The Naked Atari 800

[rubber_jonnie]

Now I have the Atari 800, PSU and keyboard, I need to identify what's missing.

So far I know that the machine is missing the custom ICs, so Antic, CTIA/GTIA and POKEY, but there are some othher supporting IC's that are missing, namely:

1. A 74LS42
2. A pair of CD4051's
3. A CD4050
4. An LM3086
5. A 3.579575 Mhz crystal
6. An adjustable inductor, L101 that i have yet to identify the values for. The schematic in the service manual doesn't help either.
7. 2x joystick ports
8. Reset, Option, Select and Start keys that are on the PSU board are missing.

I think I have most of those ICs in my parts bin, so I need to go and have a dig through.

I have already removed the board mods required for an Incognito, as originally this machine had just that, so R108 & R109 have been pulled and need replacement 680R resistors fitting and there were 2 jumper wires installed that I've removed

The joystick ports are for players 3 & 4 so not essential right now, but what may be hard to secure are the missing Reset, Option, Select and Start keys. I can use some other key switches in the meantime though, and if it remains naked it won't be a huge issue.

Most of it is present though:

IMG20251201123050.jpg (396.97 KiB) Viewed 116 times

And I have 3 complete RAM boards for 48K in total, plus a complete and working ROM board from my 800 and a CPU board, missing only Antic and CTIA/GTIA from my 400.

IMG20251201123000.jpg (468.89 KiB) Viewed 116 times

It should be possible to bring this back to life, albeit naked

[rubber_jonnie]

OK, so fishing through my parts drawers over lunchtime has moved me forwards quite a bit.

I found a Pokey from one of my machines that was upgraded to PokeyMAX, a 6520, the missing 3.5795 crystal and all the other missing logic etc bar a 74LS42, a CD4040 and an LM3086, all of which are now on order.

The Pokey is a great find because it means I only need to locate an Antic and a GTIA/CTIA from the custom chips. I can borrow those from one of my other machines to get going.

I will however do a power on test before I pop any of the ICs in to make sure nothing is looking dodgy, and I will also pull all the cards as well as I really don't want to kill any of those.

I have a sneaking suspicion that I may have a GTIA floating around, though it is the buggy one, but it would do if I can find it.

And wouldn't you know it, I found a 74LS42 about half an hour after ordering some...

[rubber_jonnie]

And I have located the buggy GTIA, it'll do for now. That just leaves me to find an Antic. Again I can borrow one from on of my other machines, but I would like to get hold of one.

I have also located what looks to be a suitable adjustable inductor that came from my 7800. It looks like it will fit and is apparently used for the same purpose relating to the modulator, so that's looking positive too. Fingers crossed it'll be up and running in no time

[rubber_jonnie]

Well glad I checked the voltage regulators before powering up the whole thing, the 7805 was taking a 12v input and sending it straight out of the output!!!

New 7805 is installed and it's the nearly the same, 10v. very odd indeed, and hooking the old 7805 up to 12v from my bench PSU, it's working and delivering 5v as expected. Curiouser and curioser.

So I think I need to hit up the service manual/schematics to figure out what's occurring.

[rubber_jonnie]

OK, some useful diagnostics done.

I lifted the output leg of the 7805 and it is indeed giving 5v.

However, looking at the pad the 7805 output is connected to, that is reading as 12v, so somehow 12v is finding it's way over to the 5v line. This is the schematic:

Screenshot 2025-12-01 at 16.31.21.png (601.67 KiB) Viewed 92 times

And from what i've measured, the input to the 7812 is 25v, and out comes 12v as expected. The 7805 gets 12v and when the leg is lifted, out comes the expected 5v, but not when connected to the board, and the only connections I can see are L201 and R203, so I will lift a leg on each to see if anything changes.

[rubber_jonnie]

Hmm, OK so I lifted one end of R203 and it's resolved the issue of 5v being 10-12v. R203 is 33R as exoected.

However that still means there is an issue of something getting through, and the only thing I can see is C204, so I will check that next.

[mrbombermillzy]

Well glad I checked the voltage regulators before powering up the whole thing, the 7805 was taking a 12v input and sending it straight out of the output!!!

Glad you checked after all that hard work! Phew!

Would have been a gutter to see all that go up in (magic) smoke.

[PhilC]

@rubber_jonnie what's the part no. For the CTIA? I've probably got one, no GTIA spare though. They are getting very rare now.

[rubber_jonnie]

@rubber_jonnie what's the part no. For the CTIA? I've probably got one, no GTIA spare though. They are getting very rare now.

The PAL CTIA part number is CO14889-03, though I do have a buggy GTIA which should be good enough. I'm more interested in an Antic (CO21698) as that is the one thing I'm missing.

Not sure if you might have one of those at all?

Cheers

[rubber_jonnie]

Well glad I checked the voltage regulators before powering up the whole thing, the 7805 was taking a 12v input and sending it straight out of the output!!!

Glad you checked after all that hard work! Phew!

Would have been a gutter to see all that go up in (magic) smoke.

Very glad indeed I tested firts rather than just plugging it all in!!!

As it turns out it's a bit more complex as it looks like C204 is leaking voltage into the 5v line, but I need to test it and see what's what.