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/

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

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/

Koen Martens <gmc@metro.cx> wrote:

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.

There's an android app, PlayZX, that can generate a suitable audio
from tap/tzx files.

It wasn't long however before the loading times became frustrating
enough that I bought myself a SMART card for my zx spectrum which
can load games in seconds from a microsd: https://retroleum.co.uk/spectrum-peripherals

I have no affiliation with either.
--

IanJ

gopher://gopher.icu

IanJ <SPAMian_jones_01@yahoo.co.uk.invalid> wrote:

Koen Martens <gmc@metro.cx> wrote:

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.

There's an android app, PlayZX, that can generate a suitable audio
from tap/tzx files.

I've used that on my ZX Spectrum before, works great indeed, although
I had to boost the audio from my phone with a headphone amp, otherwise
the signal would be too weak.

The ZX-81 is particularly sensitive to getting the level exactly right,
even back in the day it was nearly impossible to get the output of
the tape recorder just right. I'm going to measure what the ULA
is seeing sometime with the oscilloscope, see if I can figure out
what's wrong.

It wasn't long however before the loading times became frustrating
enough that I bought myself a SMART card for my zx spectrum which
can load games in seconds from a microsd: https://retroleum.co.uk/spectrum-peripherals

This I can imagine :) I've got something similar for my C64. I haven't
looked if something like that is available for the ZX-81.

Cheers,

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