Hello there,
I have a Macintosh Classic that had many different faults, and I'm now at one that I'm not completely sure how to make progress with: The machine has the RAM expansion board with two 1MB SIMMs, so I should have 4MB in total. The system only shows 2MB however. I have tried different SIMMs but the result was identical. Given that the board had some corrosion issues, it is not impossible that there is a broken trace that causes this, but I have checked a couple of things:
My first thought was that A21 would not be connected to the RAM subsystem, causing the lower 2MB to be mirrored to the upper 4MB (assuming that the RAM sizing algorithm works by probing possible RAM locations). I have, however, traced A21 from the CPU to UH6 (74LS174) and can see activity both on the input pin to the latch and also on its output to UL1 (CASPAL)
I believe that this means that the problem is either in UL1, or there is something else that I'm missing.
I've also used the ROM diagnostics to check whether I can see writes to the DRAM at 0x00100000 be mirrored at 0x00300000, and that is indeed the case. I had pondered whether I'd be able to capture writes made with the ROM diagnostics at the outputs of UH6, but given that there is a lot of activity on A21, that seems to be difficult and also of limited utility.
If the GALPAL is really gone, I might be able to do Adrian Black's conversion to 4MB on the Motherboard, but I don't have the required RAM chips at hand and before I go that route, I would like to be sure that the PAL is really the issue. Does anyone have any suggestion how I could verify that to be the case?
Thanks!
Hans
I have a Macintosh Classic that had many different faults, and I'm now at one that I'm not completely sure how to make progress with: The machine has the RAM expansion board with two 1MB SIMMs, so I should have 4MB in total. The system only shows 2MB however. I have tried different SIMMs but the result was identical. Given that the board had some corrosion issues, it is not impossible that there is a broken trace that causes this, but I have checked a couple of things:
My first thought was that A21 would not be connected to the RAM subsystem, causing the lower 2MB to be mirrored to the upper 4MB (assuming that the RAM sizing algorithm works by probing possible RAM locations). I have, however, traced A21 from the CPU to UH6 (74LS174) and can see activity both on the input pin to the latch and also on its output to UL1 (CASPAL)
I believe that this means that the problem is either in UL1, or there is something else that I'm missing.
I've also used the ROM diagnostics to check whether I can see writes to the DRAM at 0x00100000 be mirrored at 0x00300000, and that is indeed the case. I had pondered whether I'd be able to capture writes made with the ROM diagnostics at the outputs of UH6, but given that there is a lot of activity on A21, that seems to be difficult and also of limited utility.
If the GALPAL is really gone, I might be able to do Adrian Black's conversion to 4MB on the Motherboard, but I don't have the required RAM chips at hand and before I go that route, I would like to be sure that the PAL is really the issue. Does anyone have any suggestion how I could verify that to be the case?
Thanks!
Hans
