...two left to go and I will have 10 working beautiful U67's.
(...) Close visual inspection can reveal residue from cap leakage, but missing residue doesn’t mean the cap is fine.
...If you don‘t break one by plugging into a faulty PSU.Use resistive dummy loads for testing, not a valuable mic:33 Ohms / 2 W (this simulates additional 1.5 Ohms cable resistance) and 240 kOhms.
C1-C3 caps can be basically checked in place by looking at their AC ripple voltage UNDER LOAD and comparing the result to a known 100% working unit (preferably with fresh caps).
Close visual inspection can reveal residue from cap leakage, but missing residue doesn’t mean the cap is fine.
Electronic’s symbols vary all over the place, remember the 60 years that passed along since.American symbols are often quite different to european’s, even denoted for resistors in the schematic!
...and electrolyte leakage from the vent holes in the top of electrolytic capacitors does likewise not indicate damage! Lift the plus (in the case of NU67 heater rail minus) connections of the cap and use a capacitance tester to determine health.
On to another one. This is the one I mentioned above that has high voltage and has been recapped. I tested before a load and got 299v at R2 and -30v at R9.
First thing I would do: plug in a mic and monitor on your meter as the heater voltage ramps up. Immediately turn the supply off if the voltage creeps past 6.3VDC, then adjust with the trimpot down to 6.2VDC.
The second thing I would do (to this and all other NU67 like it): remove the multi-resistor* network from the circuit board (lower right on your last picture), remove any resistors* from the XLR connector (second from last picture) and connect the power supply Tuchel audio connections (pins 1, 2) directly to the XLR connector (pins 2, 3) via an appropriate shielded three-conductor cable whose ground and shield are terminated together to ground on both connector ends of the cable
if you get -30 V at R9 the Zener is toast.If it would work it would limit the voltage to shortly above -15 V even without load.Replace it.
B+ = 217Heater = 6.52Close enough?
NigelT,I don't know a tonne about tube mics, but I do know that it's best for tube life to be conservative with heater voltages. 6.52 is 0.22v above the recommended 6.3v for an ef86.
I suspect the rectifier in your NG power supply may aged and developed higher than nominal internal resistance. Be careful when attempting to replace it with a modern silicon diode bridge, its much lower resistance may jeopardize the mains transformer by excessive in-rush current upon power-up. A suitable resistor, or better yet, a NTC thermistor in series should be used.
I have a supply that has been blowing fuses. I isolated the transformer to just the fuse holder and power switch and still: poof. On the 210v tap (blue and yellow wires) I measure 2.7k ohms and the 20v tap (green wires) reads 5 ohms. I assume that means it's shorted?...