Today I built my Atari Diagnostic cartridge,so I hope to solve mystery:
Result: RS232 output data, and, with a VT100 terminal I can read this line, repeated in loop:
I1 RAM/data line error 0000000000000000 0000000000100000 000008
I searched in service manual how this output can help to identify defective RAM, but no answer.
So, in my opinion, it could indicate Ram chip n.11, lower row, counting from left to right?
I try to desolder it meanwhile..
I desoldered that chip (it is named U18 on board), put a socket and substitute with other one.
Problem remain, and output from serial terminal, via diagnostic cartridge, remain the same.
So, I tried to remove chip from socket, leaving it empty, and run test again. Output now is:
I1 RAM/data line error 0000000000000000 0000000000100100 000008
So, seems that U18 is represented by 3rd from right bit in second 16bit word.
Thus, 6th from right bit in 16bit word, what chip indicate?
I substitute U43 (6th from left, on board) but same result.
At least, I individuate failed RAM.It is U28, 3rd from right in main board.
I discovery mistery how diagnostic card indicate failed ram: if interested ask me and I will open new post explaining logic behind these "magic numbers".
But, repair is not finished. After substitution of failed ram, still 520ST doesn't work.
This time, diagnostic card outputs on RS232:
I2 RAM disturbance 0000000000000000 000000000010010 000008
This is a failure in these other 2 ram chips? (U17, U25 if you've discovered diagnostic cart logic too)
Despite my desolder gun runs out so I've to wait until I repair it to check these 2 drams....
S O L V E D
I substituted U17, U25 and Atari goes on!
Soon I create, with your permission, a little guide about how to find bad rams following numeric error codes of the cartridge.