Since no one (or maybe one person?) repairs boards you have to replace the motherboard to fix this one.
I was given a Dimension 4700 with 1 gig (two 512 modules) installed. The problem is that of the four memory slots, only slot four recognizes a memory module and allows the computer to boot. Placing a module in slot 1,2 or 3 generates a series of error beeps and nothing appears on the monitor. So is this a motherboard problem that cannot be fixed or is there some other issue I may be able to address to make these memory slots functional.

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