U87 Voltage drop when high SPL is applied

Hello everyone, I found an issue in one of my mics (80’s U87) that looks a little odd to me. I was noticing a huge signal drop everytime the singer was singing very loud (VERY). After tracking the issue to the mic himself I noticed that the FET was having a huge voltage drop at the Drain. I took a picture of the scope, from the stable 16v it drops to something like 2.5v and after a few seconds everything is up and running until I scream like a crazy near the grill and goes out again. This situation of high pressure must be mantained for about three seconds to make the mic go bad. I was thinking about a bad cap failing to provide juice to the FET during the labor of delivering under these conditions but I replaced all tants and electrolytics and the issue is still there. Any thoughs? Thank you

The most likely cause is the capsule being dirty, and getting damp from the singers breath. This will drain away the polarising voltage. You need to clean or replace the capsule.

There is definitely some dirt at the front capsule, I wasn’t cleaning it because the microphone was performing great compared to other U87s so I decided not to take the risk. Even now that I know what’s happening (thanks to you radardoug) I am not cleaning it, it only fails under those extreme and rare conditions. I may flip the capsule though. Rear side is mint. Thanks for your input