Firstly error codes follow the link below its pretty comprehensive.
E74: AV cable error... There is a problem with the AV cable, try using a different AV cable. If the cable is known to be working then there is a 90% chance it's a scaler chip problem (the "ANA" or "HANA" chip connected directly to the AV cable) in rare cases it is the GPU.
Firstly try a new AV cable. Secondly the Hana or ANA chip does sometimes develop dry joints, taking a thin tipped soldering iron to the pins of the device is the only solution to that. If its the graphics chip causing the error you could either try the X-clamp solution (google it) or you could try getting it re-flowed... whatever you do dont wrap your 360 up in a towel or take a paintstripper heatgun to it in order to try and reflow the graphics chip (You will damage the chip internally if you do so ;) ) Easy way to tell if your graphics chip is at fault is to open up your 360 and apply downward pressure to the graphics chip when turning on your 360.
Best of Luck
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