Han, I had my Yamaha Tyros worked on at CAE Sound in San Mateo. They're an official authorized by Yamaha repair shop and I believe the closest one in the SF/South Bay area:
http://caesound.com/ The senior technician there had introduced himself to me quite a while back at one of my gigs, and assured giving me extra special attention if I ever needed my keyboard repaired. When I brought my Tyros in for the warranty USB Port Hardware Upgrade, CAE Sound actually got right on it the very SAME day.
Unfortunately, a resistor they needed to put in got installed 'in reverse'
by mistake, but considering they received no Tyros keyboard schematic or any written instructions from Yamaha on 'how to' install the USB upgrade, it's not really all that surprising. In addition, because CAE Sound had no way to check (confirm) the completed installation correct, as this requires a computer with USB connectivity as well as certain specific Yamaha Tyros utility software installed (they had neither), I ended up having to take the keyboard home and test it out (unsuccessfully) myself.
In spite of the above, CAE is respected and used by many leading professional musicians performing in the SF/SJ Bay Area:
http://caesound.com/cae/links.htm so in spite of that isolated screw up, I'll probably continue to use them in the future(if required).
Good luck,
Scott
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