Mega ST dead ? Have you seen this before ?

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Marsupilami
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Mega ST dead ? Have you seen this before ?

Post by Marsupilami »

Hello,
my Mega ST1 is sick, does anyone have any ideas?

At startup I have either a white screen or lines (background colors may change)...
I have already changed :
- the keyboard
- the power supply (and checked voltages on both)
- the shifter (CO70713) and its DIP socket
- and the TOS.
At boot the floppy disk drive doesn't move at all (I changed it too).

Thanks for you help.
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frank.lukas
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Re: Mega ST dead ? Have you seen this before ?

Post by frank.lukas »

Ram faulty ...
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Arne
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Re: Mega ST dead ? Have you seen this before ?

Post by Arne »

Even if RAM is defective... shouldn't it display a stable screen? At least that's how my F030 behaves. I'd say it should not show these "frayed" borders. Look like HSYNC is out-of-sync.
GLUE: HSYNC pin 37, VSYNC pin 38. Check these with an oscilloscope.
There is a GAL listing around which re-creates these signals if GLUE is partly defective in this area.
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Re: Mega ST dead ? Have you seen this before ?

Post by Bikerbob »

Picture of the board please? Does this have the blitter patch? I was having bad memory sync issues. it could be the blitter patch issue.

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Marsupilami
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Re: Mega ST dead ? Have you seen this before ?

Post by Marsupilami »

@franck.luckas : Yes maybe, i have a spare Exxos RAM extension i could use to test.

@Arne : Unfortunatelly I don't have any oscilloscope. And no GAL progammer either :roll:

@Bikerbob : some pictures of the board : yes there is a patch.
20191020_144558.jpg
20191123_155047-2.jpg
20191020_143629.jpg
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Re: Mega ST dead ? Have you seen this before ?

Post by Bikerbob »

http://atari-forum.com/viewtopic.php?t=27509#p269814

There is a link to Exxos megaST findings as well.. which is worth a read. A diagnostic cart might be a help here. Most likely fixable.. just not sure you have the equipment to do it.

Any other machines that you can try chips in?

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Marsupilami
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Re: Mega ST dead ? Have you seen this before ?

Post by Marsupilami »

Ok so...
I cleaned the contacts with IPA on :
- CO70714 (Glue)
- S90112AD chip (near the keyboard connector)
- The shifter
- The VLSI chip above (S025BV A10483 - 4070ACPIA VLI772-02PC

Then I added 2 Exxos Kits :
- a TOS104 / TOS102 DUAL TOS MEGA ST KIT
- a ST/F/M / MEGA ST-RAM 4MB UPGRADE

And... It works. Or should i say...It's better :

- The Atari boots fine
- The floppy is running fine but some good floppies needs some retries to be able to read content. The floppy drive is OK on other STs and the floppies too.
- I have some artefacts appearing randomly on the screen.
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20191128_214105 (1).jpg
And B.T.W. do you have the reference of this resistor ?
20191128_222031.jpg
20191128_222007.jpg
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Re: Mega ST dead ? Have you seen this before ?

Post by Marsupilami »

Just an update : I removed the old RAM, it took a long time and patience but it solved the problem : No artifacts anymore on the screen.
I still need to replace the damaged resistor on the PSU. But is it a 5w ? 1 W ? 1/2 W ?
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Re: Mega ST dead ? Have you seen this before ?

Post by DoG »

Marsupilami wrote:I still need to replace the damaged resistor on the PSU. But is it a 5w ? 1 W ? 1/2 W ?
R19? It's 5W.
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Re: Mega ST dead ? Have you seen this before ?

Post by Timnaber »

:cheers: Bravo.

Exxos suggest to remove the old RAM as it does use up some of the power, which is probably why it is better.
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Marsupilami
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Re: Mega ST dead ? Have you seen this before ?

Post by Marsupilami »

DoG wrote:
Marsupilami wrote:I still need to replace the damaged resistor on the PSU. But is it a 5w ? 1 W ? 1/2 W ?
R19? It's 5W.
Thanks, now i need to check the Ohms value with my ohmmeter (when you're a bit color blind it's hard to see) :lol:
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Marsupilami
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Re: Mega ST dead ? Have you seen this before ?

Post by Marsupilami »

Timnaber wrote::cheers: Bravo.

Exxos suggest to remove the old RAM as it does use up some of the power, which is probably why it is better.
Thanks :) Yes, Exxos is always right as usual :)
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Re: Mega ST dead ? Have you seen this before ?

Post by DoG »

Marsupilami wrote:
DoG wrote:
Marsupilami wrote:I still need to replace the damaged resistor on the PSU. But is it a 5w ? 1 W ? 1/2 W ?
R19? It's 5W.
Thanks, now i need to check the Ohms value with my ohmmeter (when you're a bit color blind it's hard to see) :lol:
It should be 47 Ohm according to the schematics.
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