Hi people.
I have a Win7 PC trying to flash chinese ATMEGA2560.
I had in fact 2 boards and both of them failed with the "sync" error.
After many googling leading to bootloaders or 100nF capacitors at the DTR pin or playing with reset button, my solution came by using a much much shorter USB cable.
I changed from 1m to few cms and then everything worked flawlessly. Tested on 2 boards.
I have to say that my USB cable 1m long has no issues with other peripherals but with this chinese ATMEGA.
As a root cause, these chinese boards have a 50 ohm resistor between USB D+/D- and the CH340g pin. I guess this is causing a voltage drop that added to 1m lenght of cable, causes the signal to be too low.
Apparently there's no need for those resistors. They are not in the typical usage in the datasheet of CH340g. I presume they could be shortcircuited but I haven't tested it.
Hope I can save time to anyone who reads this.