520STfm with 20 bombs

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spett
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520STfm with 20 bombs

Postby spett » Mon Jan 09, 2017 9:32 pm

Hi all!

After reading alot of great information on this site I finaly decided to create a user account, well, because I need your help! :angel:

I got a 520STfm here which don't boot. 20 bomb instant after reset/power. Bougth this broken and it came with a 1 meg Xtra-RAM upgrade installed, this is now removed, all socketed chips are cleaned/resat but nothing have changed.
But then I tried crossing CAS0H and CAS0L resistor to swap HI/LOW ram banks, just as a test if it changed anything and then got a white/black scrambled screen! I therefore suspect a faulty RAM chip but then again I'm not an expert just a hobbyist..
Or is there more change needed to swap HI/LOW RAM banks?

I'm very happy for any suggestions/help/information bringing this back to life.
Any known faults to look for? I have oscilloscope, logic analyser, eeprom burner, etc etc... Just tell me where to probe :lol:

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Arne
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Re: 520STfm with 20 bombs

Postby Arne » Tue Jan 10, 2017 5:07 am

spett wrote:Or is there more change needed to swap HI/LOW RAM banks?

So you do have 2 banks with 512KByte each?
  • Get schematics (1040STFM) and Service manual (e.g. Download section of my homepage).
  • Read the section "Troubleshooting a dead unit" in the service manual.
  • Act accordingly
spett wrote:Any known faults to look for? I have oscilloscope, logic analyser, eeprom burner, etc etc... Just tell me where to probe :lol:

Good to know. :) You will need it!
Hardware projects & Shop @ http://atari.team-yankee.net

spett
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Re: 520STfm with 20 bombs

Postby spett » Tue Jan 10, 2017 8:44 pm

Arne wrote:
spett wrote:Or is there more change needed to swap HI/LOW RAM banks?

So you do have 2 banks with 512KByte each?
  • Get schematics (1040STFM) and Service manual (e.g. Download section of my homepage).
  • Read the section "Troubleshooting a dead unit" in the service manual.
  • Act accordingly

No, I only have one 512KByte RAM bank. What I meant was that I tried to swap LO/HI RAM byte (CAS0H and CAS0L) to see it that changed anything, I was thinking that if that changed anything that would be an indication of a fault in a RAM chip because if all RAM is good that should not change anything, or what do you think?

I have had a look in the Serivce Manual and checked the rather small list there (because I don't hva a test cartridge). My 8MHz signal is good but cpu halts after ~22 sec. But because it does output picture (altough with alot of bombs) I'm thinking "double bus error" out of the 3 listed causes in the service manual as the most plausible fault.

Couple of question for you gurus;
Are all chip revisions interchangable?
What brand/type is the original 28pin 1MBit TOS chips? Thinking of reading mine but can't figure out chip type. I do have a rather good eeprom burner but chip detect fails..

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Arne
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Re: 520STfm with 20 bombs

Postby Arne » Wed Jan 11, 2017 6:30 am

spett wrote:My 8MHz signal is good but cpu halts after ~22 sec.

IIRC Glue does assert /BERR 64 clocks (8MHz) if it does not see /DTACK asserted.
Do the following:
if /BERR is asserted then check the history before this assertion.
There should be /AS asserted and /DTACK not being asserted.
Now if you found this position then CPU wants to access some device (DRAM = MMU, peripheral (= DMA, MFP, PSG...). So there should be some /CS asserted. Check which /CS is asserted. Then you should have found the "victim" which does not respond correctly to a /CS. Either /CS line is broken, some address/data lines are broken... can have many reasons.

spett wrote:Are all chip revisions interchangable?

In general: yes. But stay away from Atari ASICs of the manufacturer IMP.

spett wrote:What brand/type is the original 28pin 1MBit TOS chips? Thinking of reading mine but can't figure out chip type. I do have a rather good eeprom burner but chip detect fails..

That's an "Atari rarity". These are PROMs that do not need typical EPROM lines like Vpp. So they omitted these - et voilá: it fitted into a 28pin package. 1MBit (27C1001/27C010) EPROMs do have 32pins. So you either need an adapter PCB, or do some pin bending and wire-wrapping or (most comfortable option) program six 27C256. Then you will need to set three solder-bridges near the ROM sockets. Check schematics.

Hope that helps.
Hardware projects & Shop @ http://atari.team-yankee.net

ijor
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Re: 520STfm with 20 bombs

Postby ijor » Thu Jan 12, 2017 2:58 am

Doesn't sound as being a BERR/DTACK problem. I would expect that if some component doesn't respond, it would be much earlier than after 22 seconds.

Note that what you describe is not exactly a completely dead unit. You get a (more or less) correct display. This means that the CPU, GLUE, MMU, SHIFTER, ROM & RAM they are all working. They might not be working perfectly though. But if you get bombs, then no of those components is completely dead either.

Halted CPU means indeed double fault. But there are many causes that indirectly can lead to the double fault. RAM problems could certainly be the source.

spett
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Re: 520STfm with 20 bombs

Postby spett » Fri Jan 13, 2017 9:23 pm

IT's ALIVE! :lol: :lol:

While probing /DTACK /CS lines etc I discovered that one address line was not acting correctly @ GLUE. A corroded via was the cause resulting in a ~100k resistance between GLUE and the rest of the address bus. A little rubbing, cleaning and some solder and it is booting :D

Thanks to both of you for keeping my spirits up!

One last question for you gurus, is there a full system test program(that test more than just RAM) floating around anywhere, like a selftest to ensure it is working properly?

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Arne
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Re: 520STfm with 20 bombs

Postby Arne » Sat Jan 14, 2017 7:46 am

Good work! :cheers:
I only know of the Atari test-cartridge. But this one needs loopback connectors/a test fixture for DMA/Parallel port.

Test fixture:
Image
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czietz
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Re: 520STfm with 20 bombs

Postby czietz » Sat Jan 14, 2017 11:18 am

Arne wrote:Good work! :cheers:
I only know of the Atari test-cartridge. But this one needs loopback connectors/a test fixture for DMA/Parallel port.



I always wondered if one could rebuild that test fixture with a CPLD. After all, it's just 74-series logic and SRAM and the schematic is available. Did anyone already try this?


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