Atari Mega ST 1 Rev 1 no boot

[rubber_jonnie]

Did some more digging and noticed u35 and u32 are different than U36 and U33. Is this supposed to be like this?

As @PhilC said, yes, that's normal:

Screenshot 2025-03-08 at 07.30.06.png

I've attached the schematics here, just remove the .txt to turn it back into a PDF.

[olivier.jan]

The adapter you have should work, what I was mentioning are the little usb to serial (not RS232) like this: https://amzn.eu/d/dCjUFvn.

Anyway, if you want to check : Pin 8 of the MFP is TX while pin 9 is RX. So, if the diag cart is sending dat, you should see activity on pin 8 when you start the ST. You can test also the other direction: if you type data in your serial terminal it should generate activity on pin 9 of the MFP.

If you have an oscilloscope, checking will be pretty easy. With a multimeter, set on DC, you should see some voltage variations when there is activity.

Just realized I missed to mention a major WARNING: Do not try to connect any pin from your existing cable directly to the MFP !! It won’t work and you would most likely kill the MFP.

The cable you have is an RS-232 cable which uses +/- 12v for signals, the MFP only supports 5V, there’s a whole circuit in the ST to convert voltage from the RS-232 connector to the MFP, if you bypass it you will bring 12v to the MFP, not a good idea.

[rubber_jonnie]

When things run on for a bit, I always like to suggest a summary of where you're at now, for example:

What are the current symptoms?
What do you see when you boot without a diag cart?
What do you see when you boot with a diag cart? How much RAM is indicated when the diag cart does eventually load?
What do you see over serial when you boot with a diag cart
What are the measured voltages when on load?

When considering the symptoms, are you booting in HiRes? If so try booting in low/med res as it will show a border that you don't see in HiRes.

When threads get long it makes it easier for people reading to pick up rather than going back over 6 pages of posts.

Plus it's always good to have a checkpoint and verify what you've done so far for your own sanity.

I'd suggest if you can to take a photo of the area you reverted so that we can review what you've done, not because we think you've done something wrong necessarily, but because a 2nd pair of eyes is always helpful.

[Dwh18]

When things run on for a bit, I always like to suggest a summary of where you're at now, for example:

What are the current symptoms?
What do you see when you boot without a diag cart?
What do you see when you boot with a diag cart? How much RAM is indicated when the diag cart does eventually load?
What do you see over serial when you boot with a diag cart
What are the measured voltages when on load?

When considering the symptoms, are you booting in HiRes? If so try booting in low/med res as it will show a border that you don't see in HiRes.

When threads get long it makes it easier for people reading to pick up rather than going back over 6 pages of posts.

Plus it's always good to have a checkpoint and verify what you've done so far for your own sanity.

I'd suggest if you can to take a photo of the area you reverted so that we can review what you've done, not because we think you've done something wrong necessarily, but because a 2nd pair of eyes is always helpful.

Totally Right! First of all, Thank you so much to everyone that chimed in on this issue. Atari's is a new world to me, been more focus on other platforms like Sinclair, Commodore, IBM.... but have couple Atari's... this one is being challenging .....

What are the current symptoms? No boot, black screen in Hi Res, White then black on Low Res
What do you see when you boot without a diag cart? same as above
What do you see when you boot with a diag cart? How much RAM is indicated when the diag cart does eventually load? Diag Cart loads all tests pass, but vert slowly, full menu takes 2/3m to load. 1Mb is showed as expected-
What do you see over serial when you boot with a diag cart: Only garbage... to be sure that cable and connection was set right connected cable and card with a 1040St and all worked as expected, full menu over serial
What are the measured voltages when on load? I did a full recap on PSU, no measures were done anywhere else

Fixed one track... all continuities tested good

WhatsApp Image 2025-03-08 at 16.05.32_5d93d152.jpg

[rubber_jonnie]

No problem, help is what we try to do well here :)

Given you say voltages are not tested, I'd suggest you just check 5v and 12v are good, though 5v should be enough to boot.

When you say white then black when booting in lo res, do you mean the whole screen is white then the whole screen goes black, or do you mean the whole screen goes white and then you get a black screen with a white border?

It might seem like there isn't a difference, but there is a difference.

I'd think you're getting a black screen with a white border, it it's worthwhile being sure. That generally means no RAM.

BTW I assume the black wire is just the fix for the bad track? I'd also suggest you have a look at the resistors, they could do with a little more solder in the vias. I've had bad vias before that were ok so long as they were filled with solder, and not all of yours are.

Given it seems probable that there is a RAM issue, I'd suggest you should have a look at the power to the RAM IC's and also look at RAS/CAS and address/data lines on the RAM, though I don't know if you have an oscilloscope.

It seems to me that it's booting, but when it comes to start it can't find the RAM. I think this may be why the cart works but not the machine, and possibly why the cart boot takes a long time.

[Dwh18]

No problem, help is what we try to do well here :)

Given you say voltages are not tested, I'd suggest you just check 5v and 12v are good, though 5v should be enough to boot.

When you say white then black when booting in lo res, do you mean the whole screen is white then the whole screen goes black, or do you mean the whole screen goes white and then you get a black screen with a white border?

It might seem like there isn't a difference, but there is a difference.

I'd think you're getting a black screen with a white border, it it's worthwhile being sure. That generally means no RAM.

BTW I assume the black wire is just the fix for the bad track? I'd also suggest you have a look at the resistors, they could do with a little more solder in the vias. I've had bad vias before that were ok so long as they were filled with solder, and not all of yours are.

Given it seems probable that there is a RAM issue, I'd suggest you should have a look at the power to the RAM IC's and also look at RAS/CAS and address/data lines on the RAM, though I don't know if you have an oscilloscope.

It seems to me that it's booting, but when it comes to start it can't find the RAM. I think this may be why the cart works but not the machine, and possibly why the cart boot takes a long time.

I've an oscilloscope, just tell me what to look for and I will.... Meanwhile I'll try to test again the Hi/Low Res.... and fill the solder on those resistors. Tks!

[Dwh18]

Already filled those resistors and tested again HI/Low Res...

With Diag card - HI boots slowly Low - Red Screen with stripes and no diag screen.
No diag card - HI Black screen on boot Low - White for 3s then black screen

[stephen_usher]

Sounds like you need to put a terminal on the serial port and use the diag cartridge. It’s failing before it can properly display anything on the screen.

However, having a red screen is a positive sign as that means that it’s reading the ROM and starting the diagnostics.

[rubber_jonnie]

Low - White for 3s then black screen

So is it a completely black screen or a black screen with a white border?

@stephen_usher is right about the the red screen.

[Dwh18]

Low - White for 3s then black screen

So is it a completely black screen or a black screen with a white border?

@stephen_usher is right about the the red screen.

COmpletely black screen