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Amiga computer related discussion => Amiga Hardware Issues and discussion => Topic started by: Dimmy on February 05, 2025, 12:41:55 PM
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I try to fix an old amiga500, and got diagrom running, using extra memory in the expansion. Only serial output, when checking the ram I notice that all the ram has an error on the same spot. I am new to all this, but I like to learn. My assumption would be, that the ram is not broken, but maybe a controler? I did swap out Agnus, that did nothing.
As you can see there are the Parallel Code $fe - Test UDS/LDS line errors and the memory has a read faults (could be writing also of course) but alway on the same spot.
I switch Agnus, CPU and Garry, but that did not change anything. I looked for oxidation, did not find any. the board looks really clean.
I get into the menu via serial, because I use the expansion ram.
I also attached the memtest result and the diagrom start output
I hope that someone could help me in the right direction, on how to get closer to a fix.
Thanks all!
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Hi Dimmy and welcome.
Have you checked for broken traces? PCB explorer can help you to check connectivity between components.
http://www.amigapcb.org/index.php (http://www.amigapcb.org/index.php)
Weed
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First thing I would say, Does it work without expansion. As you have 2mb?
So you have added 1.5mb chip?
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No, without the expansion Diagrom wil start but cant continue, because of no working chipram, This is a Rev3 board that has 512 on board and I have an original 501 expansion in there to get diagrom to work.
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Thank you,
I did try to look at traces, but pcb explorer only has A500 Rev 8A, that is completely different then the Rev3 I have here.
Maybe someone has a Kicad for Rev.3? I could not find one.
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The DigROM output is showing that D1 from the DRAMs is not working. Not sure why it's only every second word; as the data bus is 16-bits wide, you'd expect to see two errors in each longword, at D1 and again at D17.
The rev 3 A500 uses 1-bit DRAMs, same as the rev 5 A500 and rev 4 A2000. The DRAM responsible for D1 is U17. I'd start by measuring at U17 pins 2 or 14 (data in and out are connected together) to see if there's any activity present. If not, work backwards to find out where it's broken. If there is data activity at U17, then it's likely defective, so replace U17.
This is either the A500 rev 3 or 5 schematic, the DRAM detail between the two should be much the same. I don't have a better copy unfortunately:
http://amiga.serveftp.net/Schematics/A500_schematics/A500-RAM-ROM-MemExpSlot.jpg
Another way to fault find this is to exchange U17 and U18. If DiagROM shows that the error has moved from D1 to D2, then the original U17 is definitely defective.
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Thank you,
I tried the piggyback method, did nothing at all. tried on all 16, nothing changed in diagrom.
The I just removed U17, because of what you said and put a socket in there, and a new ram, then I got somehow other errors, so I stack tested the other rams and when I stacked U19, all the errors where gone!!
Thank you all for your Help,
I replaced the old roms with another brand: Intel p21256-80.