Hi, I was running Athlon AMD 1700 in the K7N2 MSI board and used it to test another psu that had a 4 pin add on to the 20 pin connector.The working psu with the board had no 4 pin socket but the board was capable of accepting this socket. After I connected the 4 pin socket to the m/b, the board failed to function although the cpu is fine. Has connecting the 4 pin socket damaged the board please?

It's not clear what you asking.

The K7N2 has a 20-pin main ATX connector & a 4-pin ATX12V connector. The ATX12V is for the CPU & is sometimes called the "P4 plug". http://www.msi.com.tw/program/produ... But the ATX12V/P4 plug is NOT the same as the extra 4-pin plug on a 24-pin (20+4) main ATX plug. Check the wiring & you'll see the colors are different...ATX12V P4 has 2 yellow, 2 black; +4-pin has 1 yellow, 1 orange, 1 red, 1 black. In other words, when using a PSU that has a 20+4 pin plug on a board that only has a 20-pin ATX connector, that "+4-pin" plug should be separated from the 20+4 pin plug & it should NOT be used. The ATX12V/P4 plug is what should be connected to the square 4-pin connector on the board. If you connected the +4-pin plug to the ATX12V/P4, you *may* have damaged something. Here's the pinout diagrams for the 4-pin ATX12V/P4 plug, the 20-pin ATX plug, & the 24-pin ATX plug: http://pinouts.ru/Power/atx12v_pino... http://pinouts.ru/Power/atxpower_pi... http://pinouts.ru/Power/atx_v2_pino...

Some PSs have a 24 position main connector capability but there are two connectors in the same wiring bundle, not a single piece 24 position one - a 20 position one and a 4 position one that clips onto the end of the 20 pin one if needed, so that the main connector can be used in both 20 and 24 position sockets on the mboard. The shapes of the plastic surrounding the pins on the connectors and the shapes of the holes in the mboard are such that the 20 position connector can only go in one way into the mboard socket, and if you have a 24 position mboard socket, the 4 position connector can only go in one place on one end, and the 4 position connector cannot fit in the 4 position socket in the mboard for CPU power. jam's references and what I see on a power supply I have to look at confirms the 4 position connector for a 24 pin socket cannot fit on the 4 position socket for the cpu on a mboard. However, the colors of wires and numbers of wires in what position in the 20 position connector must be the same as it was in the original power supply connector - most of the time it is, but there are some oddball power supplies meant for server mboards that have different wiring. (The white wire at position 18 on the 20 position connector, or position 20 on a 24 position connector, does not have to be there if the mboard has no ISA slots) Also, if you did not REMOVE the AC power to the power supply before you plugged in the 4 position connector, you may have fried the mboard and/or the power supply or something else. ATX PSs are always powering ATX mboards in some places as long as AC is live to the power supply. Try the PS with another computer.

Many thanks for all the suggestions. I have yet to check the motherboard with another psu, but if I understand correctly it would appear that I have fried the m/b. I wonder if a repair is possible?

"...if I understand correctly it would appear that I have fried the m/b." Maybe. It could be it's the second PS that is fried. Try the original PS with the mboard. Try second PS with another computer.