i am working on a u47 with a nuvistor. it came in with popping. also, the polarizing resistors 60M, and 100M seem to have been reversed, though the owner said it worked for years. this may not matter. anyway i replaced them, and put it back to what the schematic read. at that point, it was very quiet, i moved the pattern lever, and it came in strong. i took it apart, cleaned the inside of the capsule cover where pattern lever slides, and cleaned the contacts on the capsule where the lever makes/breaks contact. it is now intermittently fine, then much, much louder, then cuts in and out at a very fast, but steady pace (capacitor?) the first time i heard this, i turned the variac down out of fear for the capsule, and at about 80% it was fine, loud and quiet. albeit fading out without the proper voltage. but this has been the pattern. when this fast motor boating happens i can get rid of it with the variac. i just measured the voltage from the supply, and it is about 396vdc... not 105vdc! i do have a schematic for the mic that has written on it over the 105vdc input : 390vdc, and the capsules 63vdc has 210 written next to that. i have on order a new nuvistor, and i am wondering if the .01mfd 400v cap could be bad- which is actually in this mic a .01/200vdc cap. i am also thinking of recapping the power supply, and the 1mfd, and .5mfd coupling caps. the 1780 ohm wire resistor did not measure out at first, i needed to unsolder it before it measured correctly, while another u47 i own with vf-14 it measured fine in circuit. this seemed noteworthy. if you have any input on the voltages i should be reading from the supply, the consequences of the resistors being reversed, and the variac cure's meaning, i would feel less silly.
thanks a lot!
kevin mcmahon
