U45 is U3

[Macross_VF1]

A while back I asked for help with a problematic Atari STFM and it was concluded that the problem was most likely a faulty RAM chip, specifically the one handling bit 15 in the memory address.

However due to the inconsistent labeling between the blueprint (U45) and different motherboard revisions no one could tell me exactly which chip was the culprit.

Well, I have the solution to that question now. After removing EVERY chip and socketing them, I could easily take out one chip at a time and test it by putting a fresh chip in that position. So on a C070789-001 REV.D1 motherboard, U45 is labeled U3. Figured it might be useful information to have.

 

[ParanoidLittleMan]

Looking to schematic of any version of STFM and using ohmmeter to follow lines can be easier way than putting sockets for all 16 RAM chips. Then you can see which RAM chip data line leads to D15 of CPU bus (or any other D line) via chips 74LS373 and 74LS244 .

[Macross_VF1]

For me that wasn't a solution as I just can't make head or tails of the schematic.