ShootTheCore
Legendary
I purchased this clean Simpsons board inexpensively since it has sprite issues - the internal ROM test would pass successfully, but the title screen is obviously garbled,and during game play, the character sprites randomly glitch.
I started diagnosing by first examining the board traces very carefully under the microscope - I did not spot any bad traces. Next, I grabbed the board schematic from a PDF of the manual, Googled the Konami custom processors, and marked them on the schematic.
The schematic and part lookups showed that the Sound system (CPU, ROM, memory addressing logic,RAM) is located in the upper-left of the board, the CPU system is located in the lower-left of the board, the Background rendering system is located in the lower-right of the board, and the Sprite rendering system is located in the upper-right of the board. This board is experiencing a Sprite issue, so I next focused my diagnosis on the upper-right area of the board.
I next checked for continuity on all of the lines coming out of the two Sprite rendering GPUs – I did not detect any continuity issues. I proceeded to get out the oscilloscope and start probing the ROMs, memory addressing logic chips, and RAM chips (referring to the datasheet on each chip model to make sure I was probing the right pins and interpreting the output correctly).
It was here that Imade a discovery – the 32K static memory chip located at position 1J exhibited strange behavior on its data lines.
When functioning normally, a memory chip should have 5V power on the Vcc pin, read high or show regular activity on the Enable pin, show activity on the Addressing lines, show activity on the Write Enable pin, and show activity on the data lines.
Here is how the same model chip elsewhere on the board reads on the oscilloscope on the data lines:
And here’s how thechip at 1J is reading on the data lines:
I confirmed that all of the other pins on the memory chip were showing activity as expected. It appears that this memory chip is faulty. I have ordered a replacement chip – I’ll know soon if I diagnosed this issue properly.
I started diagnosing by first examining the board traces very carefully under the microscope - I did not spot any bad traces. Next, I grabbed the board schematic from a PDF of the manual, Googled the Konami custom processors, and marked them on the schematic.
The schematic and part lookups showed that the Sound system (CPU, ROM, memory addressing logic,RAM) is located in the upper-left of the board, the CPU system is located in the lower-left of the board, the Background rendering system is located in the lower-right of the board, and the Sprite rendering system is located in the upper-right of the board. This board is experiencing a Sprite issue, so I next focused my diagnosis on the upper-right area of the board.
I next checked for continuity on all of the lines coming out of the two Sprite rendering GPUs – I did not detect any continuity issues. I proceeded to get out the oscilloscope and start probing the ROMs, memory addressing logic chips, and RAM chips (referring to the datasheet on each chip model to make sure I was probing the right pins and interpreting the output correctly).
It was here that Imade a discovery – the 32K static memory chip located at position 1J exhibited strange behavior on its data lines.
When functioning normally, a memory chip should have 5V power on the Vcc pin, read high or show regular activity on the Enable pin, show activity on the Addressing lines, show activity on the Write Enable pin, and show activity on the data lines.
Here is how the same model chip elsewhere on the board reads on the oscilloscope on the data lines:
And here’s how thechip at 1J is reading on the data lines:
I confirmed that all of the other pins on the memory chip were showing activity as expected. It appears that this memory chip is faulty. I have ordered a replacement chip – I’ll know soon if I diagnosed this issue properly.
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