T
he ZX81 was my first computer. Mine was screwed to a board with a terrible external keyboard, and I was proud of the fact that someone had fitted 2k of RAM.
It was my introduction to BASIC and machine language. Let's not forget that the humble zeddy is a vehicle for a Z80 (which has 16-bit features) running at >3Mz. (Of course you can only harness that amazing power in fast mode, ie without display output, because much of the CPU time is usually used to generate the display.)
Other than buying some kind of box that will convert the TV signal (with which I had limited success), it's possible to perform surgery to convert the output to composite, which modern displays may be more likely to display (but still maybe not in the case of the ZX81s with the older ULA).
Here are three methods - rewiring (cost zero), using some simple components (cost pence) and a purchased modification (still less than a tenner.)
With the newer issue 81 it's possible to simply tap the signal before the RF modulator and connect that to the output socket. This worked for me with Dipsy* who has an issue 3 board (it's the C210 ULA which is important here - its output is a more compatible composite signal).
As you can see, the picture is just about usable with the brightness and contrast turned up on the display.
The next cheapest solution is to make a very simple circuit, which will cost a matter of pence. There are many versions of this circuit online. This is the one I tried which simply uses one transistor and one resistor (I used a 75 ohm resistor rather than 100 because that's what I had handy). I was amazed with the result and improved it still with a 4.7uF capacitor for decoupling. It's hard to tell from the screen photos but I would say that the quality of the picture is as good as the CBB.
I didn't build a circuit on a board and I didn't bother to gut the modulator this time (that job is not fun). But you should disconnect its inputs and output. Even with the modulator components in place, you can squeeze the modification in there. This is how I did it - the base and collector of the transistor are poking through those plastic holes and they hook up with the composite output from the board and +5v. The emitter of the transistor connects up to the resistor and the -ve leg of the cap. The other leg of the cap goes to the output socket and the other leg of the resistor goes to the metal casing.
After putting insulating tape around anything that might short, and also covering the inside of the lid of the modulator, the lid went back on and the job looks very neat.
perfectly usable, very sharp but doesn't feel as solid as the CBB mod. (Different colour casts are just because these photos were taken of a projected image at different times.)
My best result so far is from a bought composite converter - several are available but the ZX-CCB comes in at less than 10 Euros.
Here's the ZX-CCB fitted to Tinky-Winky*, who has an issue 1 board with the older ULA. The CBB works with older and newer ULAs. What sold it to me was the option to have standard or inverse video - my brown/red flying lead goes to a switch which allows me to switch between them.
NB If you buy one: even though the seller has set it up, I found that I had to twiddle the trimmers for the best picture.
And what a picture**:
[Update 8 Nov 2019] I've gone to the trouble of ordering more components, building a slightly more complex transistor circuit for Dipsy.
I always hated biasing transistors - having to choose the nearest available value and component tolerances seemed to make a bit of a mockery of doing the calculations. I always felt "why not put a trimpot in there and adjust it for the best result" and so that's what I've done here. Two in fact. The best result I've obtained is pretty good. So much so that I've gone to the trouble of gutting the modulator box and fitting the little board neatly inside. I've also fitted a heatsink to the ULA because I was concerned about how hot that gets and want to extend its life as much as possible.
However, the picture is still not quite as bright and as clean as the one from the ZX-CCB (unsurprisingly). There's a little flicker that happens from time to time.
The lid is on the modulator box, the case is screwed together and the rubber feet re-fitted. Dipsy won't be the most used but he's perfectly usable when he does get a run out.
The games shown are both new releases, Tut-tut by David Stephenson and Berzxerk (hi-res) by Bukster.
* I had 4 x 81's in various conditions. Sadly I sold my original one when my parents bought me my Vic20. But since then I have collected a few. In the 90s, 8-bit computers could be picked up at boot sales for a song. The ones I collected went into the attic with a mental note that two worked and two didn't.
** I should say that the actual display is sharper than it looks in this photo. I'm currently projecting onto a wall with light-coloured textured wallpaper. (Which is the way I like to play these games.) If I take a picture of this going through an LCD display, or capture it electronically, that's not a fair comparison with the cheap mod. For reasons I don't understand, the only thing that will display the home-made mod is my projector.
It was my introduction to BASIC and machine language. Let's not forget that the humble zeddy is a vehicle for a Z80 (which has 16-bit features) running at >3Mz. (Of course you can only harness that amazing power in fast mode, ie without display output, because much of the CPU time is usually used to generate the display.)
Other than buying some kind of box that will convert the TV signal (with which I had limited success), it's possible to perform surgery to convert the output to composite, which modern displays may be more likely to display (but still maybe not in the case of the ZX81s with the older ULA).
Here are three methods - rewiring (cost zero), using some simple components (cost pence) and a purchased modification (still less than a tenner.)
As you can see, the picture is just about usable with the brightness and contrast turned up on the display.
The next cheapest solution is to make a very simple circuit, which will cost a matter of pence. There are many versions of this circuit online. This is the one I tried which simply uses one transistor and one resistor (I used a 75 ohm resistor rather than 100 because that's what I had handy). I was amazed with the result and improved it still with a 4.7uF capacitor for decoupling. It's hard to tell from the screen photos but I would say that the quality of the picture is as good as the CBB.
I didn't build a circuit on a board and I didn't bother to gut the modulator this time (that job is not fun). But you should disconnect its inputs and output. Even with the modulator components in place, you can squeeze the modification in there. This is how I did it - the base and collector of the transistor are poking through those plastic holes and they hook up with the composite output from the board and +5v. The emitter of the transistor connects up to the resistor and the -ve leg of the cap. The other leg of the cap goes to the output socket and the other leg of the resistor goes to the metal casing.
perfectly usable, very sharp but doesn't feel as solid as the CBB mod. (Different colour casts are just because these photos were taken of a projected image at different times.)
My best result so far is from a bought composite converter - several are available but the ZX-CCB comes in at less than 10 Euros.
NB If you buy one: even though the seller has set it up, I found that I had to twiddle the trimmers for the best picture.
And what a picture**:
[Update 8 Nov 2019] I've gone to the trouble of ordering more components, building a slightly more complex transistor circuit for Dipsy.
However, the picture is still not quite as bright and as clean as the one from the ZX-CCB (unsurprisingly). There's a little flicker that happens from time to time.
The lid is on the modulator box, the case is screwed together and the rubber feet re-fitted. Dipsy won't be the most used but he's perfectly usable when he does get a run out.
The games shown are both new releases, Tut-tut by David Stephenson and Berzxerk (hi-res) by Bukster.
* I had 4 x 81's in various conditions. Sadly I sold my original one when my parents bought me my Vic20. But since then I have collected a few. In the 90s, 8-bit computers could be picked up at boot sales for a song. The ones I collected went into the attic with a mental note that two worked and two didn't.
** I should say that the actual display is sharper than it looks in this photo. I'm currently projecting onto a wall with light-coloured textured wallpaper. (Which is the way I like to play these games.) If I take a picture of this going through an LCD display, or capture it electronically, that's not a fair comparison with the cheap mod. For reasons I don't understand, the only thing that will display the home-made mod is my projector.
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