Hi there, just bought a new sempron 140 and first thing was to try and unlock the second core. Enabled the ACC on my Asus M3A78-T and voila reboot with Athlon 4400x2. Post ok but cannot boot winxp all the way as it restarts just before the appearance of desktop (I just manage to see the mouse pointer). However, booting in safe mode and in Ubuntu using live cd works fine. I installed the AMD's dual core driver just in case but still the same problem. Found that it could work with underclocking the multiplier or bumping the voltage a bit with no luck as well. Please advise.

Semprons have a locked multiplier so you wont be able to underclock.
You better increase vcore gradually and check for stability in windows.
What's the current vcore when its unlocked? Dont go over 1.5 to be safe.
Buy and Sell

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CPU: Phenom II X4 B55 3.3 Ghz
Motherboard: ECS MCP61M-M3
Graphics Card: Sapphire HD5670
Memory: 4GB Kingston DDR3 1333

Edited: 10/21/2011 at 02:21 PM by edalbkrad

Enable the complete memory dump, set the computer an no resstart e.g other registry editor set HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CrashControl value CrashDumpEnabled. Next analyze memory.dmp using windbg or send the last few lines.

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My polish processor portal: drzwi gerda and drzwi

Tried once again and got a new stop message:
STOP C000021A with error code 0x0000005
Googling came up that it's a memory error! However no dump file

If you use 2 sticks of RAM, any difference when using 1stick? Could try a round of memtest86+ also, though doubt if it'll show anything in this case.
Maybe your Sempron is not stable when unlocked. It happens. If your BIOS has option to downclock RAM you could try that also.

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Tried with one stick, still the same. Memtst pass ok. However I noticed a bit of slag while booting in BIOS when searching the disks!!!