Funny, but in all the years I have serviced NU67 I have maybe once replaced a filter cap.... Recapping, quite common and necessary in mic pres and other line level amplifying devices is rarely if ever necessary in power supplies for microphones, unless, of course, there are symptoms like hum, noise, voltage drops, and measurable deterioration in capacitance.
The resulting extra heat can cause a destructive feedback loop of accelerated aging (increasing ESR further), building heat and pressure inside right up to the point of a messy catastrophic failure.
Why have I not encountered such catastrophe in 35 years of servicing hundreds of U67 systems?
(...) I don't see any reason for not performing a full electrolytic re-cap when servicing a U67 PSU or mic.
I measured a couple of them and ESR was quite high, much and much higher than a new Electrolytic cap. With those higher ESR reading I really don't think they were up to spec after 60 or 50 years, and I don't see any reason for not performing a full electrolytic re-cap when servicing a U67 PSU or mic.
With that last aside (in bold) the ice you are treading on gets decidedly thinner. You should not replace a condenser mic's coupling cap or its cathode bypass cap before doing a deep dive into the sonic consequences.
To say this clear:I don’t usually fix stuff that is not broken.
Broken for me means there’s a real problem with the mic, not just a component has “out of specs” measurements by today’s measures.