John posted whil I was typing with a similar sentiment on sweat and hair spray.. here's the post-
Check your cables and solder connections. That's the usual culprit and soldering those hair thin signal conductors is not super easy. A compromized joint can also 'awaken' if enough juice flows to bridge the resistance of a 'sometimes but not always' intermitant connection or a cold solder joint. I'd suspect that before a bad FET. DPA can check and confim a bad FET, but will need to cut off any custom termination in their troubleshooting to do so.
Chris Church knows his stuff. He knows mics from both the technical and the practical 'real world' side. I think he's right on with his comments. It's contaminants in sweat (especially the salt) and arisol sprays that concern me the most and which I believe are most likely to kill theater, film or TV sourced mics at some point. The makup, paint, tape goo and outside uglyness don't make it past the metal mesh grid for the most part, which is easily removed, cleaned or replaced. It's the mostly unseen moisture-born contaminants that are the problem. That's what's up with the recommendation for nothing but distilled water to clean the naked capsule- distillation removes all minerals and other residual contaminants. I've had several people at DPA in the US confirm that the distilled water swizzle is OK, but I've never felt the need to do that myself yet. I had one of a pair stop working when soaked with rain at a fest, but continued working and was fine once it dried out later that day. Probably would have been wise to swizzle those afterwards but they are still working correctly 4 years later, then again it was soaked by clean rain water directly from the sky. If it was sweat saturated I would have dunked it for sure.
I have bought a batch of B3s and more recently 4061s off ebay. The seller was reputable, & claims to test for operation beforehand. They came clean. The six B3s all worked fine. I haven't had time to check the batch of 4061s yet, but bought from the same vendor as the B3s and I feel confident they'll be OK. Don't know where he gets them. I was prepared for problems or limited longevity. Caveat emptor.