Atari Mega ST 1 Rev 1 no boot

[Dwh18]

Have you left the computer booting for like a minute? It takes a pretty long time for it to show desktop if no disk or drive is attached.

I set a timer for 1m ... can leave more and try. Had that experience with a 1040 before.

[rubber_jonnie]

ROM checksums are generally a red herring because often the checksums in the diag cart don't match what's in the field. Highly unlikely to be a bad ROM, I've never encountered one in an ST of any kind.

I'd honestly suggest again you try to hook up to the serial port as you will see data that just doesn't show when you're in the diagnostics cart itself.

More likely something that was done when upgrading to 4MB, maybe some cut tracks or additional wiring, perhaps on the bottom of the board.

What are you using as a display? If it's hires, it tends to be a black screen for everything, if it's low res you will get a border, and if you then get a black screen with white border it tends to point at a ram issue. In low res an all white screen is suggestive of ROM issues. Not saying it always is but IME it often is.

Yes, different behavours with Hi and Lo Res .... Diag only loads in High Res monitor, and previous RAM error (white border whith black screen) only appeared in Low Res monitor, so RAM error I think it's done. the only patch I can see now on the motherboard seem's to be related with FDD I'll try to remove it and check behaviour. Tks!

Definitely looking like a RAM issue. I'd definitely recommend using a serial cable, it will tell you an awful lot about the RAM during boot.

One thing I would add here is that the Mega ST mainboards are pretty fragile, so handle with care, I had one board where I plugged an IDE adapter into the MegaBus connector and it popped 2 vias and broke it.

[Dwh18]

Got any pictures of the RAM expansion you removed to give us a clue?

Possible your test cartridge only tests a certain amount of RAM and beyond that things go awry?

Look for low value resistors near the RAM or MMU that have been lifted, removed or had their input redirected, perhaps.

BW

Here's the RAM expansión, couldn't find any info about it anywhere

WhatsApp Image 2025-02-21 at 11.39.40_0fb7c27a.jpg

Screenshot 2025-02-27 115745.png

[rubber_jonnie]

I have a similar mod actually, and I had to remove that little adapter board and put the mainboard back to normal to make it work.

[Badwolf]

It's not the most super-clear, but here's how it ought to look after you've removed that board:-



(from viewtopic.php?f=55&t=800)

BW

[rubber_jonnie]

My board:

IMG20250227125917.jpg

Could never make it work, Mega is fine now it's gone.

[Dwh18]

It's not the most super-clear, but here's how it ought to look after you've removed that board:-



(from viewtopic.php?f=55&t=800)

BW

Tks! Already installed new capacitors on the Motherboard, and have small progress.... before this only had the white border with black screen

[Dwh18]

Yes, different behavours with Hi and Lo Res .... Diag only loads in High Res monitor, and previous RAM error (white border whith black screen) only appeared in Low Res monitor, so RAM error I think it's done. the only patch I can see now on the motherboard seem's to be related with FDD I'll try to remove it and check behaviour. Tks!

Definitely looking like a RAM issue. I'd definitely recommend using a serial cable, it will tell you an awful lot about the RAM during boot.

One thing I would add here is that the Mega ST mainboards are pretty fragile, so handle with care, I had one board where I plugged an IDE adapter into the MegaBus connector and it popped 2 vias and broke it.

Only have DB9 serial cables.... for the Atari a regular DB25 female DB9 female serial should work, right?

[rubber_jonnie]

Definitely looking like a RAM issue. I'd definitely recommend using a serial cable, it will tell you an awful lot about the RAM during boot.

One thing I would add here is that the Mega ST mainboards are pretty fragile, so handle with care, I had one board where I plugged an IDE adapter into the MegaBus connector and it popped 2 vias and broke it.

Only have DB9 serial cables.... for the Atari a regular DB25 female DB9 female serial should work, right?

Yes as long as it's a null modem cable.

[rubber_jonnie]

My board:


IMG20250227125917.jpg


Could never make it work, Mega is fine now it's gone.

I just realised it's the exact same mod...