I'm runing into an issue with my SM23; perhaps someone might come up with a suggestion before I send it in for repair.
Situation: SM23 (mfd 1961) with 2x NT52a (heating stabilizer updated) into RME Micstasy / RMEFireface / Laptop
I wanted to check it for recording sessions next week. At the beginning everything was fine. No issue (other than the inherent self noise). Then after 1 hour or so it develloped (on both channels !!!) a kind of oszillation noise, souding a bit like cell phone interference into audio. It varies sporadically. RTA shows massive peaks 12kHz and 24kkHz. No cell phone around; not sure about WLAN in the building. Distance to Laptop does not seem to alter it. Once I move my hand around the mike (without touching it) it alters. Adjusting the polar pattern voltage makes it lounder for a short moment. Once the capsule bias is stable, it lowers to what it was before.
I checked the grounding and the connectors but could not trace the issue. I think it is RF interference. Any suggestions are welcome.