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Author Topic: NU67 puzzle  (Read 1185 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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Klaus Heyne
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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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NigelT

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Re: NU67 puzzle
« Reply #15 on: May 05, 2022, 02:21:13 PM »

This is how I got the unit, it's been modded by RCA Studios to use a 5-pin XLR instead of the Tuchel. as well as adding a 3 position pad 0, -10 and-20db. All of them are the same. I dont know if this is good or bad.
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NigelT

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Re: NU67 puzzle
« Reply #16 on: May 05, 2022, 02:22:03 PM »

Board as I received it...
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klaus

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Re: NU67 puzzle
« Reply #17 on: May 05, 2022, 02:35:28 PM »

First thing I would do:
plug in a mic and monitor on your meter as the heater voltage ramps up. Immediately turn the supply off if the voltage creeps past 6.3VDC, then adjust with the trimpot down to 6.2VDC.

The second thing I would do (to this and all other NU67 like it):
remove the multi-resistor* network from the circuit board (lower right on your last picture), remove any resistors* from the XLR connector (second from last picture) and connect the power supply Tuchel audio connections (pins 1, 2) directly to the XLR connector (pins 2, 3) via an appropriate shielded three-conductor cable whose ground and shield are terminated together to ground on both connector ends of the cable.

*These resistors were installed on U.S. - distributed NU67 supplies to attenuate the mic's output. They are entirely useless and counter-productive to good sound.
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Klaus Heyne
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Kai

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Re: NU67 puzzle
« Reply #18 on: May 05, 2022, 06:18:39 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.
if you get -30 V at R9 the Zener is toast.
If it would work it would limit the voltage to shortly above -15 V even without load.

Replace it.
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NigelT

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Re: NU67 puzzle
« Reply #19 on: May 06, 2022, 05:42:36 AM »

First thing I would do:
plug in a mic and monitor on your meter as the heater voltage ramps up. Immediately turn the supply off if the voltage creeps past 6.3VDC, then adjust with the trimpot down to 6.2VDC.
Will do. I've followed your advice on this procedure on all the units and so far, they are behaving. I'm also matching each mic to its own supply as I have read here.

The second thing I would do (to this and all other NU67 like it):
remove the multi-resistor* network from the circuit board (lower right on your last picture), remove any resistors* from the XLR connector (second from last picture) and connect the power supply Tuchel audio connections (pins 1, 2) directly to the XLR connector (pins 2, 3) via an appropriate shielded three-conductor cable whose ground and shield are terminated together to ground on both connector ends of the cable
Will do. All that has been bypassed by the big wires you see in the pic. But RCA installed their own pad, should I remove it as well? It's a rotary switch at 0db, -10db and -20db.
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NigelT

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Re: NU67 puzzle
« Reply #20 on: May 06, 2022, 05:44:42 AM »

if you get -30 V at R9 the Zener is toast.
If it would work it would limit the voltage to shortly above -15 V even without load.

Replace it.
Roger that, Kai, thanks
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NigelT

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Re: NU67 puzzle
« Reply #21 on: May 06, 2022, 05:50:58 AM »

My wall outlets are giving me 127v. I need to talk to the power company. Meanwhile I have a pretty hefty Variac I will use.
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NigelT

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Re: NU67 puzzle
« Reply #22 on: May 07, 2022, 11:09:50 AM »

On to the next unit: this one is the newest schematic (NU67u) with no trim pot. I received it in the condition you see in the picture. It looks like the legs on this rectifier were too big to fit in the holes in the board so someone hacked this in. It says RS404L. I dont know what the original rectifier part number is, is this one ok as a replacement?  The schematic says BY164 but I dont see anything online that looks like it will fit or even that package.
I found a note inside that says "Blows fuse". It was taped by the rectifier, I assume that's why it was replaced.
  The unit powers up but the voltages are pretty far off.
Under load:
R1 = 255
R3 = 244
R7 = 231

R2 = 22
R4 = 14.76
R6 = 11.46
R8 = 6.71
Other issues: the resistor at R2 is getting really, really hot. So is the Zener. Is this normal? I mean hotter than all the other ones in the other supplies in the same spot. It smells.
Questions:
1. Is it a good idea to add a trim pot?
2. Are the ceramic resistors R2, R4 and R6 original Neumann? If not, could that be a problem?
3. Should I lift all resistors and check values? I tested R7 and it is right on at 160Ω. I have not tested the others yet.

Thanks again for your help. You guys are awesome.
Regards,
David
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NigelT

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Re: NU67 puzzle
« Reply #23 on: May 07, 2022, 11:11:51 AM »

This is the schematic for this unit.
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klaus

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Re: NU67 puzzle
« Reply #24 on: May 07, 2022, 12:57:28 PM »

Change the input voltage selector to 127VAC and measure again. You then will be in sight of the ballpark, if not already in it.
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NigelT

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Re: NU67 puzzle
« Reply #25 on: May 07, 2022, 01:50:39 PM »

B+ = 217
Heater = 6.52
Close enough?
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mikezietsman

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Re: NU67 puzzle
« Reply #26 on: May 07, 2022, 06:36:15 PM »

B+ = 217
Heater = 6.52
Close enough?

NigelT,
I don't know a tonne about tube mics, but I do know that it's best for tube life to be conservative with heater voltages. 6.52 is 0.22v above the recommended 6.3v for an ef86. In practice I haven't noticed any raised noise-floor running mine at something closer to 6.1-6.25, as was recommended to me by more than one very nerdy tube-head.

When testing out tubes on one of my u67s my tech and I noticed that different ef86 seemed to have different enough current draw that heater voltage needed to be adjusted individually, per tube.

When I had a 67 done by Klaus, the power supply came back labelled "for use with serial number x only", which I suspect is because the heater voltage had been adjusted specifically for the tube in that mic. I'm sure Klaus will correct me if I am wrong.
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Kai

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Re: NU67 puzzle
« Reply #27 on: May 08, 2022, 12:29:51 AM »

NigelT,
I don't know a tonne about tube mics, but I do know that it's best for tube life to be conservative with heater voltages. 6.52 is 0.22v above the recommended 6.3v for an ef86.
6.5 V measured at the PSU is OK.
The cable‘s resistance drops some voltage, measured at the mic you will end up close to or even below 6.3 V.

Each 1 Ohm cable resistance drops 0.2 V (heater current 0.2 A).
Most cables have more than 1 Ohm.

This is why the schematic says 6.5 V instead of the tube‘s nominal 6.3 V, and why it makes sense to adjust the voltage for a certain PSU / cable / mic combination, while measuring at the mic.
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mikezietsman

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Re: NU67 puzzle
« Reply #28 on: May 08, 2022, 06:12:03 AM »

Hey! You learn something every day. Last time my tech did measurements at both sides the difference was about 0.06v over a 7m cable, but that was with an ac701.
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Kai

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Re: NU67 puzzle
« Reply #29 on: May 08, 2022, 05:01:34 PM »

Here’s an example of technical date of a high quality tube microphone’s cable, Mogami 3172.
Based on these values one could calculate the heater voltage drop using Ohms Law:

Cable Length x Resistance per Lenght x factor 2 [forward+return] x Current = Voltage

Exxample from above (AC701 has 0.1 A heater current):
7 m x 0.046 Ohm/m x 2 x 0.1 A = 0.0644 V

If the shield is used for return current “factor 2“ is not used in the equation.
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NigelT

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Re: NU67 puzzle
« Reply #30 on: May 10, 2022, 04:35:28 AM »

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.
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Kai

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Re: NU67 puzzle
« Reply #31 on: May 10, 2022, 10:47:49 AM »

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.
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NigelT

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Re: NU67 puzzle
« Reply #32 on: May 25, 2022, 05:04:44 AM »

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.
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NigelT

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Re: NU67 puzzle
« Reply #33 on: May 31, 2022, 03:23:30 AM »

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.
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NigelT

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Re: NU67 puzzle
« Reply #34 on: June 17, 2022, 05:32:08 AM »

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
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Kai

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Re: NU67 puzzle
« Reply #35 on: June 17, 2022, 04:16:00 PM »

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.
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NigelT

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Re: NU67 puzzle
« Reply #36 on: June 20, 2022, 12:06:12 AM »

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.
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