I got a IIc bare mainboard recently in hopes I could use it to salvage the IWM chip. I decided I'd give it a try before taking the IWM chip out & it had a couple RAM issues so I took out the bad MT RAM chips that showed up & replaced them with socketed chips from a battery-bombed Z-RAM card I have. The corrosion from the Z-RAM battery didn't appear to damage the RAM chips.
Anyhow, I fixed all the RAM errors by replacing 4 of the chips with the exception of one remaining error. The ROM0 (actually have ROM4 but I think the test is the same) memory test returns "RAM*: 00001000" which I believe means that the 5th chip is bad. Like I said, this is a chip I've replaced with a socket & the RAM.
- I've tried moving chips around to see if the error followed the chip but it did not.
- So I tried removing the new socket & replacing it with a differnt socket. Still the same error code.
- I've done continuity checks on all the pins & they all look correct to the neighbor chips.
- Pin 2 & 14 are connected as they should be.
- Pin 2 has continuity back to the 74LS245 chip, MMU, & CPU as it should.
- I've used my oscilliscope & confirmed I have activity on all the address lines that matches the neighboring chips.
- I confirmed I'm seeing activity on pin 2 (& pin 14) that looks the same all the way back to the CPU.
- I've tried swapping CPU, MMU, IOU, & ROM from the other mainboard I have without any change.
- I tried replacing the 74LS245 without any change.
What else can I do to troubleshoot this & figure out what is causing the RAM issue? I'm stuck.
- Ben
I figured out the RAM issue. I learned that when the self-test says "RAM 00001000" it means the 4th chip from the bottom (count from the right to the left). When it says "*RAM 00001000" it means the 12th chip from the bottom (bank-switched chips, not the lower 8).