520 STF - Blue screen - Ram issue ?

[doc]

Hello.

I am experiencing issue with my Atari 520 STF (520 ko). While playing a game the screen suddenly froze. And no way to boot again. In place I have a light blue screen.
I immediately thought of a memory issue. But after checking continuity with great care, i can not find any short. I also change all the ships for new ones but nothing changed. All the ships are powered with 5v.
My Atari does not boot...
The photo below shows the screen I have when starting the machine with the Exxos test cartridge.

520stf.jpg (51.46 KiB) Viewed 1656 times

Would you have any idea ?
Unless it's a video problem?

Thank you for your help.

[stephen_usher]

The first thing I would try is to reseat the Shifter chip.

[kodak80]

I fixed a board (STFM 512kb) on the weekend with a similar screen output, blue blocks with a white boarder and it was due to a faulty LS244 chip. I love that my TL866II programmer can test Logic chips. RAM chips turned out to be fine. Once the board booted, I also found a faulty LS373 chip and faulty DMA chip, neither of these last 2 faulty chips stopped the board booting.

[rubber_jonnie]

I'd clean the MMU and Glue IC pins and sockets as well as the Shifter.

[doc]

Thank you for theses clues

So I changed the shifter with one functioning on another 520 ST -> Nothing changed
I removed the 2 LS373 without replacing them -> Nothing changed
I changed the 2 LS373 with 2 LS274 (in case I only had that) -> Nothing changed
I piggy plugged a LS244 on one ship -> Nothing changed

I piggy plugged a LS244 on the second chip -> Something changed !!!! Same mess with many colors

Look at the pictures

IMG_3439.JPG (151.35 KiB) Viewed 1580 times

And after a reset

IMG_3440.JPG (94.33 KiB) Viewed 1580 times

Any more ideas ? Do you think I should unsolder the suspected LS244 to firmly replace it ?

I own a TL866II PLUS but I cannot find how to test SN74LS chips... How do you proceed ?

[DoG]

...I own a TL866II PLUS but I cannot find how to test SN74LS chips... How do you proceed ?...

In menu -> [Device] -> [Logic IC test] -> Choose [74244]

[Steve]

You should always reseat and clean all removable chips before resorting to soldering or replacing chips. You could cause a problem on top of your original problem.

Have you cleaned and reseated every chip yet?