MegaST 4 will not boot, but looks fine with diagnostics

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guus.assmann
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MegaST 4 will not boot, but looks fine with diagnostics

Post by guus.assmann »

Hello,

Today I've found a strange puzzle.
A MegaST 4 board will not boot, no display and no floppy activity.
Looks like an impropper reset.

So I conected a diagnostics cartridge.
Display, Ram, Timing, Blitter and also Floppy work just fine.
All tests ok.
Yet the board will not boot without the diagnostics.
I've reseated all IC's. The board looks very good and has no corrosion.

Any idea's ?

BR/
Guus
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exxos
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Re: MegaST 4 will not boot, but looks fine with diagnostics

Post by exxos »

ROM ? Have you left floppy on speed test for 5mins to see what happens ? machine got blitter ?
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viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
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troed
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Re: MegaST 4 will not boot, but looks fine with diagnostics

Post by troed »

In my experience for some reason some of the first chips to suddenly die in an ST are the TOS chips. I've had more than one "white blank screen" ST where I just replaced TOS and that was it.

The diag cart only needs a tiny part of TOS to be read correctly to execute.

/Troed
guus.assmann
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Re: MegaST 4 will not boot, but looks fine with diagnostics

Post by guus.assmann »

Hello,

Yes, the board has a blitter. I'll swap that one as well to see.
And I've replaced the Tos, MMU, Glue, Shifter, DMA. All those are on a socket.

One thing that's strange, in diagnostics doing the OS-Check.
It may work one time, and give an error the next time. Either CRC-check or 4 addresses bad.
After reset, sometimes a couple of times needed, it will display the diagnostics again.

The power supply is from Exxos, with a nice transformer.
And the board is totally bare, nothing on it is modified.
Also nothing is hot.

BR/
Guus
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Re: MegaST 4 will not boot, but looks fine with diagnostics

Post by exxos »

blitter patch installed ? if its not, maybe cut trace under CPU ? If its a national blitter then you likely dont need the patch.

I would do RAM test for half hour , see if anything shows up, also do the floppy speed test for half an hour, the you can see any pattern to crashing neither of those tests.

They could just be noise on the clock lines if it seems unstable. I had a lot of issues with that... If chips are IMP then you likely will have issues I think..


http://www.exxosforum.co.uk/atari/last/MEGAST/index.htm
https://www.exxosforum.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxosforum.co.uk/atari/store2/ - All my hardware mods for sale - Please help support by making a purchase.
viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
Just because a lot of people agree on something, doesn't make it a fact. ~exxos ~
People should find solutions to problems, not find problems with solutions.
guus.assmann
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Location: Ede Netherlands

Re: MegaST 4 will not boot, but looks fine with diagnostics

Post by guus.assmann »

Hello,
Ram test keeps working just fine. As does the floppy test.
Replacing the CPU has not helped, and removing the blitter is not a solution.
I've replaced MCU, Glue, DMA, Floppy controller and Blitter.
Using the two-tosses (Exxos) also doesn't work.
And the Shifter buffer doesn't help either.
There's no damaged traces to be found.

Then I placed a Tos 2.06 (Compo TAA4)
With this unit, the computer works just fine. :o
And the Tos test is also OK.
This shows it's relates to the OS, but I cannot understand what's going on

So I replaced the 74LS11. And due to lack of time, I've not tested with that yet.
I will also test with 6 Eproms instead of 2 Rom's

BR/
Guus
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Re: MegaST 4 will not boot, but looks fine with diagnostics

Post by exxos »

what is speed on ROMs which work ?
https://www.exxosforum.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxosforum.co.uk/atari/store2/ - All my hardware mods for sale - Please help support by making a purchase.
viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
Just because a lot of people agree on something, doesn't make it a fact. ~exxos ~
People should find solutions to problems, not find problems with solutions.
guus.assmann
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Location: Ede Netherlands

Re: MegaST 4 will not boot, but looks fine with diagnostics

Post by guus.assmann »

Hello,
The working one has 120nS Eproms.
I have yet not tested with faster 29F010 Flash, 90nS. Will do that as well.
Of course this card has a Gal. 16V8 25LNC.

I have tested with 6 Eproms, Tos 1.04NL in 200nS Eproms. Not ok.
Then I took Tos 1.04UK in 6 Eproms 100nS. Also not ok. (Yes, I changed W2- W3 and W4)

Also tested with Flash-Tos in 29F040 90nS. Also not ok.

Replacing the 74LS11 with a 74HCT11 doesn't work either.

BR/
Guus
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Re: MegaST 4 will not boot, but looks fine with diagnostics

Post by exxos »

I would change the first 2 ROM sockets and try again with 2 chips TOS...
https://www.exxosforum.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxosforum.co.uk/atari/store2/ - All my hardware mods for sale - Please help support by making a purchase.
viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
Just because a lot of people agree on something, doesn't make it a fact. ~exxos ~
People should find solutions to problems, not find problems with solutions.
guus.assmann
Posts: 94
Joined: Sat Sep 23, 2017 8:52 pm
Location: Ede Netherlands

Re: MegaST 4 will not boot, but looks fine with diagnostics

Post by guus.assmann »

Have replaced the sockets, now two quality sockets with round contacts are in.
Still with these it doesn't work either. :( :shock:

However it was a very good idea. :)
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