I’m not an expert in electronics. The above thread is my first electronics project. The project is to build a keyboard for my ZX81. The project is not assembled yet, and have not been connected to my ZX81. It’s not the cause for the failure of my ZX81.
I’ve found a webpage to help diagnostic the ZX81 http://www.worldofspectrum.org/hardware/rep3.html . The page lists how to test the ZX81 board with a digital probe to figure out which ICs are broken. Therefore I’ve bought a digital probe and tested my ZX81.
The table below list what a working ZX81 data should be (according the the link above), and what my ZX81’s data is.
Pulse
High
Low
Open Curcuit
Code: Select all
Pin ULA My ULA ROM My ROM Z80 My Z80 RAM My RAMa My RAMb
1 P OC P L P OC P OC OC
2 P OC P OC P OC P OC OC
3 P OC P L PL OC P L L
4 P L P L P OC P OC OC
5 P L P L P OC P L L
6 P L P L P L P PL L
7 PH L P L P H P L L
8 PH OC P L P PH P L L
9 P L P H P PH L L L
10 P H P H P H P PL PL
11 P OC P PHL H H P PHL H
12 P L L L P PH P PHL PHL
13 P H P PHL P L P PHL H
14 P H P PL P H P PH H
15 PH H P PHL P H P L L
16 PH PH P L P OC P L L
17 P H P PHL PH H P L L
18 P OC P OC P H H H H
19 P L P OC P L
20 L L P L PH L
21 P H P OC P L
22 OC OC P OC PH PL
23 P PH P OC H H
24 P H H L PH H
25 H H H H
26 P PH H H
27 H H P PH
28 P PH P H
29 H H L L
30 P H P L
31 H H P L
32 P H P L
33 H H P L
34 L L P L
35 H H P L
36 P L P OC
37 P L P L
38 P L P OC
39 P OC P OC
40 H L P OC
/Martin