Usually psu is not serviceable and caution needed to open psu as there could be life threatening voltage or current inside even when powered off. This is not an expensive part. Probably better just upgrade to an aftermarket psu and get a 24 to 8 pin adapter from moddiy etc.  An OEM psu replacement is also cheap.  This one costs $10 free shipping.
Dear Members and Staff, Recently I've bought a Optiplex 3020 and needed to change the fans cause they were all faulted. I've put Noctua 92mm on back and 80mm on CPU but had a problem with 80mm inside PSU. When I soldered and connected the PSU the fan started to smoke and I immediately unplugged the power. Inspected the entire PCB with microscope and haven't fount any burn mark on the components  but now the fan spins really slow. When the fan is not connected it gives out 12V but when the fan is plugged in drops on 4V. Did anyone encounter similar problem or is there any way to get the schematics of the PSU for troubleshoot?
Optiplex 3020 B290EM-01 PSU problem