The Naked Atari 800

[rubber_jonnie]

What "fun" (for certain values of "fun")! :-)

Did you find and check the hardware, power-on reset circuit in the end?

Indeed, fun is in the eye of the beholder...

The reset circuit (VIA the reset switch) is a bit weird as it goes through the Antic which sends an NMI to the CPU. That means a reset button press doesn't show on pin 40 of the CPU, however it does show on pin 6!!!

Reset on pin 40 does go high on power on, which is good. Clocks look OK as far as I can tell, but there are some test points I can't check as they don't exist on a PAL machine. Also some values seem odd, so I might have to do some checking on one of my other machines to verify. The 400 and 800 have the same clock setup and my 400 is currently assembled. I'd rather not disassemble the 800, but I may have to.

Power is fine across all the ICs, I have 5v, -5v and 12v as expected and it all goes to the RAM, which uses 4116's.

[rubber_jonnie]

OK, fun often leads to more fun :)

I decided the 400 wasn't up to snuff for testing the CPU board, too many mods I thought, so I put it all back together and pulled the 800 apart to verify things, only to find the 800 wasn't up to snuff either for the same reason.

I thought that maybe while it was apart I could verify a few things, namely:

1. Does the spare CPU work in the working 800?
2. Do the Antic and GTIA work in the working 800?
3. Since the crystal I installed was based on the service manual, does it match my PAL machine?

In short order the answers were:

1. No, it just gives a brown screen.
2. They do, but the buggy GTIA is far from perfect, it will however do for now.
3. No it doesn't. The SM says 3.579575Mhz. My working 800 says 3.546894Mhz. It only works out as about a 1% difference, however I'll get the proper crystal and fit it if I can find one.

So the CPU doesn't work. Hmm, but it does work in my Electron, lets have a look at the CPU and it has a -03 suffix. Supposedly and according to the SM it needs to be a 3Mhz part, so perhaps that's why my other CPU's don't work.

With that I borrowed the Sally from my 600XL in the knowledge that the HALT pin wouldn't be in use on the 800 and I get this:

IMG20251218190353.jpg

OK, this is good, it's working!!!

Lets try something else:

IMG20251218190431.jpg

So Miner 2049er works too, sound and all.

The image is super dark and black and white, which may be a result of the crystal and/or the GTIA which I know has bugs, but it's a damn fine place to start.

Not out of the woods yet though, it's a bit unstable - again could be crystal or GTIA related, so I will do some swapping about with the 600XL GTIA later to see if that helps.

Also pin 35, /HALT, on Sally may be floating so that may also be causing instability, so I will pull it high and see if that helps.

One last thing, I managed to test the spare PSU on the working 800 and it behaved faultlessly, so I now know that the PSU, ROM board, CPU board and one RAM board are potentially fine, but I need a 6502 CPU that will run at 3Mhz or an Atari CO14806-3 CPU, standard 6502's just don't cut it.

Very pleased with these results thats for sure.

[stephen_usher]

Getting there.... slowly.

[rubber_jonnie]

Getting there.... slowly.

Indeed.

Image quality is much better with a non buggy GTIA, just has no colour so I think there is an issue with the chroma generation. I'll double check the socket, you never know it may be something simple, or a bad trace on the PSU board where the socket is.

The iffy GTIA does give colour on my 600XL too, so it is a hint at something else.

I rather suspect it's something a bit more though as tweaking the colour pot on the CPU board does get me a little colour.

Also now running with 3 RAM cards. It's not the best, but the keyboard is also working, which is a bit of a miracle too. Also, SIO works as I can boot from my SDriveMax.

Carts work and I need to give it a thorough run through with my Super Salt card, but to get some tests done I could do with colour, so I'll probably chase that down next.

There really is a lot that's working.

[rubber_jonnie]

OK, so I've come to the conclusion that the problem with the colour is very likely down to the incorrect main crystal, and they are proving hard to find at the right frequency of 3.546894Mhz.

So I have 3 options here:

1. Borrow one from my 600XL
2. Try and find a new one
3. See if I can mimic it using a signal generator

Now I could see if my bench signal generator will do it, but it's not a long term solution, so in case I can't find the real crystal I've ordered one of thes which 'could' replace it:

Screenshot 2025-12-19 at 13.50.59.png

I think the fastest option for now is to borrow one from the 600XL, so that's next on the agenda after a little bit of probing to check some stuff.

I also have another Sally CPY on the way too.

[rubber_jonnie]

So, my assertion about the main crystal was correct. I borrowed the Xtal from the 600XL and it worked straight away in full colour:

IMG20251219151826_01.jpg

IMG20251219152459.jpg

Annoyingly the 4.433618 MHz broke on my 600XL whilst I was getting the 3.546894Mhz crystal off to test the 800 :oops: :oops: :oops:

And that's the colourburst crystal on the 600XL so that's black and white now too!!!

So I now need both and I'm struggling to find either :dizzy:

[stephen_usher]

Erk! At least you know it's the crystal now... not that it helps.

[olivier.jan]

Maybe you can find crystals with a higher frequency (multiple of the one you need) and use a simple counter or flip-flops to divide to the proper frequency?

[rubber_jonnie]

Erk! At least you know it's the crystal now... not that it helps.

I'll take it.

As it goes I managed to repair the colourburst crystal of the 600XL:

IMG20251219160004.jpg

It was 50/50 if it would work or not as the pin was broken off almost flush, but it works and I've dropped a spot of hot glue on there to keep everything steady. I managed to get some replacements from Ali Express, and if they don't work they didn't cost very much

[rubber_jonnie]

Maybe you can find crystals with a higher frequency (multiple of the one you need) and use a simple counter or flip-flops to divide to the proper frequency?

Definitely athought to consider, but I'm hoping the clock generator I have coming will do the job.