Sinclair ZX80 / ZX81 / Z88 Forums

I have a 2C158E ULA which works for a few seconds and then fails to recognise the 1, Q, A and Shift keys. Possibly others but def them.
I have put a different ULA in the zeddy having these troubles and all keys work fine for ages. So I’m assuming its the ULA - at some point I will try it in a different zeddy.
Anyone seen anything like this? Would be a shame just to have to chuck it.

I’ve got one unit which randomly works fine. Sometimes it does not work at all and the other days stops recording any key pressed. All above spread regardles the chip being cold or warm.

Do both Zeddy's have the same processors?
Some ula have difficulty with cmos CPUs.
Just a thought.
Kind regards Paul

Its just the one zeddy but two ULAs - I took the original ULA out of the zeddy when the keys stopped working and then tried another ULA in that same zeddy which then worked fine.
I will try the dodgy ULA in another zeddy at some point. Who knows - it may work elsewhere?

I have not seen this in a ZX81 myself, but some ZX Spectrum ULAs fail in such a way that the keyboard inputs no longer work.

Assuming it is just the keyboard inputs, there is a workaround if you want to do a bit of hardware hacking...!

But first, yes, try the suspect ULA in another ZX81

Weird things do sometimes happen. On Tuesday I had a ZX81 I wanted to use (a known working unit), but a Memotech keyboard produced the number 2 several times on screen at power on, and then would not work on this Zeddy. So tried it with another, it worked fine. Tried it on the first ZX81 again, now works perfectly. Could have been a dirty edge connector contact though.

Mark