Having a look at two ZX-81s

From	gmc@metro.cx (Koen Martens)
Newsgroups	comp.sys.sinclair
Subject	Having a look at two ZX-81s
Date	2026-05-11 17:39 +0000
Organization	Sonologic AB
Message-ID	<10tt49a$27u5$1@nntp.sonologic.net> (permalink)

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Hi,

I had two ZX-81s that I finally got around to have a look at.

Of course I already broke one of the ribbon connectors of a
keyboard. I was not the first to break that one though, it
already was cut off near the case and someone managed to get
replacement wires and headers on it.

In any case, was trying to get it to show an image via RF on
a cheap chinese TV tuner (MTVbox), which failed. Also tried
two older (but not old) TVs that have RF input, but didn't
manage to get a good picture there. None of them want to sync
properly.

I've jurry-rigged pin 16 from the ULA directly to the composite
input on the MTVbox, and that works reasonably if I turn the
brightness all the way up. Odd, since it *is* the newer ULA
which generatest the required backporch.

Anyway, and I'll do this with pain in my heart as I try to keep
things as original as possible usually, I'll be desoldering
the modulators and try directly, and do some research into
circuits to recondition the video signal for composite inputs.

When I probed pin 16 on the scope, it showed a large DC offset,
which isn't quite right. It did see the backporch though, so
that doesn't explain the dim picture.

Once I've got the video done, I can try out the memory expansion
module I have, as well as the ZX printer.

Cheers,

Koen

-- 
Software architecture & engineering: https://www.sonologic.se/
Sci-fi: https://www.koenmartens.nl/
Retrocomputing videos: https://retroscandinavian.eu/

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Having a look at two ZX-81s gmc@metro.cx (Koen Martens) - 2026-05-11 17:39 +0000
  Re: Having a look at two ZX-81s Pierre <null@void.net> - 2026-05-12 12:52 +0000
    Re: Having a look at two ZX-81s gmc@metro.cx (Koen Martens) - 2026-05-12 16:06 +0000
      Re: Having a look at two ZX-81s IanJ <SPAMian_jones_01@yahoo.co.uk.invalid> - 2026-05-13 18:04 +0000
        Re: Having a look at two ZX-81s gmc@metro.cx (Koen Martens) - 2026-05-14 15:07 +0000

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