T770 Mic input problem

[BobT]

I'm trying to use my 770 with an audio spectrum analysis program and an external microphone. I can't find a way to mute/turn off/disable the internal mic, so I wonder if it is an automatic function when a microphone is plugged into the mic jack?

I'm running Win2KPro on the 770.

I've tried several different mic plugs, on the theory there might be some slight dimensional difference. None of them accomplish what I want, nor do they make a solid connection with the internal circuitry.
Does anyone know the spec for the mic plug for these laptops?

Thanks, BobT