Hi Katoh,
OK, I received my sensors and hooked it one up to a MB-02V6, I used pin 10. The doide was connected between 5V (red wire) and 0V(Black wire) with a 390R resistor.
For the output, VCC (white wire) was connected to 5V, Ground (green wire) was connected to 0V, and output(blue) was connected to pin 10 input terminal.
I connected the oscilloscope to the input pin 10, and there was a very clean signal right through the rpm range. I then connected the oscilloscope to the parallel port input for pin 10 (via the 26pin box header on the board) and the signal was identical, very clean.
Do you have an optivisor or magnifying glass? If so have a look at the resistor packs on the board especially around the idc26 pin box header. Look and see if there are any solder bridges. It's the only thing I can think of that would cause a problem.
Let me know how you go?
Cheers,
Peter.