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Re: Having a look at two ZX-81s

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Pierre <null@void.net> wrote:
> Koen Martens <gmc@metro.cx> wrote:
>> 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.
>> 
> 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.

I managed to fix the composite output, without too much damage to the
original circuitry. In fact, it's easly reversible if I ever would want
to. Used a simple NPN transistor to amplify the signal from the ULA a 
bit, to avoid drawing too much current from the ULA, and then I added
two diodes to drop the voltage a bit. I think composite is supposed to
be 1volt peak-to-peak, and it was about 4 and a bit. So that would
overwhelm the composite input, leading to a dim display. It now looks
nice and crisp.

On to the second challenge: loading something into the ZX-81. I hooked
up the audio output of my desktop machine to a headphone amplifier,
and fed that into the ZX-81. Been fiddling with the volume, but without
much sucess so far. I remember from the olden days that the ZX-81 was
very critical when it comes to the input level, but no matter what
I did, it won't load. Something is being processed though, because
sometimes the loading will drop back to a prompt or a white screen
once the audio is done playing on the computer.

I tried various tools to convert .p and .txz files to audio, but
just couldn't get it right. Maybe the caps on the input circuitry
have gone bad, messing up the signal.

Anyway, had to give up for now. To be continued later.

Didn't get around yet to try the 64kb memory expansion or 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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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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