I know this is from a month ago, but I have built several systems that have used old PSU's (including my current AMD 7950X based system which is on a PSU from 2013, however it was a 1000W platinum rated PSU that is still under warranty and it has appropriate connections to the motherboard, specifically the 8 pin EPS-12V power connectors). If you don't have those EPS cables, there are some molex to EPS cables that might solve your particular issue. That is the main cable that most older ATX power supplies do not have that is required by newer motherboards. As long as your PSU has enough amperage on a molex line to handle it, this should solve most people's issues when using an older PSU.