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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 3rd Aug 2017, 12:22 pm   #41
radioman
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Default Re: Sinclair ZX81

I built my ZX81 from a kit in early 1982. Made my own power supply unit for it as well. Also added a Memotech 16k RAM pack.
Later on the Maplin magazine had a kit for an external keyboard but I decided to make my own instead, using 6-way banks of keyboard switches sold by J.Bull. (These were high quality 'reed' types)
I etched my own PCB and initially made a wooden frame for the PCB and this sat over the ZX81 case with a 16-way IC socket on top to connect to the keyboard.
The keytops I painted gloss black and used the Maplin key logos from their kit (luckily available separately) which I cut out and stuck on top of each cap and then varnished. This made entering programs much quicker !
All good fun.
I've just dug it out from it's resting place of 33 years(!) and must see if it still works.

Andy
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Old 3rd Aug 2017, 12:41 pm   #42
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Default Re: Sinclair ZX81

I bought my ZX 81 kit after "repairing" a ZX80 kit for a friend who taught maths in the school in which I worked. Before this my only experience of computing was at university were I used one in a radio chemistry experiment. It used punched cards. I also remember the pet and one made by research machines. The ZX81 was cheap. I agree that this was it's advantage. I seem to remember that the ZX80 was about 100. At 49.99 I still thought the ZX81 kit was pricey and there was the risk it wouldn't work when finished and you might not get it to work but I had started working and decided to get one. Later I bought a memory expansion board and a board to connect it to the outside world both as kits. I quite like the idea of a replacement keyboard which retains the look of the old one. Typing speed is not an issue for me as I am slow. I've even kept the old keyboard as I have a notion to try a fix on it. I think for me this is a bit of a nostalgia trip. I think a lot of members of this forum might understand that.

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Old 3rd Aug 2017, 1:02 pm   #43
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Default Re: Sinclair ZX81

I understand and agree Ken. Once you start upgrading bits, you're losing the point of resurrecting it.

Plus, nowadays you're not limited to the ZX81 itself. If you intend to do some programming (not so absurd, there is still a programming community and I did some last year) it makes sense to write it on the available emulators and/or assemblers on PC, then just use the real ZX81 to run it.
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Old 3rd Aug 2017, 4:46 pm   #44
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Default Re: Sinclair ZX81

The keyboard has just arrived by today's post. On initial inspection it looks very good. I've not compared it with the original but will do so tonight if I go out to the shed. It did bring back to mind a method to bring the keys back up when they stuck down. This tended to happen with the keys that were used a lot. The method involved holding a 25W soldering iron as close to the key as you dare. If you were lucky it popped up. I don't think this would work many times for the same key. I also now remember replacing the keyboard once before. This was quite a while ago and I think it cost about 5 but I'm not sure of this.

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Old 3rd Aug 2017, 5:16 pm   #45
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Keep us posted Ken!
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Old 3rd Aug 2017, 11:15 pm   #46
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Default Re: Sinclair ZX81

If you feel like playing with an old '81 but then remember how unstable the 16K RAM pack was, it's fairly easy to replace the 1K internal RAM with a larger static RAM. Here's one description of how to go about it.

http://blog.tynemouthsoftware.co.uk/...l-16k-ram.html

That will give you the classic, clean external looks of an unexpanded '81 combined with the rock-stable 16K of internal RAM it really needs to run any of the classic software.

I experimented with using an MP3 player (rather than a cassette player) to load files into the '81 and it didn't work initially because the maximum audio output level available from one stereo output channel of the player (a small one which runs on 1.5V) was not enough.

To get it to work I had to make a special stereo version of the file with one channel inverted, thereby using the left+right channel outputs in an antiphased 'bridge' configuration to produce double the output voltage swing (measured between the two channel outputs).

I didn't connect the player's common output ground to anything. Instead, I connected the player's left channel out to '81 tape signal-in and the player's right channel out to '81 tape ground.

This arrangement works without any problems as long as the player's own 0V/Ground is completely isolated from ZX81 0V/Ground, which in the case of my battery powered player, it is.
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Old 4th Aug 2017, 4:30 pm   #47
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Well I fitted my new keyboard today. I found it a little bit more difficult to seat it correctly than I remembered. I did not remove all the backing at first as I wanted to expose it gradually so as I could be sure the section exposed was aligned properly and had no traped air before I stuck it down. However peeling back the backing when only a small part was attached seem to dislodge it a little. It went easy enough after that. I am not sure the ZX81 is working. I tried with two TVs. The small 5'' one I already mentioned then an lcd. I can tune in a steady grey background but I cannot see the 'K' cursor. I tried varrying the brightness but still could not see it. May try another B/W tv. I'll also look for a repair. I've added some images. May have reduced the resolution too much.

1. New keyboard on left.
2. New keyboard fitted.

Regards,

Ken.
PS If I get it working I'll try the mp3 player idea to load it.
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Old 4th Aug 2017, 4:56 pm   #48
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Default Re: Sinclair ZX81

looks good, id consider converting to composite output rather than RF (just need to add a simple transistor amplifier to the ULA's video output)

https://www.youtube.com/watch?v=PFhPV5RXhFs link to a video on the comp out mod I did to one of my zx81's
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Old 4th Aug 2017, 5:14 pm   #49
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Default Re: Sinclair ZX81

Hello Jay,
I'm sitting here wondering if the reason I can't see the K cursor is that the ULA is duff. Don't seem easy to get now though there are work arounds.

Regards,

Ken.
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Old 4th Aug 2017, 5:37 pm   #50
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Default Re: Sinclair ZX81

Do you have an osilloscope? Try probing pin 16 of the ULA? Does anything change when you press a key on the keyboard while monitoring that pin? If so you could just have a bad modulator.

PS I have most parts for the Zeddy in stock including a few (and I do mean just a few) ULA's

Jay
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Old 4th Aug 2017, 5:48 pm   #51
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Default Re: Sinclair ZX81

Hello Jay
I do have a scope though it doesn't get much use. I'll try your suggestion and get back. Based on what I'd read on the web I just assumed the ula was the most likely culprit. Didn't even think of the modulator.

Regards,

Ken.
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Old 4th Aug 2017, 8:33 pm   #52
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Hello Jay,
Just watched your video. Very interesting. I tried to scope pin 16 of the ula. I have the instructions to build the ZX81 and this has a circuit diagram. I noticed that pin 16 was connected to the lead that passes through a sort of plastic grommet on the side of the modulator so I connected the scope to this and earth. This gave an output which did not appear to change with key presses. However, as I said I rarely use my scope and I was not sure I had it set up properly to do this. I could however detect a voltage of about 2.8V with a digital multimeter. This fell by a bit over 0.1V each time a key was pressed. The ula also ran fairly hot. Now I'm not sure if this suggests that the ula is working but I thought it was hopeful. I am wondering if I should just build your little circuit to give composite video and depending on whether I see the cursor or not it would tell me if it was the ula or the modulator at fault.

Regards,

Ken.
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Old 4th Aug 2017, 9:56 pm   #53
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Default Re: Sinclair ZX81

I would build the circuit and convert to composite vus ruling out the modulator. Ps a ZX81 is a grate computer to play with your scope on. try seing if the clock on the cpu is running with it.
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Old 5th Aug 2017, 7:10 pm   #54
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Hello Jay,
Built the little circuit this afternoon. I built it into the modulator box having disconnecting the appropriate parts. I left it so as the modulator could be returned to operation if I ever wanted it. There is enough room for this. However, the bad news is that I still don't see the cursor. I tried on two TVs. The first has an lcd. It did not change from the blue screen. I also tried the 5" B/W crt TV that I used before. It has a composite video input. The only changes with it was if I switched the ZX81 on then of. The screen looks the same whether the ZX81 is on or off so I think it must be the vla. I also tried the circuit without the diodes but got the same results. I suppose it rules out the modulator.

Regards,

Kwn.
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Old 5th Aug 2017, 7:32 pm   #55
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Default Re: Sinclair ZX81

It unfortunetely looks like the ULA may have bit the dust. Have you checked the CPU is running? try scopeing the clock signal on the z80

Jay
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Old 6th Aug 2017, 1:31 pm   #56
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Hello Jay,
I'm not sure that I have a scope that is working properly but I do have a device for measuring frequency built from a Chinese kit. I am not currently sure of its range. I seem to remember reading that the CPU is running at 3.5MHz but this is derived from a 7MHz source generated elsewhere. It might have been in the ula but I can't remember. Should I see 3.5 or 7MHz on the cpu or have I got this wrong.

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Ken.
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Old 6th Aug 2017, 5:11 pm   #57
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Default Re: Sinclair ZX81

You should see 3.5mhz at the cpu pin 6. Also do you have a logic probe? if not very easy to make its just a couple of led's and transistors, have a look at example 3 hear for a simple one to make http://www.eleccircuit.com/many-logi...circuit-ideas/ with this you can have a look see what is running on the ULA CPU RAM and ROM. Im hopeing its somthing other than the ULA. least all your IC's appear to be socketed.

Jay
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Old 6th Aug 2017, 5:16 pm   #58
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Default Re: Sinclair ZX81

Even an analogue voltmeter helps here, if it's 2.5V then there is a 50% clock, 0 or 5V no clock.
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Old 6th Aug 2017, 5:35 pm   #59
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Hello Jay,
Thanks for the information and the link. I don't have a logic probe but the in your link looks easy to make. I don't suppose the transistors are critical. I've got lots of bc108 but no bc171s. I'll build it with bc108s an see if it works. I too am hoping it's not the ula.

Hello merlinmaxwell,
I have an analogue multimeter so can try your suggestion.

Thanks to both. I'll report back.

Regards,

Ken.
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Old 6th Aug 2017, 6:32 pm   #60
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Default Re: Sinclair ZX81

Think BC108's should work fine.
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