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IBM PC AT Maintenance Guide
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== Identifying Your Motherboard == Before any maintenance work, identify which motherboard revision you have, because the RAM chip type, the BIOS ROMs, the speed, and the PSU connector pinout differ. * '''Type 1''' (full-size AT, 13.8" ร 12", 6 MHz). 12 MHz crystal Y2, four ROM sockets U17/U27/U37/U47 or two at U27/U47, 41128 stacked DRAM, 2 banks ร 256 KB. BIOS 01/10/84. * '''Type 2''' ("Baby AT", 13.8" ร 9.4", 6 MHz). 12 MHz crystal Y2, two ROM sockets U27/U47, 41256 DRAM, 1 bank ร 512 KB. BIOS 06/10/85. * '''Type 3''' (Baby AT, 13.8" ร 9.4", 8 MHz). 16 MHz crystal Y2, same RAM as Type 2. BIOS 11/15/85. If unsure, check the date markings on the BIOS ROM chips and the crystal frequency at Y2. The IBM 5170 service references that ship with the [[IBM PC AT Troubleshooting Guide]] include detailed photographs.
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