ZX-81

One year after the release of the (now unobtainable) ZX80, Sinclair introduced the ZX81 in 1981. Almost 1.5 million units were sold, so they are not particularly rare. I bought this one for €25,- and it came complete with the original power supply.
The ZX-81 is small. Really small, as you can see in the picture above (although I must admit I have big hands). It has a tiny membrane keyboard which has four or five functions for every key. There is a lot of mode-switching when entering a program.

Since this was a real home-computer, it only came with an RF connector so you could connect it to your standard TV-set.Which is impractical in these days so I decided to modify it for composite video. Which is actually quite simple. The video signal is already there, it only needs a bit of buffering and you'll have to bypass the RF modulator. I got my instructions from here: "Adding a composite video output.", but of course I started with this  illustrated step-bystep instruction on taking it apart. And what happens to almost everybody also happened here: the keyboard flatcable broke when I turned the PCB over. :-(

Crack...

The only solution is to cut off the broken section, reshape the end of the cable and push it in again. And the latter part is the hardest. The flatcable is fragile and flexible so it's very hard to get a grip and push it into the connector. I used a pair of flatnose pliers, grabbed the cable firmly, close to the header and pushed it it small steps at a time. It's not something you want to do often, so it's better to do this after finishing the video mod.

The video buffering circuit is really simple. It's just a NPN transistor and a 100 ohm resistor.

I chose the BC547, which is as one of the most common transistors in the world, and it worked fine.
On the side of the RF-box there are all the connections you need

After cutting off the resistor that is connected to the centre pin of the RF connector, I pushed the emitter lead of the transistor through the white hole on the side, and soldered it directly to the connector pin. I also soldered one end of the resistor to the pin, and the other end to the metal of the shielding.
I cut the 5V lead close to the box, soldered it to the collector and finally cut and connected the video lead to the base of the transistor. And this simple mod works surprisingly good:

Next I tried to write a line of code, and save it by recording the tape output on a laptop. After connecting the 'MIC' output to the microphone input of my laptop and trying to record the signals using 'Audacity' I found this does not work. Somehow the output signal is extremely weak, and it was impossible to record even the faintest sound. Apparently the output is intended for a specific type of tape recorder that supported a condenser microphone, which generates very small signals.

When checking with an oscilloscope I can see the signal is actually generated on pin 16 of IC1. But the filter, made up from R29, C12, R27 and C11, attenuates it tremendously. I tried removing R27, and replacing it with a 100K resistor, but that did not seem change much. (Nope. That makes sense with this 47nF capacitor still in place...) Probably the best solution would be to  pick up the signal from pin 16 and buffer it using an OPAMP.

Price [Original]	€25,- [ £69.95]
Processor	Zilog Z80 @ 3.25MHz
RAM	1KB
ROM	8KB
Programming	Sinclair Basic
Why ?	Iconic, as being the first super cheap computer.