I have owned a pair of M269C’s since the early 90s. I occasionally check the heater trimmer voltage to see if it needs adjustment, and the voltages have been remarkably stable over the years. I am wondering, though, if they need adjusting at this point, and if so how to go about doing it with the very difficult to reach screws that adjust the voltage.
Number 129 measured 4.05 to 4.07 V at various times from the year 2000 until today. I have notes from Klaus (from back in 2000) Stating that the voltage should be 4 V plus or -.05 V. I figure I will leave that one as it is?
Number 233 has measured from 3.91 V to 3.95 v at various times from the year 2000 until now. However, this year, after a 12 hour burn in where it hovered around 3.95 V, it’s settled at around 3.53 V this morning. This, like #129, is outside the suggested range, but still pretty close.
Considering how expensive replacing the tube on these microphones is, I’m asking here if I should adjust the voltage even though it’s a PIA? While adjustment is easy to do on my U67 pair’s power supply (The adjustment screw is easily accessible), the N52T power supply that goes with the M269’s is another matter. If I have my information right, the screws that I need to adjust are shown in the accompanying photographs (in the attached, the screwhead is covered by a resistor, making it even harder to access). Looks like I can only post one photo per post, so the other will be in a minute...
It looks like, in order to turn the screw, I have to remove the circuitboard from the power supply assembly, or I could try to turn the screw from the side, using a pair of needle nose pliers, without removing the board.
Your suggestions? Should I just leave it alone?