Neumann U67 repair

[bubbi]

Hey guys,

I have an original Neumann U67 sitting on my desk waiting for a repair. It belongs to a friend who bought it from Vintage King a few years ago. The problem with it is that after a while it starts making an "airy" noise that gets louder and louder. I immediately blamed the tube (A nice telefunken diamond series EF86, ouch) and tried changing it out for one that I had (A philips miniwatt EF86). However that did not fix the problem so I'm looking for your help to find some other solutions... It is possible though that the philips tube is bad as well as I unfortunately don't have a tube tester. It will not be easy for me to find another EF86 over here (Iceland) so I wanted to look into other  possible reasons for failure before waiting for a new tube to arrive.

I looked at the capsule and it looks like it was probably reskinned by Vintage King as it looks brand new so that should be alright. However I found that Vintage King did not recap the original power supply? Can dried out electrolytics cause this kind of a problem? Maybe I should just change them out anyway? Or not fix if it aint broken? It does have nice old Siemens caps.

Thanks !

 

[bubbi]

Alright just measured the voltages on the mic side. B+ is around 231V and heaters are at ca. 7V. Will that be a big problem or is it within tolerances? (could it be a broken tube?) The adjustment pot is very heavily lacquered so I'm not sure how to go about turning it.

-B

 

[VertigoRecording]

can you record the noise?  If you swapped the tube with a known good tube  then the problem lies elsewhere and is not the tube.  but we do not know the status of the tube you put in. I have seen it where tubes are good but happen to be noisy in one circuit but the same tube works without noise in another. Now if you didn't recap the PSU your more likely to have ground hum and voltages that are way off VS some noise of AIR. I also find it unlikely that Vintage king would not recap the PSU unless it has something like PIO caps in there as those last a super long time

See if the noise is there without a tube inserted in the mic. If it is then I would be looking towards the PSU for such a strange noise. If it's not there and only there when a tube is present, I would be leaning more towards a problem with the capsule. Does the capsule cut out at all?

All of the above is quite possibly the worst troubleshooting advice I have ever read! My other favorite is..."I don't have the manual in front of me but if I remember correctly...blah, blah blah"
This is the internet and it is FREE so what do you expect? Here's the metaphor....."i have a problem with my car, it won't start". The service tech says, "Take out the engine and tell me how that works for you". Same as testing without the tube.....groan....
Yes, it is possible that two EF86 tubes, of completely different origins have exactly the same noise but what are the odds????? Not good. Sounds like you
have some other problem inside the mic preamp, possibly a noisy resistor (ouch.....hard to locate) or possibly a noisy resistor......if you get what I'm saying.
The 7 volts on the heater is a worry, 6 would be better, 6.3 being the exact number. I would try to send it to someone who knows how to work on such a valuable and wonderful instrument. Of course, Iceland is far away from all of these normal options so I feel for you.

 

[gyraf]

[Mod mode]

Vertigo,
Over here on GDIY, we have a very basic rule - NO PERSONAL ATTACKS.
Your comments and help is valued here, but please don't get too close to the NPA line.
I'm not saying you've crossed the line, just reminding you it's there - as I can see you're a relatively new user.

[/Mod mode]

Bubbi,

In my experience, noise in u67's most often comes from contamination of some sort - rarely from the tube unless it's steady, low-level crackling noise.

The capsule is the most common problem for contamination, as it's the most exposed - but the body/amplifier is also quite sensitive to dirt from uninformed repairs.

Jakob E.

 

[moamps]

B+ is around 231V and heaters are at ca. 7V. Will that be a big problem or is it within tolerances?

Hi,
+B of 230V is on high side, it translates to the polarization voltage of 65V on capsule, what's too high.
If your power supply is set to 220V primary, switch them to 240V, and recheck voltages.
Regards,
Milan

 

[bubbi]

Alright, so I just adjusted the PSU to spit out the correct voltages (this one for some reason doesn't have a 240V setting, only 117, 127 and 220) and everything is now working as it should. The mic had been living in LA for some time and the power supply probably adjusted to that and it had this noise when we first plugged it in over here in Iceland.
So thanks guys ! Always a pleasure !

 

[VertigoRecording]

[Mod mode]

Vertigo,
Over here on GDIY, we have a very basic rule - NO PERSONAL ATTACKS.
Your comments and help is valued here, but please don't get too close to the NPA line.
I'm not saying you've crossed the line, just reminding you it's there - as I can see you're a relatively new user.

[/Mod mode]
Hi Jacob,
You are absolutely right. My apologies to Puchho and all of the other people who post useful and informative information on this forum, and
particularly to you Jacob for being so generous and brilliant. Moral of the story: never post while watching your favorite World Series (American
baseball) hungry- local Dodgers team succumb to a four game losing streak! I believe my post was ill conceived and misdirected frustration.
Sorry for that. Please keep up the good work!

 

[gyraf]

 

[Whoops]

Hello,
did you ever fixed your microphone?

what was he problem?

thanks

 

[fazer]

I have a Tube Neumann and have found that the contacts on the main connect at the base of the mic needs cleaned with contact cleaner.  The pins in the connector get contaminated over time and produce white noise.  After cleaning all is good again.