Polygon memory

This memory is reported as IC70 and IC71:

The IC it’s actually referring to is IC12.

Here is a shot of a Step 1. 5 board showing you why:

You can see they empty socket for IC12 next to IC70 and IC71. The step 2. 0 boards use a single IC in IC12 instead of 2 ICs in IC70 and IC71.

So the IC you need to replace (or fix joints) is IC12 which is a NEC D4811650GF-A12. Given the -A12 version is the slowest one you should be able to use any NEC D4811650GF (I’ve used the -A10 instead with no issues).

This ram IC is connected to the video rom bus and is likely used as a cache for polygon shapes. This is why a fault in this IC produces symptoms that look similar to when there is a rom board fault.

Here’s a photo of what things look like when this IC is not working properly:

You see polygons of distorted shapes firing off in strange directions. You’ll also note that all of the 2D image is rendered fine as it ends up getting overlayed on top of the whatever is coming out of the 6 frame buffer ICs.

As an interesting side note, its actually surprising how well the system runs with this IC completely removed, a few polygon problems but a lot less then I’d expect. And if I recall correctly it was more a case of missing polygons rather than distorted ones.

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