ST520FM screen garbage

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cirollo
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ST520FM screen garbage

Postby cirollo » Sat Apr 27, 2019 4:05 pm

Hi!

I've bought an atari 520st fm yesterday.
It turns on but I get garbage on screen, as you can see in the pictures.

This is the first atari st for me, so I need your help to try to recover it.

Please tell me what to check....

regards.
Ciro from Italy
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Chandler
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Re: ST520FM screen garbage

Postby Chandler » Sat Apr 27, 2019 5:13 pm

Funnily enough i have just repaired a Mega ST with the same screen (black square with lines / white border)

It's a memory problem

I can see from your motherboard picture that someone has added 512K to make your machine 1mb at some point so maybe the issue could be there

I'd check all the solders on and around the 256k dram chips, original and added

Ultimately though you may not "see" any problems

DoG
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Re: ST520FM screen garbage

Postby DoG » Sat Apr 27, 2019 5:29 pm

I have the same screen/problem on a Mega 1 I just bought. I was going to check the memory on that one. Great to get it somewhat confirmed.

ninjabuffy
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Re: ST520FM screen garbage

Postby ninjabuffy » Sat Apr 27, 2019 5:35 pm

I would first try to desolder R71, R72, R73 to disable the added RAM. Since that ram is missing decopling capacitors it might be others issues as well with the upgrade.

cirollo
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Re: ST520FM screen garbage

Postby cirollo » Tue Apr 30, 2019 4:04 pm

Hi! and what about the tos chip with reversed label?

Chandler
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Re: ST520FM screen garbage

Postby Chandler » Wed May 01, 2019 8:18 am

The TOS chips are correct, forget the label

It's the notch in the middle of one end of the chip that is the important part

I would turn off bank 2 of the memory (the upgraded part) and then test again

If it still does the black square then it is either the MMU chip / the MMU socket / the ram or the 2 LS244 buffers

You could either add something like a Marpet memory kit to bypass the original memory or replace all of the original memory, but replacing all the original memory may not solve the issue if the problem is elsewhere
Last edited by Chandler on Wed May 01, 2019 9:43 pm, edited 1 time in total.

6BQ5
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Re: ST520FM screen garbage

Postby 6BQ5 » Wed May 01, 2019 1:58 pm

I have recently had a strange experience where faulty keyboard and faulty floppy drive were overloading the available power lines. Unplugging them both allowed me to boot to the desktop. It's not practical for everyday use :) but it is easy to test and may help isolate the problem. If you still get bad result then also check the power supply itself.

Another instance of garbage screen happened to me with a 1040 STe. After replacing the supply with a new unit everything is fine. Keyboard and floppy drive are just fine.

Also, if you do cycle power then be sure to wait 20 seconds or so for internal voltages on motherboard to really go down to zero. If you cycle too fast then you may have some residual data stored in memory which could cause a crash and garbage screens

Boris

Atari 130XE
Atari 1040 STe

cirollo
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Re: ST520FM screen garbage

Postby cirollo » Thu May 02, 2019 4:11 pm

Chandler wrote:The TOS chips are correct, forget the label

It's the notch in the middle of one end of the chip that is the important part

I would turn off bank 2 of the memory (the upgraded part) and then test again

If it still does the black square then it is either the MMU chip / the MMU socket / the ram or the 2 LS244 buffers

You could either add something like a Marpet memory kit to bypass the original memory or replace all of the original memory, but replacing all the original memory may not solve the issue if the problem is elsewhere


I can use the kit instead of the original ram? I thinked it can be used only to add up to 4mb the ram on board.....

where I can find this kit at a good price? I need to only remove the original ram or other soldering?


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