, being a ZX81 it must output a PAL signal....or they were also marketed as NTSC units in the USA (not talking about the TS1000)?.

billdeg
February 3rd, 2014, 02:18 AM
I have one or two ntsc zx81s, at least one was built from a mail-order kit. Not sure the breakdown, but most ntsc zx81s were kits.
b

vwestlife
February 3rd, 2014, 04:37 AM
Btw, being a ZX81 it must output a PAL signal....or they were also marketed as NTSC units in the USA (not talking about the TS1000)?.

It says "ZX81 USA" on the bottom. Sinclair sold NTSC ZX81s by mail order in the USA before and during the time that Timex sold it as the TS1000. The difference was that the Sinclair version only had 1K of RAM while the TS1000 had 2K RAM built-in and more internal shielding, and a real FCC ID number rather than Sinclair's "fake" FCC ID.

falter
February 13th, 2014, 02:57 PM
It's here. A few pics:

17262
17263
17261
17264
17265

falter
February 13th, 2014, 03:00 PM
17266

No power supply unfortunately -- and it looks like he's rigged up something kind of custom here.. he's got a C64 like connector on the end. Don't know if I should attempt to create a 9VDC adapter from it or just patch around it for now somehow. There is also that gold wire that runs from one of the RCA jacks to the upper left most key on the keyboard. No idea what that does.

falter
February 13th, 2014, 06:41 PM
A question if I may too -- I had a 'Radio Shack' 1500ma 9vdc adapter here... I decided to give it a try with the ZX81. It powers on, but the screen immediately goes dim and the text on the screen becomes illegible. The computer is alive for sure, I can see things coming up on the screen as I type. Just super dark. This is on an old school CRT TV too. Wondering if it's my power supply being out of step (I think zx81 needs around 700ma?) or some kind of weird screen issue.

This machine has another port, big thing like the one on the back of my Sanyo VM4509. I tried hooking up a monitor to that but nothing came of it.. not sure how that all works.

falter
February 13th, 2014, 08:22 PM
I decided to open up this ZX81 and see where that suspected composite out to strange bnc connector went. Wire soldered to motherboard (big black wire on left).. I wonder if I can just cut in a different wire with RCA plug on it? I know ZX81s were modified to composite.. but this looks rather simpler than what I've seen. It's the only thing I can think of it being for -- it's exactly the same as the connector on my Sanyo monitor. But all I have are RCA cables and simply plugging the tips in at both ends isn't working like it did with my Challenger 1P.

17277

falter
February 13th, 2014, 09:40 PM
https://www.youtube.com/watch?v=iK3upG0ZnM4&feature=youtube_gdata_player

Went ahead and disconnected from the bnc or coax video connector and attached an rca plug, then plugged into my Commodore monitor. Voila! Video. But I can only see it with the brightness turned all the way up. And it distorts as you type. Not sure what thats about. Maybe my iffy ac adapter? If I slide the channel switch.. it briefly light and straightens up. Just for a split second. Here's a video I made:

jltursan
February 14th, 2014, 03:35 AM
Not sure if you have the correct composite video mod with the amplified signal (I suppose the original owner already did it); but the ultra-dark image is usually related to having a "bad" ULA, being "bad" just incompatible with modern TVs due to incorrect black porch. You can use a older TV set or replace the old ULA with the good one.

The old one was 2C210E, if I'm not wrong the good one is 2C184E (issue 3 PCBs). Try to find more info about this before replacing the IC...

carlsson
February 14th, 2014, 04:28 AM
There even are modern replica ULA's, but those tend to cost as much as a whole ZX-81 is worth so only upgrade if you're really eager about it.

As for upgrades, I think it was fairly common for home computer magazines to publish DIY instructions how to manufacture a better keyboard, put it in a case, install internal memory expansion, improve the power connection and so on. I have at least one or two different magazines with such DIY instructions. Of course it could be questioned if the materials and labor to do it yourself would be less than what a better computer had cost to start with, but homemade always tastes better and it would make it more hobby like to perhaps solder the ZX-81 kit first, then put together an own keyboard etc.

falter
February 14th, 2014, 07:47 AM
Thanks guys. Just to be clear.. 'modern' when referring to TVs.. that means lcd, etc right? Would a 20 year old CRT count as too modern?

One other weird thing I noticed.. when I had it hooked up to TV.. it worked better if I had the TV on channel 3 but the zx81 switched to channel 2.

And you don't think this power supply I have could be an issue? I'd always bwen undsr the impression you could have more milliamps than you needed with an adapter.. just not less.

falter
February 14th, 2014, 10:57 AM
Not sure if you have the correct composite video mod with the amplified signal (I suppose the original owner already did it); but the ultra-dark image is usually related to having a "bad" ULA, being "bad" just incompatible with modern TVs due to incorrect black porch. You can use a older TV set or replace the old ULA with the good one.

The old one was 2C210E, if I'm not wrong the good one is 2C184E (issue 3 PCBs). Try to find more info about this before replacing the IC...

I have the 2C184E. Here is a picture of my mainboard:

17289

So someone upgraded that at some point. I'm wondering if this is some kind of capacitor issue or something. When you flip the channel switch, it briefly reaches full brightness, but then quits. It's either something like that, or this universal power supply isn't cutting it (9V 1500ma)

daver2
February 14th, 2014, 11:17 AM
From my investigations the Sinclair ZX81 was supplied with a 9V 1200mA power supply if used with a Sinclair printer. Clearly an unexpanded ZX81 will consume less current than an expanded ZX81 which will consume less current than one equipped with a printer. The positive connection was the tip of the 3.5mm connector.

If your power supply is capable of delivering 1500mA you are good to go.

The rule is the power supply must be capable of providing more current than the item of electrical equipment consumes. The other way around could lead to under voltage on the electrical equipment or a burnt out power supply - or both!

One possibility for your lack if brightness could be the 75 Ohm termination impedance of the monitor. The output from the ULA is probably at 5V level and resistors will be used to drop the signal level to more like 0.7 or 1.0 Volts. If you stick a 75 Ohm resistor across the divider chain (the input impedance of a monitor for example) you may find the video signal level severely compromised.

Do you have any switches on your monitor marked termination ON/OFF?

Dave

falter
February 14th, 2014, 03:34 PM
Not that I can see.. I've found the other improved ULA chip online for $14.00. I've tried just sticking the end of the rca connector into the large round port on my vm4509.. *that* monitor delivers a bright white background.. bur it crunches it into 1/3 of the screen in the middle and is illegible. I am still trying tonfind the adapter that sdapts that old style coax to rca. I just dont know what the connector type is.

There wouldn't be a sufficient difference between a 20 year old crt tv and the ones in existence when this came out, would there?

falter
February 14th, 2014, 03:50 PM
On my Sanyo VM4509 there is a switch that has 'High' or '75ohm'. I switched it to 75ohm and after jacking up the brightness, same thing as the other monitor. Again this is sticking the male RCA into the coax connector, which isn't ideal. But it does produce some picture. Gotta find that darn adapter!

17297

vwestlife
February 14th, 2014, 04:16 PM
The problem is that you are taking the unbuffered, unamplified video signal directly off of the ULA chip, so it is very sensitive to the impedance and synchronization of the monitor you're using. A good video output mod would include some additional circuitry to boost and stabilize the signal before feeding it to a monitor. (And that's speaking in relative terms, since even the signal from the RF modulator doesn't follow the NTSC or PAL video standards exactly, and thus requires an old TV with manual V-hold and fine tuning controls to get the best results.)

daver2
February 15th, 2014, 07:18 AM
The schematics for the ZX81 are here if you didn't know: http://www.zx81.us

Various websites describe problems with connecting composite monitors directly to the ZX81 by-passing the built-in modulator. Exactly as vwestlife has said - the output from the ULA is not designed to drive the outside world.

Have a look at these sites for details about making your own very simple composite video buffer/driver:

http://computers.mcbx.netne.net/8bit/zx81/index.htm

http://www.sinclairzxworld.com/zx81-composite-video-problem-t248.html

http://www.zx81.de/english/video_e.htm

Obviously I can't vouch for these circuits - nor for any damage they could cause to your ZX81 - but they seem logical to me.

Dave