ZX81 image problems

[paulwilco]

My meter is reading 10.73v when measuring across the wide tracks directly below the power supply connector

[1024MAK]

My meter is reading 10.73v when measuring across the wide tracks directly below the power supply connector

Okay, that’s fine.

I still suspect that the problem is the set-up of the video board. The video board that you have looks like it’s based on the 555 timer chip design on this site.

But as I said earlier, it’s definitely worthwhile trying another TV. The video output from a ZX81 is not fully compliant with the broadcast video/television standard, so some TVs don’t work properly with it. Sinclair designed it for black and white CRT (the heavy type with a curved glass screen) televisions. These used analogue technology and were far more tolerant of out of specification signals. Modern LCD (also described as LED) TVs process the signal completely differently.

Mark

[paulwilco]

Ahh I see. Bit of a strange development. Just plugged it all back in again and it appears to be all working, picture is fine now! Thanks very much for your help. The computer is heading for a museum so it will be connected to a different TV anyway, an LCD but older than the one I have here. Hopefully when I set it up there it will continue to behave itself!

I haven't attempted to load any software on it so far so hopefully I won't have any further issues!

Thanks again,

Paul

[paulwilco]

I seem to have another problem but I am a complete novice when it comes to the ZX81! To load software, from the research I have done, you attach the audio source (in my case a tzxduino) to the ear connector, press 'j' for the load command and "" then press the 'new line' key and start the tape/audio source. When I press the 'new line' key I get the attached image and no other signs of anything loading. Is this correct?

Many apologies if I'm asking a very obvious question!

[1024MAK]

Did you forget to attach your photo?

Yes, the correct command to load a program is

Code: Select all

LOAD “”

The ZX81 can’t display a normal picture while loading, as it has to devote all CPU time to loading and hence there is no time to produce a normal display. On a old CRT TV, it would display diagonal stripes. But a modern LCD TV may not display any picture.

And yes, on the ZX81, you use the Ear connector for audio input.

The ZX81 was designed for use with monochrome “shoe box” cassette tape recorders. These normally ran on a 6V battery or mains and the output volume could be quite loud.

One problem that many people have when using modern audio devices, is that the output is not loud enough.

Does your TZXDUINO work with other Sinclair computers like a ZX Spectrum? If it’s output operates at 5V logic levels (>3V appropriately) it should be okay.

Mark

[paulwilco]

Thanks again for the reply Mark,

Yes I did forget to attach the photo! I have now though. All I get is what's on the attached photo, there doesn't appear to be any other signs of anything happening.

The tzxduino is supposed to work with spectrums but the spectrums I have are also not working so I am unable to test it.

[desiv]

One thing that threw me off a bit was the double quotes over the Q. I tried those and that didn't work.

So for me, I type J (for LOAD) and then Shift-P twice for two quotes "".

[bobs]

Are you getting the double quotes using SHIFT + Q? You need to use SHIFT + P twice - it’s two single quotes.

[paulwilco]

That was exactly the problem! Using the "" key instead of shift p twice. So now when I type that the screen goes completely blank. Nothing appears to be loading, the counter on the tzxduino counts down but the screen continues to be blank until I press the space button then it goes back to the command prompt. I might try changing the firmware on the tzxduino.

[1024MAK]

Ahh, now that you have attached the photo, it’s easy for us to see the problem. The inverse S means syntax error. While the machine detects a syntax error, it will refuse the command or program line.

Code: Select all

LOAD “”

is a shortened version of the command.
LOAD requires a string expression. A simple literal string in a program or command has to be enclosed in double quotes.
So the following are all valid:

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LOAD “GAME”

LOAD “PROGRAM”

LET A$=“GAME”
LOAD A$

If a string expression is given, the machine will look for a tape file/program with that exact name, then load it.
It will not load a program that has a different name.

However, if the string expression has no characters, the machine will load the first tape file/program that it detects. In other words, the name does not matter.

And because of this, and because it’s quicker to enter,

Code: Select all

LOAD “”

and is now the most common usage.

The double double quote symbol on the Q key has a special usage, so it’s not valid for use in place of two separate double quote characters.

Note that LOAD on it’s own is also not valid.

Mark