I could do with some guidance for an old nu67 psu

[Bo.b]

Hi,

I’ve got a lovely old u67 that I hadn’t used for a while.

A few hours into a vocal tracking session, on the last of the punch ins, I started getting a low noise problem. A bit hard to describe, but it felt like it could have been a loose connection or something.

So I checked all the cables, opened the psu, poked around carefully with a long wooden skewer and still managed to give myself a couple of pretty nasty shocks.. the noise was present even when the microphone was not connected. The noise also came and went over longer periods of time.

I’ve found:

-that the psu looks to have been worked on at some point. I can’t say it’s the cleanest work I’ve ever seen. I suspect that there were possibly issues with lifting traces on the pcb.

-That the one different looking capacitor(older?) appears to have failed(white crust out of one side). It is marked C4 on the pcb

-That there are only two prongs of the kettle lead connectors wired up. The psu is not grounded. There is a banana plug socket for grounding on the unit that is still wired up from original, but I’ve never even thought about it, and just took it for granted that the kettle plug was grounding the unit.


I live on an island with very limited tech support, and have done a fair amount of work myself so I’ve got a view to replacing the capacitor myself. I’ll try snip the wires close to the failed cap and soldering the new cap to the old wire, to avoid messing with that pcb.

I’ve come across a few different schematics for nu67 psu’s, so I’m not 100% sure of the value of the capacitor. Until I get it off the board I can’t be certain that I will be able to read the value directly off the cap.

My questions are:

-Should I replace the capacitor with a generic electrolytic cap with suitable values that I can source locally, or should I look for a fancier capacitor for this role?

- the other big psu capacitors don’t look particularly fancy, one of them seems to have the plastic end degrading and cracking. Should I be looking to totally recap the psu at this stage or just replace the visually failed “odd-one-out” cap for now, and see if I solve the issue?

- should I be grounding the psu? Is it simply a matter of hard wiring the “earth” banana plug connector to the ground pin on the kettle lead? Should I be thinking about adding a ground lift switch if so?

Any guidance would be much appreciated.

Thanks!

Rob

 

[garp]

Big rule : always do several power cycles before using such vintage baby otherwise dried electrolytics will never scar properly (you can also use variac for that) and the unit will inevitably fail after a few hours. Judging by the pictures i'll probably begin by a complete restauration of the PSU before using it further, the job done was awfull and certainly not at the level this gem requires ! (I'll also probably change the white 22ohms ceramic resistors and replace them with high quality Dale aluminum body, and in fact probably replace every component on the psu board because everything appears to be butcherized on it with broken traces and mods). The ground banana plug is for measuring devices. The ground is usually located at the power plug, maybe modified in your case.
.
PS: Lucky you !

 

[Bo.b]

Thanks Garp!

This is feeling to me like something I should pass on to someone who knows these mics inside out. Maybe I should ask one of the “big name” techs associated with these mics if they’d be able to go over the supply and bring it up to spec

 

[Cqwet Dbdfte]

Those old electrolytics have about a 103% chance of making a bad day.
The extra 3% comes from the juice leaking over the circuits
Even old film caps ("plastic") have a high probability of leakage.
Pull em out like starting a lawnmower.
Replace with recognized brand name parts.

 

[Accelerator]

@Bo.b​

The Neumann NU67u P.S.U. schematic is this:

You can/should replace the entire 25μF/260 Volt electrolytic capacitors with the:

Mundorf TubeCap 47uF 600V

https://www.banzaimusic.com/Mundorf-TubeCap-47uF-600V.html

for the HT Line and therefore you can/should replace the entire 500μF/260 Volt electrolytic capacitors for the LT Line with the:

Mundorf ECAP63-680 680uF 63V

https://www.banzaimusic.com/Mundorf-BR63-680uF-63V.html

And you can thank(s) me later…

 

[Khron]

@Bo.b​

The Neumann NU67u P.S.U. schematic is this:

You can/should replace the entire 25μF/260 Volt electrolytic capacitors with the:

Mundorf TubeCap 47uF 600V

https://www.banzaimusic.com/Mundorf-TubeCap-47uF-600V.html

for the HT Line and therefore you can/should replace the entire 500μF/260 Volt electrolytic capacitors for the LT Line with the:

Mundorf ECAP63-680 680uF 63V

https://www.banzaimusic.com/Mundorf-BR63-680uF-63V.html

And you can thank(s) me later…

... Or just some 105C rated Panasonic / Rubycon / Nichicon / Elna / United Chemi-Con from the major distributors, for 1-10% of the price of those Mundorfs...

22-47u / 400v & 1000u (or more, whatever physically fits) / 35v, but selenium rectifiers definitely need to go.

 

[Cqwet Dbdfte]

Series connected electrolytics need some balancing resistors or zener diode to limit voltages on caps.

 

[Khron]

Series connected electrolytics need some balancing resistors or zener diode to limit voltages on caps.

That's true, but that shouldn't be the case here...?

 

[Accelerator]

... Or just some 105C rated Panasonic / Rubycon / Nichicon / Elna / United Chemi-Con from the major distributors, for 1-10% of the price of those Mundorfs...

22-47u / 400v & 1000u (or more, whatever physically fits) / 35v, but selenium rectifiers definitely need to go.

Yes, all of them (105C rated Panasonic / Rubycon / Nichicon / Elna / United Chemi-Con from the major distributors) they are –cheaper- for 1-10% of the price of those Mundorfs, but they are not "Deutsch", neither are they metal film capacitors with (extra) long life…

So, actually it is not Bo.b but his grand-grand-children that will thank(s) me later for these Mundorfs TubeCap Capacitors…

Now if money it is an object, there are also these German Electrolytic Capacitors…

F+T LFA 220uF 350V

https://www.banzaimusic.com/f-t-lfa-220uf-350v.html

for the HT line and

F+T 4700uF, 63V

https://www.banzaimusic.com/F-T-4700uF-63V.html

for the LT Line...

I diddo that selenium rectifiers definitely need to go.

also, there are not “bleeder” resistors with each one of capacitors in this ancient topology (like there are not any “bleeder” resistors in the Neumann U-47 P.S.U.) and this in our modern age it is a theme…

 

[Cqwet Dbdfte]

That's true, but that shouldn't be the case here...?

Oh... I looked at the pics.

 

[Khron]

Oh... I looked at the pics.

Ah, my bad in fact, i guess i subconsciously chose to mentally block the carnage that's currently physically existing in that PSU...

 

[k brown]

The work doesn't require a Neumann specialist, or even a vintage mic one - just someone who can do solid, clean work on tube device PSUs; there's nothing exotic going on in there.

 

[tomas.borgstrom]

I’m not shure all the big expensive hifi-capacitors will fit in the case. Buy some Japanese high quality radial electrolytic capacitors with the correct spacing between the leads.

My experience is that old plastic capacitors work fine but paper and wax drift and leak.

The original siemens electrolytics (they have been replaced in your mic) in the old Neumann PSUs are actually very reliable. I rarely find a faulty one when I replace them.

If you replace the selenium rectifiers the voltage will rise a bit and you will have to change some resistors to get the correct voltage to the mic.

Clean all the contacts and also the tube socket in the microphone. There’s an electrolytic capacitor in the microphone as well but it’s tricky to replace. Finally I recommend lifting S2 for a better low end.

 

[Accelerator]

@Bo.b

HORCH Audiogeräte NT2106U67 power supply for Neumann U67 tube microphone €618.45 *

https://www.sonicworld.eu/brands/a-...-power-supply-for-neumann-u67-tube-microphone