PCB Repair: The Simpsons

Problem

Jail-bars through sprites:

Simp.jpg

Diagnosis

Running the ROM test revealed ROM errors:

Simpsons3.JPG

The 053246 and 053247 custom ICs are responsible for the sprites. These customs aren’t shown in the schematics but they are visible in the schematics for Over Drive, an earlier Konami game:

053246_7.jpg
Excerpt from the Over Drive schematics

The 16-bit data bus of each sprite ROM is interfaced to the 053247 via two 74LS374s. After some probing around with the oscilloscope, bit 1 of the 053247 CD bus was found to be stuck low. There was a low resistance path of around 3Ω between this line and ground, suggesting that a pin had failed internally inside either the 053247 or one of the 4 74LS374s that share this line. Lifting pin 73 (CD1) of the 053247 caused the short to disappear. It would be the bloody custom, wouldn’t it…

Fortunately I happened to have a junk Dragon Ball Z 2 PCB on hand, which uses a 053246A/053247A pair. The 053247A appears to be compatible with the 053247, so I transplanted it onto The Simpsons and all was well in Springfield once again.

SimpsonsCustoms.jpg
Not my best QFP soldering attempt but I’m improving…

Fix

Replace 053247 at 7J.

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