My First ZX 81

I started my computing with a ZX Spectrum 48k.  I never really noticed the ZX80/81 and I don’t remember any of my friends having one.  But they both played a significant part in kick-starting home computing.  For this reason they were next on my hit-list for acquiring, off to eBay.  As with the ZX Spectrums the prices vary significantly, especially if they are known to be working.  What I hadn’t realised was the price ZX80’s are going for.  I decided to set my sights on getting a ZX81 first.

It didn’t take long to win two auctions, the first for a ZX81 console, the second for a 16k RAM pack; I remember these being legendary for falling out and resetting the console.  

I started with opening it up and doing a visual check, the immediate issue was the keyboard.  Something had obviously happened to it with ‘ripples’ all the way around and one corner peeling up.  On opening it up I also found that the membrane connection to the motherboard was also snapped.  There was no saving this keyboard, luckily new ones are available for £10.

The voltage checks and it all looked good so I decided to try powering it up.  I plugged it into my analog tv and turned it on.  The boot ‘K‘ was sort-of displayed, although the TV was obviously having issues tuning the channel; but the good news was that it was working. 

I decided to start the future-proofing while waiting for the new keyboard to be delivered.

The changes were to replace the 5v regulator with a switched regulator to reduce the output heat, replace all the capacitors and to add a heatsink to the ULA.  The keyboard was fairly easy to remove with the help of a heat-gun.

Initial strip down
ZX 81 motherboard

The final mod was to upgrade the video to output a composite signal.  This is slightly more involved that the Spectrum requiring a transistor and a few resistors.  I created a prototype board to check the circuit, unfortunately I didn’t have any 33 Ohm resistors to used 3*10 Ohm in series for the prototype.

The results were a little bright, this is due to the lower value resistors being used.  I decided to order the correct values and do a more professional job when they arrive.  

I also replaced the keyboard, another 2 minute job, but it looks great and all the keys passed my first test.

Fully refurbished

Job complete.