MXL V67i no output

I have been given a MXL V67i FET mic to have a look at. It has not been used much and is in excellent condition. When last plugged in to be used in the studio there was no audio output. I checked it out on my mixer to see if there was any difference and found a strange phenomenon. Sure enough, when 48v. was switched on, there was no audio, but when I switched off 48v. the sound level came up to normal for a couple of seconds before fading out. That was a consistent pattern from either side of the mic (two diaphragms) and I repeated the same test a number of times with the same results. I have not had much chance as yet to check voltages, etc. inside the mic. The red LED inside the grille to indicate which diaphragm is in use lights up, which would show that power is getting to the circuitry. Could it be one of the decoupling caps in the polarizing section is bad? What else should I look for?

Anyone had this same experience, or has anyone have any ideas about why the mic might be behaving like this? Any suggestions or help would be much appreciated.

Kindest regards,
zephyrmic

I have been able to do some testing on this mic today. It is a double sided capsule, with a switch to "Warm" and the other to "Bright", using either side of the capsule as well as low cut and -6dB pad switches. The circuit of the  main boards is as per the attached pic, to which I have added the measured voltages, and would indicate that they are normal, from what I can tell using a DMM. The mic was plugged into a small Behringer mixer I have. I found the schematic  on the microporium website under MXL V67. This circuit unfortunately does not include the switching circuitry.

In tests today I found that when it is switched to "Warm", it operates normally, with reasonable output, but when switched to "Bright", there is only very low output - almost none - until the phantom power is switched off. Then the level increases to normal, and fades as the phantom voltage lessens. So it does not seem to be anything on the main circuit boards above the transformer that is at fault. Must be in the board accommodating the switches, from what I can tell. I can take some internal photos of the mic and post them if that would be useful for anyone who would like to comment.

At this stage, I cannot figure out what may be going on in the mic to produce this effect/fault. I would be most grateful for any help.

Kindest regards,
zephyrmic

Problem solved!! The poor thing was suffering from the chills of a Melbourne winter in the hills, where it can be cold and damp. I am told that the mic had been stored at the back of a cupboard which was against an external wall. No wonder the mic was suffering from the flu!

So I brought it in to the warmth of my study, unscrewed the outer casing, and left it in the warm dry atmosphere of the study for a day or so. On reassembling the mic, it worked like a charm! No problems whatsoever. So a lesson reinforced again that condenser mics need to be treated with TLC and not left out in the cold, so to speak. Needless to say, I encouraged the owner to find a place in the house to store the mic in its box where he himself would be comfortable. He was most relieved to find out that there was nothing more serious wrong with the mic.

Kindest regards,
zephyrmic