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[NigelT]

Thanks Kai.
An update. All ten supplies are now working and in spec, sort of...my biggest problem now seems to be fluctuating line voltage. Today I am at 121vac. I have found that some of my units have a 110v tap and others at 117v. My procedure is switching between the high (127) and low (110 or 117) tap and swapping mics between all ten PSU's and by doing that, I am getting all of them within spec. I have also found that these need at least 30 minutes to warm up and stabilize. Thanks for all your help, I have fallen in love with these mics! Although I am blessed with all of the great ones,  (251x2, U47x2, M49, C12x2, C24) and they all sound amazing, something keeps pulling me back to the U67. Out of ten, there are four that are just outstanding but only when compared side by side and by very minute differences. Any of them alone are amazing.

[Kai]

Your mains voltage seems to be on the high side, even a bit out of standard.

Maybe your supplier can do something about that, switch to another transformer tap for your individual household supply, if exists.

For the tube’s heater supply:
In general little undervoltage is more preferable than overvoltage, if you can’t get it stable.

Anode voltage (the high one) is uncritical.

[NigelT]

Guys, thanks again for all your help. The mics are all working great. I ran into a problem I didnt expect but should have, and that is sourcing proper replacement parts. For example, the B30C500. I cant find it anywhere. Someone has hacked in a black one that has a different layout and way thicker pins. Are there still direct drop-in OEM replacements available?  If not, maybe a list of alternate parts would be good.
For example, I found Zeners, but not the exact same one.
I replaced part # Zx15 with
 Newark part #: 1N3314B Zener Single Diode, 15 V, 50 W, DO-5, 5 %, 2 Pins, 175 °C
https://www.newark.com/solid-state/1n3314b/zener-diode-50w-15v-do-5/dp/10P4821?gclid=Cj0KCQjwhLKUBhDiARIsAMaTLnHJXQ3u2KPr_XJzfzIDeKF_GDrq46xk1HiA_Kiy-xlJ1ZRhERVN2tMaAi38EALw_wcB&mckv=_dc
If anyone wants to add to this, please do.

[NigelT]

Uwe,
I read this and I have an OEM B30C500 in one of the NU67's that has been replaced by a modern silicon rectifier exactly as you say below. What would you consider a suitable resistor? Thanks.

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.

[NigelT]

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? The bridge rectifier GR2 shown as a BY164 was replaced by exactly what Uwe described above as modern silicon diode bridge and I wonder if it has caused this.
https://www.jameco.com/z/BY164-Major-Brands-80V-1-5A-Silicon-Bridge-Rectifier-Through-Hole-KBP_2286503.html

[Kai]

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?...

Try to remove the neon bulb.
I never had one with a short, but it‘s possible, the two internal electrodes could touch each other.


The secondary 2.7k and 5 Ohm seems reasonable, no indication of short from this values.


Measure for shorts Mains against Ground / Case.

Measure the primary, each winding:
green - red,
red - brown,
brown - white,
white - black,
should be in the same range.
Compare to a known working specimen.

This does not completely exclude internal winding to winding shorts, but can give a hint.

[NigelT]

Thanks Kai, I will try that. I am worried due to Uwe's post that I quoted above regarding the transformer being damaged by use of a modern rectifier, That is what mine has. But you give me hope.