ZX81 kit assembly

Discussions about Sinclair ZX80 and ZX81 Hardware
sP1d3r
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Re: ZX81 kit assembly

Post by sP1d3r »

1024MAK wrote: Fri Oct 07, 2022 10:35 am That picture looks just like the symptoms of a ULA that does not generate a back porch signal, displayed on a device that needs the correct back porch signal to automatically calibrate its contrast to the correct values.

Mark
It's strange, that's the display from the vLA81 with extended sync signal and back porch enabled, without these there's a black screen.

With the TV card, it's worked with the modulator signal from 48K and 128K Spectrums.
Both had the right contrast and brightness but the signal was blurry in comparison with a composite signal.
Last edited by sP1d3r on Fri Oct 07, 2022 10:26 pm, edited 1 time in total.
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1024MAK
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Re: ZX81 kit assembly

Post by 1024MAK »

The display from a ZX81 should be a white background (including the border) with an inverse K (black square with a white letter K) in the area of the bottom left corner. Any non-inverse video test is black text on a white background.

Some modern replacements for ULA chips (like the vLA81) also offer a choice between normal video or inverse (black background with white text) and a choice of a black or a white border.

If the display system requires the back porch part of the signal (some systems will work even if this section of the video waveform is not at the correct level) then you get a very poor picture that is very dark, and which has very poor contrast between the parts of the picture that are supposed to be white and the parts that are supposed to be black.

Another setting on the vLA81 that can affect the brightness and contrast levels, is the output impedance / video signal level. The web page for the vLA81 describes it like this:
Video level selectable between standard video (suitable for direct connection to a 75 ohm monitor input) and high sync level (suitable for the modulator input or some video mod boards).
Are you using a composite video connection or the VHF/UHF output from the modulator?

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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derek
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Re: ZX81 kit assembly

Post by derek »

Hi,

Where can I buy the ZX81 kit?
Regards,

Derek
sP1d3r
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Re: ZX81 kit assembly

Post by sP1d3r »

1024MAK wrote: Fri Oct 07, 2022 3:38 pm The display from a ZX81 should be a white background (including the border) with an inverse K (black square with a white letter K) in the area of the bottom left corner. Any non-inverse video test is black text on a white background.
The output is from the UHF modulator and the video signal isn't inverted.
I can't do any more modulator testing, I'll have to wait until I get a composite mod.
sP1d3r
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Re: ZX81 kit assembly

Post by sP1d3r »

derek wrote: Fri Oct 07, 2022 3:42 pm Where can I buy the ZX81 kit?
If you've got 400 sovereigns to spare you can get one from eBay.
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1024MAK
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Re: ZX81 kit assembly

Post by 1024MAK »

sP1d3r wrote: Fri Oct 07, 2022 4:28 pm
derek wrote: Fri Oct 07, 2022 3:42 pm Where can I buy the ZX81 kit?
If you've got 400 sovereigns to spare you can get one from eBay.
Search for “zx81 computer kit”…

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
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sP1d3r
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Re: ZX81 kit assembly

Post by sP1d3r »

1024MAK wrote: Fri Oct 07, 2022 8:32 pm Search for zx81 computer kit”…
There's one here also:

https://www.retrogames.co.uk/028474/Sin ... it-Nr-MINT
sP1d3r
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Re: ZX81 kit assembly

Post by sP1d3r »

Here's the newest vLA81 manual:

https://www.vretrodesign.com/pages/downloads

I'm glad to say that after cleaning up the motherboard edge connector on my Performa the flicker has gone and it seems unaffected by the 'Tommy Cooper' trick.

Edit: The flickers maybe unrelated to the sudden motherboard removal because now they still happen but only when the computer gets warm!

Because this computer was battery damaged, I was able to repair it but this was only the motherboard, it's in need of a much more complex power supply and CRT refurbish, which the flickering would appear to be a symptom of.
sP1d3r
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Re: ZX81 kit assembly

Post by sP1d3r »

Here's a picture that'll make you laugh, it's the kit in the case.
Motherboard in case with original membrane
Motherboard in case with original membrane
Shouldn't laugh really, I can imagine that the individual that was assembling this kit didn't want any harm coming to his/her precious Zeddy and conscientiously provided adequate ventilation to avoid the hazard of the heat from the ULA and 7805, only to have an inexperienced assemblers nightmare, a partially soldered in socket with a pin trapped.

I think that it shows considerable character that he/she dropped it from his todo list rather than attempting some difficult desoldering.

It might have caused him/her a long lasting trauma, but I think that I'll stick with this case and I might try to 3D print an appropriate grille to fit in the hole.

The original membrane was stuck in and the legs weren't bent at all, it's smooth as silk although I'm not sure that it'll last long before the inevitable cracks occur.
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