exxos's Atari, Amiga & retro forum

stephen_usher wrote: 22 Sep 2026 18:40 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.

I did subsequently find better quality scans of the schematics (Attached) but need to have a proper look, but they still don't seem to show the two expansion connectors, which seems a bit odd.

The schematics do show a 4164 based internal RAM expansion, but again, no connectors shown, just an extension of the address, data and control signals from the main schematic.

I've seen noisier too, so I'm not convinced that's it either. Pretty weird though. I'd assume from looking at the schematics that if Nick and Dave were bad it would be unlikely that anything was working.

Will look again later in the week but I'm in London tomorrow, so won't be for a day or two.

The problem on the schematics is that the connectors are broken up into parts.

Also, the memory expansion connector and EXP2 are just replicas of the expansion bus.

stephen_usher wrote: 22 Sep 2026 23:42 The problem on the schematics is that the connectors are broken up into parts.

Also, the memory expansion connector and EXP2 are just replicas of the expansion bus.

Indeed, it's not terribly clear.

@stephen_usher
@PhilC

I note that the FDD PCB has a spot for RAM, I presume if the internal slot is in use then the external FDD board RAM is either not necessary or cannot be installed?

rubber_jonnie wrote: 23 Sep 2026 09:27 @stephen_usher
@PhilC

I note that the FDD PCB has a spot for RAM, I presume if the internal slot is in use then the external FDD board RAM is either not necessary or cannot be installed?

I believe it can be configured as rom or ram depending on the jumper settings. If Ram, its meant to be used instead of the internal.

Of course, if the data lines aren't working to the RAM board then they won't be working to the expansion port either as they are just alternative connections.

OK, so I decided to go back and look at the address and data lines, I figured I must have missed something, and this is what A2 looks like at the RAM:


Not sure how I missed this earlier, but it doesn't look good and is on the main expansion header. I need to see what it looks like at the header and also without the board plugged in.

That's not a nice address trace. Duff pull-up somewhere?

stephen_usher wrote: 24 Sep 2026 11:01 That's not a nice address trace. Duff pull-up somewhere?

Definitely possible. I traced it back to Dave and it looks nasty there too. Looks just fine on the onboard RAM.

I'll try it with a pull up and see what it looks like.

I added a 4.7K pullup and now A2 looks like this:


And it passes the boot up RAM test now and shows aw 512KB on screen :)


That's a result, will look to see if there are supposed to be any pullups on the address lines shown on the schematic.

I will try with 128KB too, see if that works now. I may also give that pin on Dave a quick reflow.

That said it could be that A2 is having a problem in any number of places, so I'm not blaming Dave, as that is likely an input.