It's long overdue - Cosmac Elf II build

[HigashiJun]

@rubber_jonnie

:girldance:

Good to hear that !

[PhilC]

@rubber_jonnie I thought my monitor cabling was bad but you're clearly winning in this case :lol:

Glad to see you got it working.

[rubber_jonnie]

@rubber_jonnie

:girldance:

Good to hear that !

I was very happy to see it run, though I expect it is mirroring top and bottom. Not sure if that is something weird due to mine being an ELF II remake or some other weird artifact, but since it does actually give a display and it does show RAM contents, I'll take it for now.

Bit weird that my SMD versions aren't working, as I ordered all the 100% correct parts shown on the BoM from Digikey so I'm confident the parts are all good. Only one, my 2nd built with the Digikey parts, is fully working and a 3rd I built with all Digikey parts doesn't.

Its a bit perplexing.

@PhilC That's just the cabling at my workbench, the cabling at my retrodesk is something else entirely...

This lot:

IMG20260608120136.jpg

Feeds 3 layered video switches:

IMG20260608120149.jpg

So I can pretty much hook up anything retro. That is only whats going through composite though, I have an OSSC buried at the back that also does anything SCART/VGA or component to HDMI. I also have a 5 port HDMI switch in there for other stuff.

Does my brain in looking at it sometimes...

[rubber_jonnie]

Interesting, the other 74HC4040 I ordered arrived in just 4 days. Might shift to this supplier as I've used them a few time so far and they've been good.

[rubber_jonnie]

OK, so I had another look at the SMD CDP1861 and why my 1st and 3rd builds don't work, and it turns out to be this little fella:

IMG20260612151357.jpg

It's a 74LVC2G07 open drain buffer and I think the hot air/hot plate installation is killing them, either that or I have a bad batch. Pinout is this:

Screenshot 2026-06-12 at 15.24.47.png

With it fitted the Elf hangs and I can't enter data. Even if it's just that one single IC.

If I test from 1A-1Y and 2A-2Y in diode mode, with the positive on the A pin, the working one gives a result of 2.8v. Non working examples do nothing at all. So next I need to test a new one before I solder it on, and I may manually solder the next one on rather than use hot air, though I don't think it should really make much of a difference.

I'll also pull that one off to see if I get a different result with it off the PCB, in case its the PCBs that are bad.

More to follow.