Fixing a Macintosh Classic with memory issues

[hans23]

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

 

[bibilit]

there is a jumper on the board for 2Mb > 4 Mb options, is this ok for you ?

 

[hans23]

there is a jumper on the board for 2Mb > 4 Mb options, is this ok for you ?

Do you mean the "SIMM installed" jumper on the memory expansion board? That is installed and set correctly.

 

[bibilit]

Do you mean the "SIMM installed" jumper on the memory expansion board? That is installed and set correctly.

Yes.

So probabaly a broken trace somewhere

 

[hans23]

So probabaly a broken trace somewhere

I would agree, but all traces in the relevant areas are intact. There are a couple things that make me doubt the "broken trace" idea: The machine is working with 2MB without any problems. The address lines are going to UH6 (74LS174) and they show activity there. I also see activity on the output side of that chip on all address lines. Using the ROM diagnostics, I can write to memory in the 2MB that are recognized and see the values mirrored in the 2-4MB address space. I have desoldered the CASPAL so that I can try validating that it works, but I'm not quite sure what to make of pins 6 and 7 which go to the BBU. They are probably inputs and it is not unlikely that the PAL is completely combinatorical, but I'm a little bit out of my depth at this point and am looking for specific knowledge and advice how to continue.