The problem might be cured by replacing C21…. originally this was a 47uF 100V capacitor. On later versions I increased this to 100uF or higher.
If you still have problems, send me the unit and I will fix it free of charge…; no guarantees as I don’t know the current demand of those particular mics. But I would only charge just the cost of returning it to you.
(If you do change that capacitor, it’s worth changing the regulator transistor Q4 (2N5550) as well.)