Yes, you can use a separate power supply unit for the motherboard compared to the rest of the system.
Yes, you can use a separate power supply unit for the motherboard compared to the rest of the system.
While improving my PC, I found that the rm750x I purchased from Facebook Marketplace lacked the 24-pin ATX motherboard cable. That’s definitely my fault for not verifying earlier, and I’m ordering a replacement from Corsair. In the meantime, could I run the old 500-watt non-modular power supply to power the motherboard and the rm750x to handle the rest of the system?
It's doable, but you need to move the pins on the RM750X to reach T3 and it has to be very precise.
the cpu power is not required you can start up easily with a 24 pin if it's a low power model
It's a hassle, but there are real problems that can happen from power mismatches. Make sure you pair the right components—8 pin CPU with 24 pin PSU—to avoid damage. Just be careful; skipping this step could ruin things.