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Author Topic: NU67 puzzle  (Read 763 times)

NigelT

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NU67 puzzle
« on: April 28, 2022, 12:37:00 PM »

Hello,
I'm new here and really enjoy reading all the great posts. I have a small studio with mostly analog vintage gear that I am struggling to keep alive on almost no money. I could really use some help. A long time ago I acquired a bunch of U67's from RCA Studios when they shut down. The mics all work great but the PSU's not so much. I have four that work fine and six that have various problems. I'm taking them on one at a time and am already stuck on what I thought would be easiest one. I have read that there are two versions of these but in my research I have found there are actually three and of course I have some of them all. This is the correct schematic for what I'll call unit #1. The circuit board says N-084-07. http://www.tuberadio.com/robinson/museum/Neuman_U67/NeumanPSUcct.gif
It powers up and here is what I see with mic hooked up.
Gr 1 = 230vac
R1 = 252v
R3 = 233v
R7 = 208

Gr2 =23vac
R2 = 22v
R5 = -15.7v
R6 = -15.7v
R8 = -15.7v
The trim pot has no effect at all. It appears to be functioning, I measured one of my working units and it measures 0-24 ohms in circuit as does this one.
Question #1: I have to measure with the mic plugged in, will it harm the tube running it at 15.7v while testing? I hope not. If so, is there a way to hook up a dummy load till I get it sorted?
Question #2: What is F 1 and what is it's function?
Question #3: What is  Gr3? I think it's a Zener diode but I have looked everywhere on the web and cannot find any symbols that have the line at a 45 degree angle. If it is a Zener, why use that symbol?
Question #4: Is there a way to measure the caps and determine if they are still OK?
Notes: This supply has all the original parts and all resistors measure almost exactly as specified with one end lifted.
Any ideas on this guys?
Thanks in advance.
Regards
David
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RuudNL

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Re: NU67 puzzle
« Reply #1 on: April 28, 2022, 03:52:17 PM »

1 - Without a load, there is no voltage drop over the series resistors.
To play safe, have a competent technician- see my comments below (K.H.) connect a resistor of 33 ohm/2 Watt to the filament output.
Measured voltage will now be a bit higher than 6.3 Volts, but it will give you a rough indication.
2 - F1 is a mains filter.
3 - This is a 15 V. zener diode that functions as a kind of 'pre stabilisation'.
4 - The only way to check the capacitors is to disconnect one side of each capacitor and measure them. (Capacity, leak current and ESR)
Most capacitors will have a different value than specified after many years and also have developed significant leak currents.
Usually they should be replaced after many years. (I just 'revitalized' a power supply from 1951...)

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NigelT

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Re: NU67 puzzle
« Reply #2 on: April 28, 2022, 05:34:07 PM »

Not sure I follow you. I am getting 15.7v at pin 4 with the mic plugged in. What will the 33 ohm resistor do? Do I connect it at the supply from pin 4 to ground? I dont know why there is no change when I turn the pot with the mic plugged in, which means it has a load.
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klaus

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Re: NU67 puzzle
« Reply #3 on: April 29, 2022, 12:52:03 AM »

If that high a heater voltage is measured when the mic is plugged in, then the heater circuit is not being completed.

I do not mean to be disrespectful, but your comments lead me to conclude that you do not know how to repair this unit. The presence of high voltages and possibility of a lethal shock prompts me to recommend you hire a competent technician to further troubleshoot this issue.

So have a competent technician analyze the entire heater supply rail to detect where there is an open ground. Likewise, a broken heater filament in the tube will also prevent the voltage to go to ground, i.e. the heater supply sees no load, as if no mic were connected.

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NigelT

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Re: NU67 puzzle
« Reply #4 on: April 29, 2022, 05:19:41 AM »

Klaus,
Thanks so much for the reply. Ruud also, thank you. I found a broken trace on the board and now all is well. I am amazed at how well these mics and PSU's function even after 60 years. The mics sound truly amazing. I'm on to unit #2 and I hope it's similar.
Regards
David
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NigelT

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Re: NU67 puzzle
« Reply #5 on: April 29, 2022, 06:50:03 AM »

Klaus,
I see you changed your post, I dont take what you say as disrespectful at all but the exact opposite. I will say I work with other electronics, for the most part digital in avionics but where there is also lethal 115vac 400hz. So I am extremely cautious.

I am not very familiar with older stuff though and maybe I wasn't clear. For example, I know what a Zener is, I just never saw the symbol drawn that way. I am still curious about that, is it an older obsolete symbol? I am sure it's obvious to you, but I am way behind in this area.

Regarding caps, I meant that you guys must have seen about everything that can wrong in these units by now and thought maybe you would know signs of bad caps without having to pull them. I am willing to replace them no problem if necessary. I guess I'm asking if it's something I should start with, given the age of these units, they look all original. In my limited experience with my Neve console, I was always told to leave the electrolytics alone until there's a problem. I found that not to be true though.
     
I am trying to learn but nothing beats experience and from your one line explanation about the supply rail I knew where to look and I was able to figure out what was wrong in minutes.

Again, thanks. 
Regards
David
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NigelT

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Re: NU67 puzzle
« Reply #6 on: April 29, 2022, 06:52:26 AM »

Oh, and I am working with a known good mic and tube so I knew the filament was OK. Out of the six dead units, I have repaired three of them with various problems so far and now the fourth one thanks to you. The simple things get me sometimes...two left to go and I will have 10 working beautiful U67's.
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Kai

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Re: NU67 puzzle
« Reply #7 on: April 29, 2022, 07:48:19 AM »

...two left to go and I will have 10 working beautiful U67's.
...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.

Electronics symbols vary all over the place, remember the 60 years that passed along since.
American symbols are often quite different  from European’s, even denoted for resistors in the schematic!
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klaus

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Re: NU67 puzzle
« Reply #8 on: April 29, 2022, 11:56:31 AM »

(...) Close visual inspection can reveal residue from cap leakage, but missing residue doesn’t mean the cap is fine.

...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.
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NigelT

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Re: NU67 puzzle
« Reply #9 on: May 04, 2022, 01:42:02 AM »

...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.
Which is exactly why I am here asking questions. The next unit I am testing has voltages way to high to even plug a mic in. It's been recapped but something else is wrong. See below. So I will test using the dummy load before I plug in a valuable mic. Thank you.
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).
Sadly my scope died. Although I do have eight working NU67's, all are really old. The voltages are in spec but I only have an RMS Fluke to measure ripple. I'll check them all but I dont have one I can say for sure is good enough to compare to.
Close visual inspection can reveal residue from cap leakage, but missing residue doesn’t mean the cap is fine.
All of the NOS caps visually look ok but some of the circuit boards are in pretty poor shape, they look like they have had a lot of crappy soldering done.

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!
Understood. I learn every day. I haven't had to use many 60 year old German schematics, in fact this is the first time. I assumed it was a Zener but again, that's why I ask. You know what they say about assume...

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NigelT

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Re: NU67 puzzle
« Reply #10 on: May 04, 2022, 01:42:53 AM »

...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.
Will do.
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NigelT

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Re: NU67 puzzle
« Reply #11 on: May 05, 2022, 01:56:54 PM »

The next unit i repaired was easy. It showed no voltage on the heater rail. I had 20.3v in to Gr2 but nothing coming out. I suspected it was faulty so I removed it and found that one of the pins was dry and never been soldered at all. I soldered it back in and checked it (with a load first) and all is well. 209.8v and 6.2v heater.
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NigelT

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Re: NU67 puzzle
« Reply #12 on: May 05, 2022, 02:16:09 PM »

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. However, when I added the 33ohm load, R2 dropped to 208.9v and R9 dropped to 6.3v variable. I am wondering why this would be and is it ok to plug in a mic? Thanks guys.
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NigelT

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Re: NU67 puzzle
« Reply #13 on: May 05, 2022, 02:16:57 PM »

Pics of caps
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NigelT

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Re: NU67 puzzle
« Reply #14 on: May 05, 2022, 02:18:04 PM »

Pics of caps
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