Optiplex 755 Motherboard Replacement?

[coastergraham]

I'm doing some large upgrades to my Dell Optiplex 755, (the larger size) and I have read multiple sources that say if I'm going to get a standard ATX motherboard, I'll have to get a new case and psu as well. Is this true?

 

[imsqam]

I think you can break things down to can't, could, should and must. Let start with the PSU. You say "the larger size", I will assume you mean the mini-tower which is the largest size of the four versions: Mini-Tower (MT), DeskTop (DT), Small Form Factor (SFF) and Ultra Small Form Factor (USFF). FYI, here is the tech guide
https://www.dell.com/downloads/global/products/optix/en/opti_755_techspecs.pdf

Page 28 of the guide shows the MT ships with a 305W and looks to be 264W on the 12V. PSU is of questionable origin, it appears that Dell has sourced a variety PSU manufacturers
http://www.stuartconnections.com/3103-thickbox/Dell-Optiplex-755-Dimension-E520-MiniTower-305-Watt-Power-Supply-NH493.jpg
http://www.stuartconnections.com/8371-thickbox/Dell-Optiplex-320-755-Dimension-E520-MiniTower-305-W-PSU-Power-Supply-XK215-NPS305KB.jpg
http://data.stuartconnections.com/object/image?p=8794&i=2

This will be a cheap, poor quality PSU which will not support much of a build. FYI, as power supplies age they lose capacity, especially if components like capacitors are cheap. If this thing is more than 5 years old your 264W might be closer to 210W. Also it only comes with a
- 24-pin ATX mainboard power
- 4-pin ATX mainboard power
- 4-pin Berg Floppy
- and four 15-pin serial ATA (SATA) power connectors.
http://www.stuartconnections.com/en/power-supplies/5556-Dell-Optiplex-755-Dimension-E520-MiniTower-305-Watt-Power-Supply-C248C.html

There are no PCI-e connectors for running video cards. For these reasons you should replace it and you might have to if you require a GPU 6 or 8 pin PCI-e connection.

The case appears to be an issue as well, I believe that this is a picture of the back
http://www.stuartconnections.com/5627-thickbox/Dell-Optiplex-755-Mini-Tower-System-w-Core-2-Duo-E8400-30GHz-2Gb-Ram-DVDROM.jpg

Note the location of knock-out slots for expansion cards, only a single slot is available for the GPU and thus you would be restricted to single slot cards only. This severely limits your options for GPU's. It also appears that the opening for the mobo connectors is an odd size/shape but I can't tell for sure. Top mount for PSU, okay but not optimal, bottom mount is now the standard. The motherboard does appear to be micro-ATX
http://www.pc-specs.com/mobo/Dell/Dell_GM819/3322

but I don't see any other mounting holes in the case so you would be restricted to a micro-ATX replacement mobo. Must you replace it, maybe not. But for the hassles you will face in trying to use this I would just get a good budget case and be done with it. You will get better space/cable management and thus better air flow/heat management.

 

[coastergraham]

I think you can break things down to can't, could, should and must. Let start with the PSU. You say "the larger size", I will assume you mean the mini-tower which is the largest size of the four versions: Mini-Tower (MT), DeskTop (DT), Small Form Factor (SFF) and Ultra Small Form Factor (USFF). FYI, here is the tech guide
https://www.dell.com/downloads/global/products/optix/en/opti_755_techspecs.pdf

Page 28 of the guide shows the MT ships with a 305W and looks to be 264W on the 12V. PSU is of questionable origin, it appears that Dell has sourced a variety PSU manufacturers
http://www.stuartconnections.com/3103-thickbox/Dell-Optiplex-755-Dimension-E520-MiniTower-305-Watt-Power-Supply-NH493.jpg
http://www.stuartconnections.com/8371-thickbox/Dell-Optiplex-320-755-Dimension-E520-MiniTower-305-W-PSU-Power-Supply-XK215-NPS305KB.jpg
http://data.stuartconnections.com/object/image?p=8794&i=2

This will be a cheap, poor quality PSU which will not support much of a build. FYI, as power supplies age they lose capacity, especially if components like capacitors are cheap. If this thing is more than 5 years old your 264W might be closer to 210W. Also it only comes with a
- 24-pin ATX mainboard power
- 4-pin ATX mainboard power
- 4-pin Berg Floppy
- and four 15-pin serial ATA (SATA) power connectors.
http://www.stuartconnections.com/en/power-supplies/5556-Dell-Optiplex-755-Dimension-E520-MiniTower-305-Watt-Power-Supply-C248C.html

There are no PCI-e connectors for running video cards. For these reasons you should replace it and you might have to if you require a GPU 6 or 8 pin PCI-e connection.

The case appears to be an issue as well, I believe that this is a picture of the back
http://www.stuartconnections.com/5627-thickbox/Dell-Optiplex-755-Mini-Tower-System-w-Core-2-Duo-E8400-30GHz-2Gb-Ram-DVDROM.jpg

Note the location of knock-out slots for expansion cards, only a single slot is available for the GPU and thus you would be restricted to single slot cards only. This severely limits your options for GPU's. It also appears that the opening for the mobo connectors is an odd size/shape but I can't tell for sure. Top mount for PSU, okay but not optimal, bottom mount is now the standard. The motherboard does appear to be micro-ATX
http://www.pc-specs.com/mobo/Dell/Dell_GM819/3322

but I don't see any other mounting holes in the case so you would be restricted to a micro-ATX replacement mobo. Must you replace it, maybe not. But for the hassles you will face in trying to use this I would just get a good budget case and be done with it. You will get better space/cable management and thus better air flow/heat management.

Thanks so much!

 

[imsqam]

You are most welcome

 

[william p]

The Dell Optiplex 755 is a "Micro BTX" form factor. No aftermarket coolers, or cases are made for it. Dell uses proprietary I/O front cables, headers, and control boards. These have Dell extra features like ambient temperature sensors, and 4 led diagnostic displays. The PSU is a quality part, and the 305W was used in computers with 130W Pentium D's. It can make 4000W on a test bench, and shut down safely.
https://www.hardwareinsights.com/dell-h305p-01-power-supply-review/
In the Mini Tower aftermarket PSUs fit just fine. Be sure there's enough power on the 5V. rail. BTX was based on P4 and still used that quite a bit.
A built up 755 would have a Q9650, 8 GB DDR2-800 RAM (low density x64)and a ZOTAC MINI GTX 1050Ti 4GB GPU that runs off of the MB PCIe slot.
The Zotac Mini is the only 1050 that fits without modding due to it's short length, and 1 slot bracket on a 2 slot card. You've been warned! The base 1050 fitsalso. Newer AMD GPUs can't display the dell BIOS screen.
D9729 heatpipe cooler from the old Pentium 4/D will work and should bolt right in.
Here are 1000+ builds for that system. So you can see what upgrades are possible and how much performance they add.
http://www.userbenchmark.com/System/Dell-OptiPlex-755/3939
If you see a Q6600 running mid 30% scores it's overclocked to 3GHz and is still slower than the Q9550 which is also popular.
Any GPU over the Zotac will require some trimming and was probably an ITX version to start with. The builds I suggested will run older games at 1080P OK. Newer titles will choke on the CPU which doesn't result in low FPS, but causes freezes in the game while the GPU waits for data.
It's an old office machine which can become a cheap low level gamer. The PSU and GPU can both be moved forward to a newer system so it's OK to spend there.