from C2D E8400 to C2Q Q8400 causing no boot, orange power light and fullspeed fan

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afex54

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Sep 21, 2014
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So a while back I made a thread about swapping out a Q8400 ,that I happen to own anyways, for the existing E8400 in my Dell Optiplex 755.
I have tried updating the BIOS but Im still getting the orange power light and fullspeed fan when I turn it on with the Q8400 installed.
it doesnt display anything and it will just stay like this until i disconnect the power.

The 275W PSU is definitely sufficient for this CPU because the optiplex 780 that the Q8400 was originally taken from had only a 235W PSU

any ideas what the problem is?
 
Solution
cub_fanatic

If you were looking at Intel's ARK website, unfortunately it does mean nothing. Unless Dell says that the Q8400 is supported by the 755, then it isn't. I found this out the hard way when I ordered a CPU for an old Dell tower like yours that Intel said was supported by the chipset in it. Despite them both being 800 MHz FSB chips and both being 65 nm chips, the Core 2 I ordered didn't work because that PC didn't support...
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InvalidError

InvalidError

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Dell's motherboards often have very limited CPU support built into their BIOS - usually limited to little more than whatever CPU models were offered in the original product lineup. Since the Optiplex 780 were only marketed as dual-core, the BIOS likely lacks support for any quad-core model.
 
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cdabc123

cdabc123

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i did get a quad core core 2 duo working in a 780 without modifying anything bit back.
 
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cub_fanatic

cub_fanatic

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Just the fact that it is a Dell I am going to agree with the other two posters. You can try checking if there are any updated for your BIOS but even then, it is unlikely to work unless one of the BIOS revisions specifically states that the Q8400 is now supported. Check the changelogs before running over to the BIOS updates since it is sort of a risky process if you haven't done it before.
 
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afex54

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Sep 21, 2014
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its Q35 chipset which has the Q8400 listed as a compatible product.
that probably means nothing but i felt it was worth noting
 
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afex54

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Sep 21, 2014
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have tried reinstalling multiples time, and the socket is fine because Im using the PC right now with E8400 back in it
 
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cub_fanatic

cub_fanatic

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If you were looking at Intel's ARK website, unfortunately it does mean nothing. Unless Dell says that the Q8400 is supported by the 755, then it isn't. I found this out the hard way when I ordered a CPU for an old Dell tower like yours that Intel said was supported by the chipset in it. Despite them both being 800 MHz FSB chips and both being 65 nm chips, the Core 2 I ordered didn't work because that PC didn't support anything over the Pentium D.
 
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Saberus

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Aug 23, 2013
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Then I feel it is more likely Invalid Error's diagnosis, Dell's BIOS is simply limited on which chips it will acknowledge as valid, and since the 755 was released without any quad core options, it will simply refuse them as they aren't on the list, so the BIOS will not start POST.

If there is a quad core that works with it, it might be the Q6600, which is the same generation as the E6750 the thing was configured with as it's peak processor.
 
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synphul

synphul

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Mar 11, 2011
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The chipset in general may support different cpu's but like InvalidError and others mentioned, it could very well be limited by dell. Proprietary companies are notorious for doing things like that, running an oem style board with a custom bios suited to their retail builds. In a sense similar to how apple operated as well, they had specific brands and models of components (ram etc) that were on an 'approved' list as tested compatible and simply having another brand of the same hardware would cause instability or boot failure. There's very little freedom when dealing with oem/proprietary hardware.

They didn't really intend for the home user to go changing the build around, they sold machines figuring people would use them and if they broke they'd be sent in for factory repair/replacement so only their technicians would be dealing with parts exchange. Found that out the hard way with my old compaq (before hp bought them). Trying to find replacement parts and driver/bios support was a nightmare and I became a quick member of the diy club buying my own parts.
 
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