Hi
I have two (nominally) identical Arduino Megas. One works fine, but when I plug the other one into the USB the PWR, Tx and Rx LEDs all flash at about 1 Hz for a few seconds, and then all go out. That one doesn't work (i.e. doesn't establish a USB connection).
Can anyone tell me what that means, please?
Thanks
Adrian
My first thought is your USB interface chip (FT232RL) is short-circuiting the power supply, and that's causing the polymeric fuse to open, then cool down and work again, then open, then cool down, etc.
With two identical Megas, one of which isn't recognized as a USB device, that's pretty strong evidence for a broken USB interface chip. Now, by "doesn't establish a USB connection" do you mean that it doesn't even show up on your computer? (that's what I've been assuming)
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Yes - that's right. A few minutes ago I discovered a fix: plug the USB cable in very very quickly (so quickly that one hurts one's fingers) or have everything already plugged in when you power up the computer.
From this I tentatively deduce that it is actually the plug and socket that's wrong, and that one of the power connections is making contact intermittently as they slide together, causing confusion as the processor powers up.
Nice detective work! What about plugging in the USB cable on the Mega side only, then plugging in the USB cable on the PC later? That should work too.
Perhaps look for little bits of metal inside the connector.
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Just checked - no debris in the plug or socket. Blew it out with a compressed air line to make sure. The other end of the USB cable snakes down behind a load of kit on racks to the back of the computer, so plugging and unplugging that end is a minor-house-move operation. 
I can make it work with the quick-connect trick, so I'm OK for now. Thanks for your help.