exxos's Atari, Amiga & retro forum

Hello all,
I was enjoying my upgraded Atari 520 STf for about a month while, a few days ago, suddenly I started having some new issues... :(

The first symptom was vertical lines while loading a game... then I did try my Diag Cart and I got a Red Screen with no menu at all. I did some research and the issue seems caused by a "dead" MMU/Glue C025915-38A Chip on my C103175 Rev.1 Mainboard.

Before buying a new one to see if, actually, it is the problem I would like to know from someone that already encountered this behavior if a new C025915-38A Chip could bring back to life my A520 STf.

It seems that it is not able to manage the video RAM, for example if I start lemmings I can hear the music of the cracktro but I only see a black background and the two horizontal lines but the computer doesn't show the image and the writing in the middle of the screen.

I did test the computer either with 1024 and 512 KB with the same result. I also refreshed all the solder joint of the chip without success.
Any hints will be much appreciated, thanks!

Just a few pictures to better explain the issue.

jurassicman wrote: 30 Nov 2025 09:15 Just a few pictures to better explain the issue:

Can I please remind you that from an admin perspective all images should be uploaded to the forum via the attachments tab and not linked to external sites.

External sites go down, or disappear leaving dead links in posts and incomplete information for future readers, hence why we ask that images are uploaded.

As for your issue, I'd suggest hooking up the serial port via null modem cable to a PC and using a terminal there to see what the diag cart is spitting out.

Details of how to do this are here: Diag cart basics

Hello, thanks for your message.

I'm sorry but I didn't know that linking the images from an external website wasn't allowed... if I click on the "image icon" I'm asked to insert an URL so I thought that was the "suggested" way to show an image.
I'm going to emend my post and I'll attach my pictures to my messages from now. on.

As per your hint about the serial port, I could try that but I haven't a serial cable at the moment and also I only use Linux on my PCs so I should find a suitable program to receive the image.

If someone could confirm that already had this issue in the past and solved it replacing the MMU/Glue Chip that would be enough for me. :)

jurassicman wrote: 30 Nov 2025 10:35 If someone could confirm that already had this issue in the past and solved it replacing the MMU/Glue Chip that would be enough for me. :)

Until you have the information from the diagnostic cart output from the serial port we don't have enough information to pinpoint the MMU or GLUE chips. It could be anything at this point, possibly even memory. Please don't jump the gun.

With regards to a terminal connection via Linux, that's pretty easy. You'll need a USB-Serial adapter, such as this: https://www.amazon.co.uk/StarTech-com-U ... B000HVHDJ8 and a null modem cable. You can then install a serial terminal program such as minicom.

jurassicman wrote: 30 Nov 2025 10:35 Hello, thanks for your message.

I'm sorry but I didn't know that linking the images from an external website wasn't allowed... if I click on the "image icon" I'm asked to insert an URL so I thought that was the "suggested" way to show an image.
I'm going to emend my post and I'll attach my pictures to my messages from now. on.

As per your hint about the serial port, I could try that but I haven't a serial cable at the moment and also I only use Linux on my PCs so I should find a suitable program to receive the image.

If someone could confirm that already had this issue in the past and solved it replacing the MMU/Glue Chip that would be enough for me. :)

It's not a problem just doing my job as admin, it happens from time to time especially for new posters.

As @stephen_usher has said, some diagnosis is needed before making any call as to what the problem is and you should be able to get a null modern cable cheaply.

Ok, thanks both for your messages. :)

I'll buy a serial cable and see what the Diag Cart says. I thought that the errors I described could easily be traced back to a “dead” MMU/Glue C025915-38A Chip since I asked "Grok" about it and it said that it was almost certainly the cause (it said that, on the forums it checked, 90% of times that Chip is the guilty for such a behavior).

I looked myself on the Web to find some more info but I couldn't find anything so I decided to ask here.

I'll get back as soon as I get the output from the Diag Cart on my Linux PC.

The first rule of AI interaction is never take the output of an AI to be true, without corroborating evidence. They are merely digital hallucinations.

stephen_usher wrote: 30 Nov 2025 13:25 The first rule of AI interaction is never take the output of an AI to be true, without corroborating evidence. They are merely digital hallucinations.

I totally agree and, in fact, I looked on the web and asked here before replacing the chip.
AI are useful sometimes but they say a lot of wrong information.

Yeah I fried an Apple IIGS because I trusted AI for the PSU pinout to be correct. It had been the previous time, but it seems I asked just a little differently the second time...