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Re: Having a look at two ZX-81s

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Koen Martens <gmc@metro.cx> wrote:
> 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
> 

Hello Koen,

Your ZX-81 project sounds good.

I had a similar observation with two ZX Spectrums, one is an unmoded 
original and the other has been composite moded.  On a 14" colour TV 
from early 2000, the OG ZX Spectrum had a brighter picture using RF than 
the composite moded ZX Spectrum.  Although the OG ZX Spectrum using RF 
had more interference lines.

Cheers
Pierre

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Thread

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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