I have a zx81 computer. I tried it and it did not responded to my typing so well...about half of the letters and numbers came correct on screen when i typed.
Ok...i thought it was a membrane error and bought a new one. Installed it,but the problem is the same!
Example. I push the J key and it should type LOAD,but instead it types LOA5
About half of the keyboard types correct letters and numbers and the other half does not.the same number can come from 3 buttons example.
What can i do? What can cause this other than membrane?
Really want to fix this little computer.
No replacements to be found anywhere?
If not,then it was end for that computer. It gives a normal white picture with the letter K at the bottom.
Only that i can not write anything good on the screen.
So many letters writes wrong letters or number or some symbol.
Thank you anyway for answer!
I need to ask him how we fixed it.
There is hope!
kind regards Paul
I had a machine with pretty much exactly this fault a couple of weeks ago. A switched ULA fixed it
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The ZX81 internally stores "keywords" (command words and function words) as a special number. When the code that is in the ROM finds one of these code, it expands the code to the correct characters and then prints these on the screen.
If you try to type in LOA5, the system would not accept it as a valid command.
So, as far as I can see, the keyboard is not the problem. Instead, the problem is the ROM, the ULA, the Z80 CPU or the circuitry that links them. As it is these that do the generation of the display. It is unlikely that the cause is the Z80 CPU, as most failures of this chip would prevent even getting to the "K" start screen.
So to test this, can you enter a simple BASIC program, then RUN it.
So for example:
10 PRINT "Hello World"
20 GOTO 10
Ignore any incorrectly displayed characters. If this works okay, then it confirms my thinking.
It would also indicate that the connections between the ROM, the ULA and the Z80 CPU are all okay.
So that then leaves a very strange ROM chip fault, or a ULA chip fault.
I am a fool.... sorry guys.... so that leaves....
Faulty ula, or if your lucky maybe just a bent pin or dicky comnection if the ula is socketed....