Problem in connecting PC to LCD projector

I am an IT teacher of a school. Our school has several NEC MT1040 and
MT1050 LCD rojectors. Recently, a company installed all the projectors
to the ceiling for us. However, I found all the MT1040 projectors give
only display of 800x600. They cannot give display at 640x480 nor
1024x728 when connecting to a desktop or a notebook. When we change the
MT1040 with MT1050, all the resolutions can be displayed. The signal
cable used is a three cores shielded wire (~15m).

The company reconnected the signal cable but could not solve the
problem. They claimed that the signals from my notebook and desktop are
too weak. However, the same result was given when we added a video
separator (it can enhance the signal).

I am not sure who should be responsible for the problem.  Can anyone
give me advice such that we can fix the problem? Thx!

Rgds,

SK Chung

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