Acorn Atom Replica build

[rubber_jonnie]

So, I tossed a coin to decide if I should start the Cosmac Elf build or the Atom. You can see from the title which one won, though the Atom did have the advantage of some soldered sockets thanks to the donor @PhilC who sent it to me as he managed to get the real thing.

So stay tuned for some Atom Antics :)

[rubber_jonnie]

A little progress with the obvious and quick to add parts today. Before:

IMG20260208175244.jpg

And After:

IMG20260208185537.jpg

Not a huge amount I know, but it's a start and I now have the BoM so I'm ready to start getting all the parts together. I should have quite a few already. I'll get the passives done next whilst I wait for the RAM sockets to arrive.

[HigashiJun]

Good stuff !

Reminds me that I have to populate my A3000 new motherboard...

[rubber_jonnie]

Good stuff !

Reminds me that I have to populate my A3000 new motherboard...

That I'd like to see :)

I really need to get all my outstanding builds done...

I do have a keyboard from a BBC that I need to strip, however I think I'll probably leave that until last thing.

[rubber_jonnie]

So some more progress yesterday, I finished installing the remaining sockets which arrived yesterday afternoon:

IMG20260215083530.jpg

I usually prefer to do the passives first, but since the board arrived partially built with quite a few sockets already installed, I decided to just go with the flow.

Now they're all done I'm going to do the 47 decoupling caps next, but the BoM didn't show the values:

Screenshot 2026-02-15 at 08.40.25.png

So a little more digging was required. I had made an assumption that 100nF/.1uF would be about right, but had no evidence and the Atom schematic I have doesn't really match the repro board at all. So I dove into the schematics on the project github page and found the power section, and sure enough 100nF was whats needed:

Screenshot 2026-02-15 at 08.43.50.png

I have enough for the task at hand, so that's next and then I'll work my way through the BoM and will likely do the resistors next followed by the larger caps. They keyboard will be last of all as I don't want any of the switches to get damaged during assembly of the rest of the board.

Once all the passives are installed I'll do a smoke test and check voltages are going where they should at the sockets.

[stephen_usher]

You can tell from their locations that they're just decoupling caps. :-)

Nice project.

[rubber_jonnie]

You can tell from their locations that they're just decoupling caps. :-)

Nice project.

Yeah definitely, it was just the value that was unclear.

Reading the github project notes what's interesting is that the decoupling caps were added specifically and it looks like the original machine didn't have a lot of them and the Atom schematic seems to back that up.

It is a nice project for sure and I'm looking forwards to getting it finished. I'll start on the Cosmac Elf when this is done :)

[stephen_usher]

Yeah definitely, it was just the value that was unclear.

Decoupling caps are always the same value for 5V chips. It's just physics. :-)

With respect to the original design not having many. I've seen a lot of old PCBs which didn't have them at all. Obviously people in the past were not as fussy, and maybe 1970s chips were not as sensitive.

[rubber_jonnie]

Decoupling caps are always the same value for 5V chips. It's just physics. :-)

I didn't want to assume, though I had already to decided to us 100nF/.1uF if I couldn't find the actual values, but good to know that :)

With respect to the original design not having many. I've seen a lot of old PCBs which didn't have them at all. Obviously people in the past were not as fussy, and maybe 1970s chips were not as sensitive.

Indeed.

[rubber_jonnie]

More work done today:

IMG20260215152544.jpg

Most of the caps and resistors are done bar a few that I didn't have enough of, and some resistors I just didn't have. All on order now and that should complete the bulk of the passives.

There are a few other items such as transistors and crystals, but I know I have a lot of those to hand and likely do have the correct types available. I probably have most of the ICs if not all, but I'll get to those once I've got the passives done.

I need to see what of the other items I need and it may be time to start recovering the switched from the BBC keyboard.

Annoyingly there is one item, a coil, L1, in the video circuit for which there is no value in either the replica or original schematics. Not sure where I'll get the value for that from at the moment...