01 May 2024, 01:18 | #1 |
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Location: Montreal / Canada
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A3000 Green screen with a failing OVL test
Hi, all.
I recently de-mothballed my Amigas only to be reminded of why they were mothballed some 20+ years ago in the first place. While I eventually plan to repair all of them, I figured I'd start with the 3000 since Green Screen = ChipRam issue was/is common and likely easy to fix. But that is not the world I live in where nothing is ever simple. (Mind you I have limited experience fixing my computers so you may have a different opinion on my efforts.) Here is a summary of the most helpful(?) tests/action I performed: - Board is bare (ChipRam and Daughter board, no extras - removed socketed ChipRam as well) - Oscilloscope shows activity on ChipRam Data/Address/WE/CAS/RAS lines - all "look" good - Oscilloscope shows activity on 74646 (Data lines) - all look good but chips get very hot...is that normal? - DiagROM - OVL check FAILS - Test words FAIL - Test Addr FAIL (Specific addresses are tested with 8 write/read sequence. Each read for a given address is the same, as if read worked but write failed. Ie: it just reads the same value that was in the memory location being tested. The next address does the same, but the read value will be different implying it's reading a different value from the new memory location and hinting that read is working. But I'm guessing) - DiagROM continues and shows the main menu (both RGB or Serial, if I press a button), but then it freezes. - Menu screen shows 512K of ChipRAM. (Should show 1M of the onboard RAM.) - It also shows 1.5M if I added the additional socketed 1M of ChipRAM - CPU not detected just displays 68xxx - Because DiagROM hinted the issue was the onboard ChipRAM, I removed it and tested it in RetroChipTester...All RAM passed. - Put sockets where onboard ChipRAM was, and tested with various Chips, always get to menu and then freeze - Rechecked continuity: ChipRam, 74646, Agnus, CIA (U350) All passed. - Assuming the OVL (overlay) issue (as reported by DiagROM): - Swapped the CIA chips (U300 & U350) - freezes - Replaced both CIA chips with working A500 CIA chips - freezes - Lifted pin 2 of "Odd" CIA (U350) [the OVL out pin] : OVL check still fails but no longer reporting Ram issues and menu shows 68030 and 1MB of ChipRAM...but still freezes. And if you were going to ask, I did recap the board and...same result. (The caps tested fine upon removal, but I changed them anyway). I even modified the DiagROM code to test the OVL earlier to see if the register was being set and then unset - it is not being set. Appreciate any ideas. Let me know if you want/need more details. Thanks for reading! |
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