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[klaus]

Siemens/Bosch/AEG electrolytics often look like they are leaking: Each cap has a vent hole at the top, for excess electrolyte to escape. This is often mistakingly diagnosed as a "leaky capacitor".

Capacitance tests of each capacitor are the only valid determination of deterioration. In my experience, almost all of these caps last for many decades without measurable deterioration- probably due to lack of any significant current load on them.

[Kai]

...all of these caps last for many decades without measurable deterioration...

Same here, the older and bigger they are, the longer they last.
In contrast to the miniature ones used today which can dry out within few years, no matter if running hot or not.

You can use an inexpensive capacitance meter (of sufficient range) to test.
If they are within or not far off specs (usually nominal capacity +50/-20%) they are fine.
Don't forget to discharge them before using the meter, or you will fry it.

Regards
Kai

[method1]

The caps all look fine to me, but I haven't measured them.

I was able to get an original neumann PSU on loan, so I will test and compare with my replacement PSU and see if there are any noticeable differences and report back with my findings.

[Jim Williams]

I recently repaired a U-67 supply for Big Fish recording here in San Diego County.

The caps were old and tired, too small as well. I used Panasonic FR series 2200 uf 35 V for the heaters, 3 of them. The B+ got 220 uf 400 volt caps, 3 of those as well. The originals were only 30 uf. I also added some Wima MKP-3 polyprop bypass caps to lower the impedance at higher frequencies.

The results are this mic has never been quieter.

[Kai]

The results are this mic has never been quieter.

I once made a recording with an original 60's Stratocaster.
It had a lot of hum and noise.
So I carefully screened it with aluminium foil (from the outside, letting the pickups stick thru).
The hum and noise was gone, so was the sound.

We ended up removing the foil, finding a room position and orientation where the hum was acceptable and enjoyed the tremendous sound of this great classical guitar played by a gifted player.
Nobody ever complained about the noise, it wasn't obviously audible in the song, but without it the mix worked much worse.

Regards
Kai

[klaus]

Forgive me if I am extrapolating the wrong message from your post. Are you trying to say that:

1. Properly working U67 power supply filter capacitors will detrimentally alter the sound of the mic?

2. Changing parts on a collectible item will affect its value, even if that change will restore functionality?

3. Listeners will not mind hum in a recording created with a defective power supply?

4. Something entirely different?

[Kai]

1. Properly working U67 power supply filter capacitors will detrimentally alter the sound of the mic?

I suppose that a properly working PSU will give a good S/N ratio, so nothing wrong with that.

2. Changing parts on a collectible item will affect its value, even if that change will restore functionality?

Of course the collectible value goes down as soon as anything is changed, even if it's possible to obtain the original parts.
In this case the PSU wasn't restored to original, but redesigned using modern parts.
If someone wants to go this way I would suggest to buy a new, modern PSU or build one from the scratch.
It's not too complicated to do so, or modify a cheap chinese PSU readily available.

May I remind you to the "fixing" Blue does to vintage mic's?
This is an extreme example, but one has to be careful, too many people aren't conciuos of what they are doing to the few left examples of a now almost historical era.

3. Listeners will not mind hum in a recording created with a defective power supply?

A certain amount of noise really can be useful in a mix, how strange this concept might even be.
It depends on the music type and style, of course, you would't want that in a recording of classical music.
But imagine the sound of a blues harp through a guitar amp - without the noise, breath and pops it would almost sound like a child playing melodica.

4. Something entirely different?

The mic we discuss here seems to be broken more likely then it's PSU, so taking efforts into "fixing" the PSU let's me come to mind the old rule "Don't fix it if it ain't broken".

The mods made to the PSU might cause problems:
the bigger caps lead to higher switch-on charge currents, which can break the rectifier or even worse, the line transformer on the long run.

Regards
Kai

[Jim Williams]

If you cover the outside of an electric guitar with aluminum foil, that will affect the resonance of the instrument, I don't think an acoustic guitar would sound so great covered with foil either.

A properly screened Stratocaster is done inside the instrument. Mine are lined with adhesive copper foil, even the pickup covers are lined with it. The results are no buzz, even if you take your hands off the strings. The tone is also intact. Screening does not affect the tone unless all that noise is also considered tone.

The U-67 power supply was repaired. Repaired because the original caps had failed. This is not uncommon on 40+ year old designs.

I suppose I could have searched the web for NOS caps, but why bother? The mic is used in a professional recording studio. It's not an item they intend to sell. It must perform and perform well. It's not a museum piece needing original restoration, it's a tool.

As for a certain amount of noise being really useful in a mix, that's a new concept I havn't heard before. My harp player uses a Shure Green Bullet wired low impedance into a Shure step up transformer into a Fender Twin. The amp is rebuilt with good caps, it's silent. The mic is silent. Blues is heard, sans noise. Nothing wrong with that, it's prefered around these parts.

The caps I installed are larger. They remove much more ripple, especially the B+ line. The output impedance of the supply is also lower. The inrush current is still low, these are not 33,000 uf power amp caps, they are 220 uf. If the rectifiers fail down the road, I have a bunch of fast recovery diodes I can use, those would also remove a bit of additional rf noise that's generated by the rectifiers. If the power transformer fails, I'll put a stronger one in. So far, none of these fears have come to pass.

I can imagine if the original designers had access to these modern parts, they would have used them. You can respect the original design, but that doesn't mean you have to get married to it. People have work to get done.

[radardoug]

+1 Jim, couldn't have said it better myself.

Regards,
             Doug Jane.

[Kai]

If you cover the outside of an electric guitar with aluminum foil, that will affect the resonance of the instrument, ... A properly screened Stratocaster is done inside the instrument. ...

This Strat was an collectible item, even at that time.
I would never had opened and modify it, but some shielding with foil on the pickup board and from the back did the job of removing the noise.
I didn't wrap the whole instrument into foil, just little pieces.
The sound change wasn't due to killing the resonance, it's really been the missing noise that made us record without that temporary mod.
Everyone involved into that production, btw. had the same opinion about this.
You can imagine my face gettin longer when that happened, I'm usually trying to do clean recordings, but I had to agree, the song needed that noise, don't ask me why.

The U-67 power supply was repaired. Repaired because the original caps had failed. This is not uncommon on 40+ year old designs.

If something is broken, it needs to be repaired or restored, of course.
I only have learned the hard way that some mods are what we call a "Verschlimmbesserung" (making it worse by making it better).
This often turned out much later. One has to be carfull sometimes changing prooven good  (or working) designs.

Regards

[method1]

An update:

I did some recording using an original Neumann PSU and the mic sounded a lot better! Looks like the PSU was the culprit after all.
Currently looking into modifying the Telefunken PSU to output the correct voltage.

I'll post some samples soon and hopefully other ears will verify what I'm hearing.

Thanks for the assistance.

[Jim Williams]

The second U-67 came through here last week from Big Fish, the other is so quiet now, Paul wanted them to match. Those modern caps do the job very well and they will not need replacing for at least 40 years. Ripple is very low on the scope now. The mics are quiet, no hum leakage at all. These were a pair from Wally Heider in LA, probably used on many recordings back then.

[Mário Leite]

Hello.
I was following this post because i have the same problem with mine U67. I can´t hear nothing but a 50Hz hum (i mesured with a scope). Not a single reaction on singing or scratching it to hear audio. Nothing! Just the 50Hz hum.
Do you guys have any clue on this? I measured all the voltages and they are correct. The mic´s tube voltages are also correct...

Thanks a lot
Mário