520 STM - video

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damanloox
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520 STM - video

Postby damanloox » Sat Feb 10, 2018 9:37 am

Hi,

I have the original 520 ST which I would like to keep for future generations but I'll need to fix it first :)
The Atari powers up but generates garbage as shown on the attached picture.
I swapped MMU and shifter already but it didn't help. Video cable/plug/socket and the tv - all double checked and verified.
Does anybody have any idea what could be wrong (before I start desildering DRAM which is rather painful process...)?
IMG_2910.JPG
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Atari030
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Re: 520 STM - video

Postby Atari030 » Mon Feb 12, 2018 12:15 am

I'd be interested to see if anyone has come across this one too. I have a Mega4 doing near the exact same thing.

I'll let you know if I work it out.

damanloox
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Re: 520 STM - video

Postby damanloox » Thu Feb 15, 2018 8:24 am

Atari030 wrote:I'll let you know if I work it out.


thanks. I'll update this topic if I find the source of the problem ;)
I'll try to do some work on it on the weekend so fingers crossed this topic will be updated with good news on Monday ;)

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Re: 520 STM - video

Postby gilles504 » Thu Feb 15, 2018 9:20 am

this is not a "video" problem, video is generated and seems stable. but it can be many things.
Does this pattern appears immediately at boot? does a reset makes something?
Are your roms in the correct sockets and in good condition?

damanloox
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Re: 520 STM - video

Postby damanloox » Thu Feb 15, 2018 9:31 am

gilles504 wrote:this is not a "video" problem, video is generated and seems stable. but it can be many things.

That's correct but the "video" is affected. My guess (at this point of troubleshooting) is either DRAM or ROM.
Does this pattern appears immediately at boot? does a reset makes something?

Right at power up. Reset doesn't change anything.
Are your roms in the correct sockets and in good condition?

They are in correct sockets. As for their condition - not sure. I'm expecting a delivery of working roms today so will check...

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Re: 520 STM - video

Postby gilles504 » Thu Feb 15, 2018 10:09 am

it could also be a processor stuck on reset state, or bad bus buffers. do you have a basic logic analyzer or at least a multimeter to check voltages?

damanloox
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Re: 520 STM - video

Postby damanloox » Thu Feb 15, 2018 10:21 am

gilles504 wrote:it could also be a processor stuck on reset state, or bad bus buffers. do you have a basic logic analyzer or at least a multimeter to check voltages?

Multimeter - yes, logic analyzer - no.. My multimeter has basic frequency check option and it seems like the clock on pin 15 is correct (or perhaps there's some other clocks I can check - but again - it's just simple frequency meter...)

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Re: 520 STM - video

Postby Atari030 » Thu Feb 15, 2018 10:29 am

From the looks of it, RAM. I have progressed to a black screen with white boarders toying with it. Unfortunately, with a Mega 4 that is a ton of RAM. What board revision are you running?

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Re: 520 STM - video

Postby gilles504 » Thu Feb 15, 2018 10:40 am

check also voltage on 18 (reset (also check when pressing reset)).
and voltage (and frequency if possible) on A1 to see if there is some activity on address bus.
It's just to eliminate a reset issue.

damanloox
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Re: 520 STM - video

Postby damanloox » Thu Feb 15, 2018 10:41 am

gilles504 wrote:check also voltage on 18 (reset (also check when pressing reset)).
and voltage (and frequency if possible) on A1 to see if there is some activity on address bus.
It's just to eliminate a reset issue.

Thanks, will do!

damanloox
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Re: 520 STM - video

Postby damanloox » Thu Feb 15, 2018 10:43 am

Atari030 wrote:From the looks of it, RAM. I have progressed to a black screen with white boarders toying with it. Unfortunately, with a Mega 4 that is a ton of RAM. What board revision are you running?

C070243 rev. H

damanloox
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Re: 520 STM - video

Postby damanloox » Fri Feb 16, 2018 7:49 pm

So - as I didn't have anything better to do on Friday evening - I took the challenge and opened up the Atari again...
Good guys at atari.org.pl suggested that this might be memory (bit 15-16 whatever that means) because of the pattern of black vertical lines. So I started from dram 16 (U45 - piggy back working dram) and - while it didn't help - I noticed another set of black lines. Moving to next chip - the new lines moved again... So I went straight to 1st dram (U16) and... nice GEM desktop appeared on my screen! Not only I know it was dram- I know which chip exactly (and only one fortunately)!!!
As for reset issues - I checked voltage on pin 18 - 5v and 0 on reset (which seems to suggest it's OK). Voltage on A1 - 5v. Can't measure frequency (or my multimeter says 0 - but again I don't think it's advanced enough to detect _any_ activity on the bus as it only works with stable, constant frequency - like pin 15).
Anyway - thanks for all hints (I learned a few things). My precious 520 ST is like new now ;)

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Atari030
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Re: 520 STM - video

Postby Atari030 » Fri Feb 16, 2018 8:59 pm

Good to know I'm on the right track. :-) I buzzed out the RAM on me Mega4 and all good, so it looks like a case of faulty chip. Mega4 has 32 1mbx1. This might take a while. I tried piggybacking too. I'm half tempted to get a marpet board and disable the onboard RAM.

I have to go get spares today to try a few things. Well done, mate, good result.

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Re: 520 STM - video

Postby Foxie » Sat Feb 17, 2018 1:28 am

I had almost the same problem with my 520STFM back in the early 90s. It displayed animated garbage on the screen at power-up and wouldn't do anything at all. In the end, I fixed it by taking the machine apart to the bare board and running it off a different power supply. Maybe one of those two things fixed it (if the board, then surely a bad contact somewhere). Or it could have been coincidence. The bare board still works to this day.

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Re: 520 STM - video

Postby Atari030 » Sat Feb 17, 2018 2:55 am

I just got mt Mega2 up and running, but the PSU went poof. Luckily I had some PSU's for external CDRoms floating around. Hotwire it and away it wet. I got desktop. Cost me a few new caps. But RAM it was. Now for the PSU.


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