Atari Mega ST 1 Rev 1 no boot

[Dwh18]

Hi Everyone
Trying to fix a Mega ST 1 that I got... when I received the computer, it had a weird 3Mb expansion which I removed and restored to its original state with 1Mb.... Now I'm able to boot it with Bombs away diag card (which loads very sloooow) but shows no error in RAM. If I remove the diag card no boot to TOS... only black screen. Any clue where I should look next? Tks in advance.

[rubber_jonnie]

Hi Everyone
Trying to fix a Mega ST 1 that I got... when I received the computer, it had a weird 3Mb expansion which I removed and restored to its original state with 1Mb.... Now I'm able to boot it with Bombs away diag card (which loads very sloooow) but shows no error in RAM. If I remove the diag card no boot to TOS... only black screen. Any clue where I should look next? Tks in advance.

Is it a black screen or a black screen with a white border? Usually indicative of no RAM.

I'd reseat any chips in sockets and clean their contacts, use some contact cleaner if you have any. Also I'd suggest you hook up to the serial port with a null modem cable to a PC and see what data gets sent out over serial as that can often give some other info you don't see in the diag menu.

[Dwh18]

Only black screen... had the black with white border before and fixed moving all resistors to motherboard from expansion board. Already removed all IC's and cleaned all. Diag card seems to detect RAM and ROM.

[Dwh18]

I've this mod in C16 ... don't know the objective of this, but seen it in other systems. Can this be the issue?

WhatsApp Image 2025-02-26 at 17.21.23_8908327e.jpg

[stephen_usher]

It's a blitter timing patch adding a delay to (?)/DTACK. It's a factory fitted patch.

[Dwh18]

It's a blitter timing patch adding a delay to (?)/DTACK. It's a factory fitted patch.

Tks!
Could it be that any other change was made to the Motherboard that I'm not seeing to support the 4Mb? Saw some posts where additional jumpers were needed. But not seeing any in this issue 1 Motherboard. Also thinking if could be any Rom issue, Diagnostics show the attached result but couldn't find any info if ROM diag shows a good or bad ROM

WhatsApp Image 2025-02-26 at 15.32.51_54161edb.jpg

[Badwolf]

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

[rubber_jonnie]

It's a blitter timing patch adding a delay to (?)/DTACK. It's a factory fitted patch.

Tks!
Could it be that any other change was made to the Motherboard that I'm not seeing to support the 4Mb? Saw some posts where additional jumpers were needed. But not seeing any in this issue 1 Motherboard. Also thinking if could be any Rom issue, Diagnostics show the attached result but couldn't find any info if ROM diag shows a good or bad ROM
WhatsApp Image 2025-02-26 at 15.32.51_54161edb.jpg

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.

[DoG]

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.

[Dwh18]

Tks!
Could it be that any other change was made to the Motherboard that I'm not seeing to support the 4Mb? Saw some posts where additional jumpers were needed. But not seeing any in this issue 1 Motherboard. Also thinking if could be any Rom issue, Diagnostics show the attached result but couldn't find any info if ROM diag shows a good or bad ROM
WhatsApp Image 2025-02-26 at 15.32.51_54161edb.jpg

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!