If you used the same cable, input channel, mixer (?) and computer when you tried the MXL990 and the problem was not there, it points to a fault with the MXL770. To verify this, you could take just the microphone and try it with a friend's setup, or take it to a music store to test. It is just possible that it's a problem with the phantom power on your system that is not showing up with the MXL990. Note that the MXL990 is specified as having a 6dB better S/N ratio than the MXL770, although the sensitivities and the noise floor are meant to be the same.

Does the MXL770 sound the same with the 10dB pad engaged (allowing for the attenuation, of course)? Have you checked it using a different channel of your interface or mixer? Has the microphone been in a damp atmosphere?

What type of interface or mixer are you using for this test?

Problem with that hum occurs of course in other channel in mixer, it's same like in channel 1
MXL 990 sound normal, noise is normal, a little bit quieter than mxl 770 while normal working..
But now, 770's have really bad peak's in noise, like hm.. electric noise? It never be so loud like now.

Sure. I used -10db pad, and low pass roll... and -10db only reduce that noise by -10, and nothing more..
But Low pass roll when is switched to "ON", noise disappears, same when i switch 150Hz low pass roll in mixer, noise disappears.

You asked me has the mic been in a damp atmosphere.
When you asked me so i thought to myself... and it's happend after storm and rain.
I have opened window all this time.
Maybe it make mic wet?

Yes, that's why I asked. We had a similar thread recently.

You know what?
I don't really know, but now is everythink ok.
I turn on phantom power and decrease volume on mixer and that way mic was turn on.
I go look some movie after few hours.. and? Magicialy mic come back to normal noise
Maybe solution is, I close window in room, make some hot air in room, and mic stay in this atmosphere.

Thanks you for help!