ZX81 finally at home ;-)

After doing some research on the popular modifications to be made on a ZX81, I decided to modify the video output in order to provide a composite video signal directly usable on a monitor with this type of input.

Also, rather than increasing the RAM size to the usual 2KBytes, I decided to implement a 16Kbytes extension.

As a reminder, the keyboard connectors are cut. So I ordered a new membrane keyboard (see previous post). Moreover, after checking the video signal at the output of the PLA (pin 16), I know that the video signal is correctly generated :

In fact, after looking more closely at the video output, I could see that the cursor was indeed displayed. I was able to see 8 successive lines with this type of signal:

So, I decided to set up inside the HF modulator, a small circuit intended to improve the quality of the composite video signal, in particular to solve the problem of the back porch.

This type of circuit (ZX8-CCB) is available at this address. 

It is also possible to mount yourself a small transistor-based assembly which will have roughly the same effect, except that the back porch is not treated, which can be a problem on some monitors : 

Good, information about how the ZX8-CCB circuit works can be found there.

So here is what the modification of the HF modulator looks like after inserting the circuit  ZX8-CCB:

As I said at the beginning of the post, I also took the opportunity to increase the internal RAM to 16Kbytes. There is enough information on this subject on the web that it is not useful here to describe the implementation process. I used 16KBytes of a 32KBytes static RAM :

Voilà, once these modifications have been made, it remains to carry out the first operational tests.

And the tests to verify that the 16Kbytes are present :

PRINT PEEK 16388+256*PEEK 16389

So let's go :

And the result:

Conclusion: These small modifications require a bit of time but do not present any difficulty. With a working ZX81, these modifications allow a very interesting update of the machine for a very modest price.

Also, I didn't use a new membrane keyboard. I just trimmed the original connectors a few millimeters to remove the damaged part. It works perfectly.

Note that I have never been able to obtain an image with the RetroTink 2X Mini converter that I own.

The images shown here are from a ViewSonic brand monitor.

Finally, here I am, the owner of a fully functional ZX81. A computer that I first touched (in the literal sense) in 1983. I will always remember it as a very intense moment. I never bought one, however I was able to do a few programs on it at school the following year. As it was impossible for me to have access to a television set in my bedroom as a teenager, I fairly quickly bought a Sharp PC1500 which had its integrated display and which I had a lot of fun with.

sweet home ;-)