Atari Mega STE Black screen in color only. Monochrome ok.

[Roberto]

Hi everyone,
I was finally thinking of re-solding the connector on the back of my Atari SC 1224 color monitor (since I can't see the image anymore), when I said to myself: maybe it's better to try it on another Atari.. The monitor works great, so it's my Mega STE that doesn't work properly when connected to the color monitor. I always use it on a monochrome monitor and assumed that the color monitor itself was the cause of the problem.
Having established that the problem is on my Mega STE, I tried the diagnostic cartridge, but unfortunately it seems to have problems (it no longer works on any of my Ataris... meh), so I searched a bit on the net and discovered that the U504 it is an integrated circuit (placed under the shielding of the expansion card slot) which takes care of selecting the type of monitor to be managed. One user says: ...

"you need to test this defect by cutting pin 7. This will disable mono mode and hopefully it will now work in color. This way you know the chip is bad."

The fact is that in my Mega STE you can't hear the keyboard click when connected to a color monitor, so it seems strange to me that this U504 could be the cause; among other things I made some measurements with my oscilloscope and it seems to work correctly (the U504 is a 74F257A-03 SL86189). I also changed TOS and reseated RAM and other chip sockets.

Basically, when I turn it on, for the first 3 seconds, I see a white screen, then I permanently see a black screen, but only when it is connected to a color monitor.

[Badwolf]

Basically, when I turn it on, for the first 3 seconds, I see a white screen, then I permanently see a black screen, but only when it is connected to a color monitor.

If you have a scope then you should be able to check if it's stuck in mono mode by measuring the vsync. 50 or 60 Hz for colour, over 70 for mono.

A little less invasive than cutting tracks.

BW

[Badwolf]

Also, if you're software-inclined, I believe the switch to mono mode is merely an OS check every vertical blank.

It should be possible to compile a custom version of emutos, bootable from floppy, that doesn't perform that check or doesn't act on it. This would force the monitor into colour despite the monodetect. This would be a sanity check before any attempted hardware fix.

See: https://github.com/emutos/emutos/blob/m ... en.c#L1221

BW

[Roberto]

If you have a scope then you should be able to check if it's stuck in mono mode by measuring the vsync. 50 or 60 Hz for colour, over 70 for mono.

A little less invasive than cutting tracks.

BW

Thanks for these directions. I think it's time to print some wiring diagrams relating to the operation of the video of my Mega STE, so as to understand how it works. I would like to use my EXXOS diagnostic cartridge but it just doesn't work: black screen or it is bypassed. This happens with all my Ataris. I think I will open a new topic for this to understand the reason for this malfunction.

[Roberto]

Hi everyone, these days I haven't had much time to dedicate myself to the tests you suggested. Also because I have to understand on which chip and in which pins to perform the vsync test. However today I managed to get my diagnostic cartridge working. Inserting the cartridge and turning on my Mega STE I get this screen. Don't let me get to the menu.
Is there any change compared to the tests you recommended me to do?

IMG_0042 2.jpeg

[rubber_jonnie]

Hi everyone, these days I haven't had much time to dedicate myself to the tests you suggested. Also because I have to understand on which chip and in which pins to perform the vsync test. However today I managed to get my diagnostic cartridge working. Inserting the cartridge and turning on my Mega STE I get this screen. Don't let me get to the menu.
Is there any change compared to the tests you recommended me to do?

IMG_0042 2.jpeg

Comparing to my own MSTE with a different diag cart, immediately after that screen I get the screen flickering and flashing and it drops me down to the menu. So you're definitely getting the first part of the diag cart startup.

I have had diag cart failures as a result of bad/dirty edge connectors and the socket on the machine, so if you haven't already cleaned them, I'd suggest you do that next, and double check for solder whiskers where you were working.

If you have a suitable serial cable, hook up to a PC and terminal program to see if anything additional comes through over that.

[Roberto]

Comparing to my own MSTE with a different diag cart, immediately after that screen I get the screen flickering and flashing and it drops me down to the menu. So you're definitely getting the first part of the diag cart startup.

I have had diag cart failures as a result of bad/dirty edge connectors and the socket on the machine, so if you haven't already cleaned them, I'd suggest you do that next, and double check for solder whiskers where you were working.

If you have a suitable serial cable, hook up to a PC and terminal program to see if anything additional comes through over that.

Yes, I cleaned all the contacts perfectly (even those relating to the Mega STE cartridge socket), so I'm sure it's not a contact problem (let's say 99%). Today I will try to see what happens if I connect my Mega STE to a serial socket: it's a good idea.