Mario Bros Repair Logs
Symptom: No sprite graphics
Board would play blind, show text, and floors but had no moving graphics, characters, pipes, or title graphics. The Video EPROMs had no activity on the address or data lines, except A0-A3. Traced it back to the input edge of the Video PC Board. Data was coming into the 74LS245 but not out from it. The enable and direction lines were stuck high. Problem was on the CPU board.
Tracked the signals back to the CPU board: "OBJ WR*", "OBJ RD*", and "OBJ RQ*" were all stuck high. Replaced the 74SL138 at 3B to no effect. Tested the PROM at 5B by substitution even though the signals looked good. All the signals to the 3A, 3B, and 4A chips looked good except for the MREQ* signal. It was pulsing, but not the same as a known working board. This signal came from the Z80A DMA controller at 5D. The "BUS REQ*" and DAI* signals lines between it and the CPU were stuck high. Replaced the DMA Controller IC to repair the board.