Hello,
i try to get a (big) USB HOST shield working with a Uno. I bought the shield from a well known seller here in germany but it does not want to work. USB HOST lib V2.0, the 3 solder jumpers are configuered, tried the examples with Mouse, Keyboard, XBOX360 cable controller. After some troubleshooting i realized, the 3.3V rail on the shield is at nearly 4V. The cause seems to be the connection of ARD MOSI leading to pin 9 of the 74AHC125 levelshifter (5V to 3.3V) - at least leaving this connection out drops voltage to regular 3.3V. The schematic sais this levelshifter from 5V to 3.3V should be a "74AHC125" but on the board i spot a 74HCT125 - which is also used for the other levelshifter from 3.3V to 5V. There are slightly different datasheets for both chips but my electronics knowledge is not sufficient to judge if that's the cause. Fotos of shields on the web also have the same chips populated (74HCT125). Is it possible the wrong levelshifter is populated and makes voltage crawl from pin9 into it's supply line causing to raise it up to 4V? Or maybe the chip is simply broken?
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An answer to myself, may it be helpful for someone with similar issues:
I desoldered the 74HTC125 and replaced it with a 74AHC125. Afterwards the 3.3V rail stayed on 3.3V and the USB Host examples worked right away. Horray 
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