So you were having problems with your Athlon, and now the Sempron won't boot? I think the brand/model of your motherboard, and perhaps the manufacturer of your power supply would help. If you have access to another s754 mobo, try both chips in that.
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AXDL2600+, 11x202 @ 1.6v OCCT stable | EPoX 8RDA3I+ Pro | 1x512 MB Samsung TCB3 2.5-3-3-9 | Sapphire Radeon x1600Pro/256 MB | WD1600JB IDE | Enermax 420w | Ultra Aluminus
sA+ - Getting the most from older hardware