GabeShack

@DrVenkmanAny recommendations for a good one?

Thanks for the through response! Is the cost of a logic probe greater or less than the MMU chip? If less, I’ll grab one and start probing lol. Any recommendations for an inexpensive model?

I spoke with the best electronics shop who suggested removing the MMU and cleaning it again and putting it back 3 or 4 times to help clean out the sockets. Going to try that next. Last resort is buying MMU and OS chips.

Basic chip back in it boots to “ready” so yes basic boots to basic. But holding option down boots to self test

Tested with star raiders with basic chip out still only boots to self test. So could it possibly be the OS chip U5? If thats also a possibility then I'll order a spare of that as well with the MMU.

I just received the A/V cable set and was able to test with the original GTIA chip and its the same issue. So at least its not the Sophia 2 chip. Let me try star raiders with the basic out. I was testing with 2 known good carts that worked on my 400, empire strikes back and caverns of mars.

@DrVenkman I had tried swapping the PIA with the one from my 400 and nothing changed. The self test shows all green on the ram and roms. And with the basic chip pulled and a cartridge in, it just boots to the self test. Im hopeful that Simius is correct and its just needing a replacement MMU, but still have to find one to test with. I just reached out to best-electronics in hopes of ordering one. Looks like its only about $14 so not too bad.

Thanks for the suggestions. I found what looked liked a dried piece of grass in the cartridge slot, but still not reading anything. According to Simius, the MMU looks to be bad if its not reading the input from the card slot. I reviewed the soldering for the bottom of the sophia board riser, and each pin is separated without any bridging as tested with continuity. I just need an MMU to swap with to test Simius's theory. Ive also cleaned the slots extensively, even using a very fine emery board to clean each contact head with no effect. I have tested each contact using the pins outs referenced earlier by touching the actual cartridge port pins to the legs of the respected chips (or ground), and each seems to connect properly which again leads me to the chips themselves. I just need to get my hands on a replacement MMU to test with then move on to other chips. BTW with the basic chip pulled, the system boots right to the self test.

Testing the pins on the MMU with a cartridge in the slot shows 5v (4.9) on pins 6-9. So it looks like it is seeing the cartridge in the slot.

Simius is thinking its the MMU. I guess I can look for that chip individually or just get the U1MB. Thanks again all for the help with this. Really frustrating to just not know what the issue it.

thanks again for the help. I did try it with a cartridge before I removed the rf with the same issue. I’ll definitely scan the board again for any loose solder or bridges although there isn’t much I actually changed aside from the GTIA and those joints all look clean.

Thanks again. Just seems so weird that everything else works pretty normally. I reached out to Simius to see if there is any chance the Sophia 2 could be causing it, but since video is working I’d guess not.

Well doesnt look like there is a spare U3, U2 for me to test from my 400. May have to get a spare 800xl from eBay, or just be happy with the 400 I already modded that works.

I resoldered the pins. Still no change. I swapped the PIA for the one from my 400. Still no change. Looking at U3, U2 and cpu next.

Can I use a PIA from one of my spare Atari 400s or is it specific to the 800xl?

Thanks for the suggestion. I’ll resolder them and try again. However it seems like all the points are showing continuity based on the schematic above but still worth a shot.

Im not sure if it worked before the sophia 2 since i didn't have an older monitor to test with. I'm almost through continuity testing and so far its all matching up...so Im now wondering if its a bad U5 OS ROM chip possibly since the basic chip is working to load the Ready screen.

Attached are close ups of the cart port and the bottom of board with the cart port connections

So if thats the case how do I fix or test it?

Thanks for the reply, Ive tried most of the carts in the picture attached. Since I'm not sure which work with the 800xl I figured Id try them all. So far it only boots to the "Ready" blue screen or the self test if I hold option.

Hi All, I’ve recently worked on an Atari 400 and 800xl and installed the new Sophia 2 chips with dvi connections. Both are outputting great video now. My 400 had some issues at first running cartridges but after a few tries and cleaning some carts off it worked. My 800xl is another story though. I can’t get a single cartridge to start. Ones that I already tested as working on the 400 do not work on the 800xl. I ran the self test and all items pass. Wondering how to test the cartridge slot itself. I did use some canned air to blow out the port as well as a little 99.8 isopropyl. Anyone have any thoughts to verify if the cartridge slot is working? Are there any continuity points I can check from the cartridge pins? Any suggestions appreciated. GabeShack

Hi all, I saw pictures for an install on an 800xl but I didn't see anyone with a 400 install (the reg 800 seems to have more room on the i/o area on the right side, than the 400). Can someone share pictures on how they did the cut out for a 400? Also wondering if anyone created documentation with pictures for installs on each various model? If not, I'd be happy to attempt it.

Looks like I made a duplicate request. Sorry for that. Just looking for the 1 for a 400 and 1 for an 800xl. Sorry for the duplicate post earlier.

Hi, Id be interested in one for a 400 and one for an 800xl Thanks Simius!

Hey Simius, I’d love to order one for a 400 and another for an 800XL Thanks!