Why did my power fuse blow?

My power fuse blows when I switch the amp from standby to play. I checked all 7 internal fuses and the 4 tube fuses and they are all good. I even replaced all the tubes and tried different modules. Any ideas before I send this amp out? Thanks

Sounds like the board arcing issue. Had that problem when I got my RM100. My understanding is that the traces are too close for the voltage on them. Some boards fail...others don't. I had read that it was more prevalent on an earlier run of the amp but mine was not that old nor did it have the issue while in the hands if the original owner. Got to me and POOF with the mains fuse every time. Local tech/amp builder fixed it for me. Fairly cheap repair.

thanks for the reply. I have had the amp for 4 years or so. It was made in 2006.