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The chip marked in yellow may be important, but I found it curious. I also found that I could not find information explaining what its function was. I would guess it has something to do with either the RAM expansion or the Internal modem.
Perhaps someone out there can share some insight concerning the chip in question?
The yellow box shows all of the primary RAM chips from Mostek. The red arrows show where the infrared module connects to the mother board.
The red arrows point to the expansion slots and expose the fact that our PC Jr. is old. The connections, once bright and shiny, show definite signs of oxidation.