exxos's Atari, Amiga & retro forum

If it's detecting 512K but then saying that it's bad suggests a data line issue rather than an address line, which is what's on EXP2. Well, it could be the SRAM chip being faulty as well I s'pose.

I had a different design of RAM board.


P.S. Do I see a dry joint one one of the header pins on your board?

@rubber_jonnie yes that's the rom chip I meant. Do check that I put the ram chip in the socket correctly :lol: and not impossible the ram chip is bad. It's new old stock from China so never know.

PhilC wrote: 21 Sep 2026 20:46 @rubber_jonnie yes that's the rom chip I meant. Do check that I put the ram chip in the socket correctly :lol: and not impossible the ram chip is bad. It's new old stock from China so never know.

Well that's the ROM taken care of at least.

The RAM was correctly inserted BTW, will see if I can find a similar RAM ic, do a cross check.

With respect to the ROM...

stephen_usher wrote: 21 Sep 2026 22:14 With respect to the ROM...


IMG_5274.jpeg

Thanks, that's as mine is and aside from the initial issues with a bad ROM ic early on, it's been running 100%.

I need to dig through my parts bin to see if I have a suitable replacement, it's also possible that one or more of my SBC's has something I can borrow.

I'll report back later.

PhilC wrote: 21 Sep 2026 18:45 Are you going to CL5? I'll try and remember to bring a PCB if so.

Unfortunately I don't think so. I am on my own with my 10yo that day and unfortunately he won't want to come. If it's not 3D, texture mapped and ray traced he's not interested.

OK. This is mine:


And the ROM installation:


Yes, that's a smaller RAM, just using it for testing, it recognises 256KB in this case and 512KB in the case of the RAM that @PhilC included but the result is the same - 64KB OK - Bad 256KB RAM.

I have tried another GAL too, same results. I have checked continuity from the bottom of the board to the RAM expander and all connections are fine.

5v is arriving at both ICs OK too.

I tried unplugging each address line connection to see what happened, each one made a difference of some sort, so I think addressing is likely OK, and as @stephen_usher suggested, the problem lies with the data lines.

I may need to buzz out the data lines from the RAM to the mainboard, see what's what. Also I should hook up the keyboard see what the diags say.

I'd pop a scope on the data and address lines and see if they all look good or not. Probably be a 74LS244/5 in need of changing?

PhilC wrote: 22 Sep 2026 13:36 I'd pop a scope on the data and address lines and see if they all look good or not. Probably be a 74LS244/5 in need of changing?

I did and they all look normal.

What seems to be bad is /WE on the expansion port, and by association the RAM as it's a direct connection, it's really quite noisy. At least I think it is.


It's also having maximum values into the 6.5v range. If I trace back on the mainboard, it's happening there too, so I need to now find out where it comes from so I can see if I can figure out if the mainboard is bad, or something on it.

I think address and data look OK TBH.

I seem to remember finding a schematic on-line

I’ve seen far noisier lines and the noise doesn’t go near TTL switch voltages. Then again, the SRAM may be more sensitive.