Hello,
I have a Macintosh 128k which has some RAM related fault. With all the original RAM, it starts up with a sad mac code of 040001, which I understand suggests a bad RAM chip at position F5 (I've done some probing with an oscilloscope and don't see any other obvious issues). The original RAM is all Motorola MCM6665BP20 with the Apple logo. I ordered a full set of replacement Samsung KM4164B-12 chips, which based on what I can see should be compatible. All I've done so far is socket F5, but when I put the new chip in instead of getting a sad mac code and a slightly audible startup chime, the screen is just garbage. Putting the original chip back in the socket brings back the sad mac code.
I tried using another chip from my Commodore 64 and that does the same thing with the garbage on screen. The Samsung chip seems to work in the C64.
I've seen some suggestions that mixing different DRAM chips could cause issues, my understanding is that the new chips are 120 ns parts and the old ones are 200 ns parts, but I don't know how likely that is to really be a problem. However I'm hesitant to just rip all the rest of the chips out, especially since I'll probably clip out any others I remove to avoid risking damage to the pcb. I'd like to get some suggestions from someone who might have experience with this before I proceed. Should I just go ahead and replace all the RAM, or do I perhaps have a marginal or incompatible replacement?
Thanks
I have a Macintosh 128k which has some RAM related fault. With all the original RAM, it starts up with a sad mac code of 040001, which I understand suggests a bad RAM chip at position F5 (I've done some probing with an oscilloscope and don't see any other obvious issues). The original RAM is all Motorola MCM6665BP20 with the Apple logo. I ordered a full set of replacement Samsung KM4164B-12 chips, which based on what I can see should be compatible. All I've done so far is socket F5, but when I put the new chip in instead of getting a sad mac code and a slightly audible startup chime, the screen is just garbage. Putting the original chip back in the socket brings back the sad mac code.
I tried using another chip from my Commodore 64 and that does the same thing with the garbage on screen. The Samsung chip seems to work in the C64.
I've seen some suggestions that mixing different DRAM chips could cause issues, my understanding is that the new chips are 120 ns parts and the old ones are 200 ns parts, but I don't know how likely that is to really be a problem. However I'm hesitant to just rip all the rest of the chips out, especially since I'll probably clip out any others I remove to avoid risking damage to the pcb. I'd like to get some suggestions from someone who might have experience with this before I proceed. Should I just go ahead and replace all the RAM, or do I perhaps have a marginal or incompatible replacement?
Thanks