There could be two things wrong. Your phanotm power supply could be faulty, or your microphone could be faulty, or both. Condenser mics can demand anything between 3 milliamps (mA) and 12 mA from the phantom power supply depending on whether they have a phantom power LED or not.
The Samson C03, for instance, which has a phantom power LED draws around 8mA from the phantom power supply, whereas the Behringer B1 which doesn't have a LED draws only around 3mA. If your phantom power supply has gone 'soft', that is it's regulation has failed and it can't supply the current demand, then when you plug your mic in the phantom power voltage will drop dramatically, sometimes as low as 25 volts, and the mic's LED won't come on, though you'll probably still get an output from it. On the other hand if your mic has gone faulty in a way which demands excessive current from the phantom power supply then you'll get the same result.
What you need to do is find someone with a voltmeter and get them to check the voltage across pins 1 and 2 of the XLR cable with no mic connected. It should be 48 volts +/- 4 volts. Then what you'll need to do is take the top off your PS400, ( there's only 4 screws holding it on), connect the mic and measure the voltage again inside the PS400 and it should be same as before.
For what it's worth a condeser microphone contains, guess what, a condenser or capacitor in modern language which has one fixed and one moving plate. When sound pressure strikes the moving plate of the capacitor it vibrates with the sound striking it and this varies the capacitance value in sync. with the sound. With a fixed 'phantom power' voltage aplied across the capacitor this change in capcitance is detected by accompanying electronics in the body of the mic. Now if you're asking if the oputput from a condenser mic varies with varying phantom power voltage, the answer is some do some don't, it depends on the accompanying electronics. Generally speaking if a mic's spec says it needs 48v phantom power then you can thake that to mean 48v +/- 4v, outside which the mic won't function to spec. If, on the other hand, a mic's spec says something like 'Phantom Power 36v - 52v, then you can assume that it means just that without affecting the spec of the mic.
Hope this helps.
