Low frequency circuit noise in a condenser mic

[ntnsystems]

I recently purchased a "broken" Earthworks SR69/SR20 microphone for $20 because it mostly works except for an increased noise floor in the bass frequencies. If I apply a high pass filter around 300Hz, it cleans up most of it. Alternatively, I applied a high threshold noise gate and it works fine for louder instruments, such as a drum or room mic for a live band. Nice purchase overall!

However, I would still like to understand the possible cause of this problem for my own learning. I have read a couple posts online that it could possibly be a bad capacitor or FET. I ran the rumble through a frequency analyzer and was surprised to find that it almost looks like an inverted highpass filter. Is that possible? (according to the seller, this mic used to work fine... so it wasn't wired wrong). I will try to attach an image of the frequency analysis. I increased the level by +24dB so it is easier to see the noise floor.

Any thoughts?

 

[Tomm Williams]

Don't have the knowledge to contribute to the cause but $20????? even with a repair by EW, that is a screaming deal.

 

[mixsit]

Interesting a rise into the basement'. I was expecting 60 cycle hum maybe.