This might indicate a problem with the motherboard.
This might indicate a problem with the motherboard.
Your second PC stopped working unexpectedly even after changing the power supply. Even when you turned off the CPU fan, other fans started up, but only the CPU fan reacted again when the CPU fan was plugged back in. This issue might be related to power delivery or fan configuration.
It looks like a brief task. I’d stop doing this. Take out the motherboard, make sure all connections are secure and nothing is blocking it, preventing a bridge or short circuit.
This is either a bad PSU, or there's a short circuit somewhere. That could mean a dead motherboard, but I'd go through and check to make sure there's no punctured cables and check the system on an open test bench (a cardboard box is fine) to eliminate the case as a problem, as well as trying a different PSU before coming to that conclusion. Also, some motherboards will do this no matter what if you don't have RAM installed, so put in a stick before testing on a test bench.
Did you verify its performance outside the case? That’s the key concern before labeling it a dead motherboard.
Also, verify that you're using the correct RAM slot on your motherboard as per the manual. Generally, a single DIMM requires a dedicated slot.
These two issues are quite distinct and probably stem from separate causes.