Sinclar ZX81
Model: Issue One / 2C184E ULA / Dual RAM ICs
I purchased this computer in a lot together with several other ZX81s. They were sold as faulty, but this specific model was working perfectly as designed. Unfortunately, 'as designed' in this case meant there was no usable video on modern TVs.
Work done:
- Inspection and testing
- Keyboard replaced
- Reversible composite video upgrade
- Power supply capacitor replacement
- Full cleaning inside and out
And here is the final result:
Inspection and initial troubleshooting:
When I tried powering it up, I found that there was no image on the TV at all. When I opened it up I found that this is a really early issue one board and 2C184E ULA which doesn't generate a correct video signal (more on the below). I rigged up a temporary workaround for testing and found it was actually working perfectly.
Keyboard replacement
The original keyboard membrane was completely dried out and cracked. I ordered a replacement and installed it.
Here is a picture of the original keyboard removed:
Power supply
I opened up the power supply and found it in overall great condition.
The original capacitor showed some discolouring and tested marginal and was quickly replaced
Looking at the markings, I suspect that this was originally a UK power supply which had the power connector changed to an unearthed CEE 7/2 connector. These plugs are not compatible with the CEE 7/3 sockets used more commonly in the Netherlands these days.
I decided to replace it with a modern CEE 7/7 plug, without using the earth connection. Using a Europlug would have been better, but it would mean losing the strain relief on the power supply side.
Composite video upgrade
My TV wasn't able to sync with the image generated by the computer, leaving the screen just blank. This is caused by an early 2C184E ULA, which is known to generate a video signal which is incompatible with most colour TVs.
I worked on a few different designs, which are documented on this page. In the end, I found that the solution of sinclairworld.com user Pokemon gave an absolutely amazing result by not just adding the required back porch, but actually recreating the whole signal. Unfortunately, it wasn't available for sale anywhere at the time of writing, so I ended up designing a similar circuit using the 74HC04 logic IC instead.
Here is the final board ready to be installed:
Some pictures of the installation:
And lastly, the end result on a TV:
