PCB Repair: The Simpsons
Problem
Jail-bars through sprites:
Diagnosis
Running the ROM test revealed ROM errors:
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:
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.
Fix
Replace 053247 at 7J.
Thanks for the post and most importantly for sharing your knowledge and experience doing arcade repairs. I’m also trying to repair a PCB The Simpsons with graphics problems, and thanks to you i know that overdrive schematics includes information about the custom chips 053246 and 053247. The excerpt from the Over Drive schematics that you posted is very helpful, but sadly the bottom part is incomplete . For that reason I take the boldness to ask you please, to publish the link to download the complete schematic diagram of Over Drive or the same image with the missing part. And once again I thank you for your kindness.
You’re very welcome. The schematics are available here: http://arcarc.xmission.com/PDF_Arcade_Manuals_and_Schematics/Over_Drive_Manual_and_Schematics.pdf
Thank you very much.