PC won't pass POST - one long beep

Any help appreciated

I have made no changes to the PC but it will not get to windows load up.

POST tone sounds like 1 long then silence though possibly 2 short to follow which would suggest a graphics card problem.

I have changed the graphics card for a working one in another pc and it also does not work in this pc.
I have tried my card in the other pc and it works fine
I have used another pci-e slot with the same results
I have tested the monitor with another pc and it is fine
I have removed all peripherals
I have removed RAM stick by stick and also in pairs
I have checked all internal wiring

I am a bit stuck now and suspect a motherboard problem

Help appreciated
 
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