exxos's Atari, Amiga & retro forum

(the latest chapter in the ongoing saga of the ST that hates me: viewtopic.php?f=14&t=6967)

I have a 520 STFM C103088 REV 2.2 TOS 1.02UK (surface mounted RAM, non-socketed Shifter, Glue & MMU in sockets) with what I think is ram fault.

The machine worked fine then started crashing when running games or any memory-intensive software. SYSINFO now refuses to load but ram tests and a diagnostic cart show the following errors:


I have removed and cleaned the MMU, Glue and their sockets. The PSU is brand new.
As I get the same error every test I assume it's a RAM fault, the thing is that although I am handy with a soldering iron I have no clue with the theory behind what I'm doing, especially when it comes to translating error codes etc.

So, help! How do I work out which one of the RAM chips is at fault (if it is that)?

Thanks in advance all.

I did a video on finding the faulty RAM chips, but it wasn't on that board specifically.

I think it says bit 11 is bad so the Ram chip connected to the Video Shifter Pin15 is bad. So replace it ...


... see the 520/1040STFM Schematics

Thank you, I was wondering how to decipher the labelling in the schematics...

On my machine pin 15 from the shifter goes to DATA IN on U34 (my board doesn't have U38 as per the diagram) so I know which chip to replace. Unfortunately Samsung km41256aj-15 DRAM seems to be unobtainium online so it looks like I may be exploring other options. Maybe it's time to get myself the Exxos 4mb ram expansion.