Home » R/E/P » Klaus Heyne's Mic Lab » nuvistor u47 popping
| nuvistor u47 popping [message #73610] |
Wed, 22 June 2005 15:19  |
 sparechessroom
Messages: 5
Registered: June 2005 |
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i am working on a u47 with a nuvistor. it came in with popping. also, the polarizing resistors 60M, and 100M seem to have been reversed, though the owner said it worked for years. this may not matter. anyway i replaced them, and put it back to what the schematic read. at that point, it was very quiet, i moved the pattern lever, and it came in strong. i took it apart, cleaned the inside of the capsule cover where pattern lever slides, and cleaned the contacts on the capsule where the lever makes/breaks contact. it is now intermittently fine, then much, much louder, then cuts in and out at a very fast, but steady pace (capacitor?) the first time i heard this, i turned the variac down out of fear for the capsule, and at about 80% it was fine, loud and quiet. albeit fading out without the proper voltage. but this has been the pattern. when this fast motor boating happens i can get rid of it with the variac. i just measured the voltage from the supply, and it is about 396vdc... not 105vdc! i do have a schematic for the mic that has written on it over the 105vdc input : 390vdc, and the capsules 63vdc has 210 written next to that. i have on order a new nuvistor, and i am wondering if the .01mfd 400v cap could be bad- which is actually in this mic a .01/200vdc cap. i am also thinking of recapping the power supply, and the 1mfd, and .5mfd coupling caps. the 1780 ohm wire resistor did not measure out at first, i needed to unsolder it before it measured correctly, while another u47 i own with vf-14 it measured fine in circuit. this seemed noteworthy. if you have any input on the voltages i should be reading from the supply, the consequences of the resistors being reversed, and the variac cure's meaning, i would feel less silly.
thanks a lot!
kevin mcmahon
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| Re: nuvistor u47 popping [message #73750 is a reply to message #73610 ] |
Thu, 23 June 2005 12:04  |
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Klaus Heyne
Messages: 2974
Registered: April 2004 |
Platinum Member |
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Kevin,
There comes a time into a repair project when I need to admit that a specialist who has taken apart a few of these convoluted, idiotic electric window mechanisms in a Vanagon will deal with its repair faster, cheaper, and with less aggravation than I can muster.
Your report sounds to me that you may be a bit in it over your head with this project.
Your approach to troubleshooting seems like that of a studio gear specialist, and that just does not work on these idiosyncratic beasts.
1. Capsule condition? If you have an M7, there may lie your answer for the erratic popping and volume fades.
2. Supply voltages? More than 300VDC? Obviously you ran the power supply without load. Forget any measurements without the mic plugged in-
With the Nuvistor model you will need exactly 110 VDC at the red wire coming off pin #5 of the Tuchel connector in the mic to have correct supply voltage.
3. Regarding capacitors, regardless of where they are located: If these are original Neumann stock, forget about trouble shooting- they literally never fail. (I should say: They COULD fail, but then this would be the first time I have witnessed it in 25 years)
4. I suggest you drop your pride and let someone who is familiar wiht this model take a look at it.
Best of luck,
Klaus Heyne
German Masterworks
www.GermanMasterworks.com
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| Re: nuvistor u47 popping [message #73786 is a reply to message #73610 ] |
Thu, 23 June 2005 15:21 |
sparechessroom
Messages: 5
Registered: June 2005 |
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hi, thanks for getting back to me.
the capsule is a k47 not an m7. there are no cracks in the gold, and very little build up, though there is some dust visible. the capsule did work fine with another body.
i measured voltage without the mic, and suspected that i should measure with it, but was fearful for the safety of the capsule since it was behaving very strangely in a way that sounds like the capsule turning on and off at the steady rate of a cap charging/discharging. superstition is not good in teching, but it may be better than taking a chance with someone else's $5000 mic that is acting weird.
the caps in all are not original. the .01mfd/400v decoupling cap
is actually a .01/200v which concerned me, and the coupling caps are both replaced with very ordinary run of the mill caps that are reportedly over 15 years old. besides neuman parts, i have generally been told that electrolytics in general may need to be replaced after some time around 7-15 years depending on who you talk to.
the 60M and 100M bias resistors were not in their right places, they were reversed- can this damage the capsule? it may only mean that it is reverse biased and that may pose no threat to the capsule at all, but i am not completely familiar with the consequences of this when there are also incorrect values of capacitors that are really old added to the equation. i looked into this resistor thing as i read that popping can be, if you are lucky, bad bias resistors. these were not original, were put in really sloppy, and were 1/4 watt so i got originals from telefunken, and put it back to the schematic which seemed like a logical step. is it? if no, why?
i was unable to find any info about the amount of voltage the capsule can take, and if there is a rational fear for the safety of the capsule with regard to too much voltage (i have to assume that there is but do not know at what voltage the danger starts) so i have adopted a very cautious posture with this mic unitl i can answer these questions.
i am not sure what to make of the studio gear specialist thing with regards to learning anything from this exchange, i am definately not a hobbyist though i have a lot that i need to learn, and have been/will continue learning what i can where i can.
i am much more interested in knowing what other approach i may have used with troubleshooting this mic than knowing that i do not know everything that some others do. i always wonder how one is supposed to learn when people deal with you like you should have already known what you asked. i appreciate that i may very well go to someone more qualified if i cannot answer these quesions. but without pressing further in this exchange, i would not walk away from this knowing more than i did originally which is my only motive for writing in the first place.
i hope i do not sound disrespectful, i appreciate whatever time you have dedicated so far, i just need to say that however one learns, they rarely came up with the ideas themselves, and may have learned soley because noone killed their instinct to ask a "dumb" question of someone who will actually answer it. now more than ever, there is a big need to pass info on -i think. as i see it, the ideals that are replacing the older ones are not steeped in quality at all, and sadly there are not many whop care... mp3's anyone?- i have been trying to learn things about the older gear and how it is made. and why it sounds often real compared to the things that are being sold today. it is not easy to find people who will talk about these things in a truly educational way at all, but plenty that will point out that you do not know something, which is annoying. and i know that people trying to find things out now that you may know from another time, are hard pressed to get the type of exposure to the very ideas that may have made it possible to know what you do about this type of gear... because the time is gone, the parts are not there now, even the resources to make some of them may not be the same, or findable... i doubt if there is anything about a mackie or digidesign product that is relevant to why a u47 sounds good- which frustrates me. i could hazard a guess that if someone were not in a financial position to choose to do otherwise, and they had an interest in making or fixing gear, that there would be a larger chance that they would go into working for a newer company... most of which will teach their employees how to replace boards on things mass manufactured with parts that do not have a character let alone a sound, and are designed to fail. without forums like these, and other forms of interest by people like you, people like me have a really hard road to travel to keep what i think are suprior ideas alive- because you cannot naturally find a lot of these ideals in place now. if i am wrong about this, i would be overjoyed to know it.
thank you for your time. i do not mean any disrespect.
sincerely,
kevin mcmahon
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| Re: nuvistor u47 popping [message #73817 is a reply to message #73786 ] |
Thu, 23 June 2005 19:01 |
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Klaus Heyne
Messages: 2974
Registered: April 2004 |
Platinum Member |
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| Quote: |
i am much more interested in knowing what other approach i may have used with troubleshooting this mic than knowing that i do not know everything that some others do.
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I made my comments because long distance trouble shooting of a difficult situation like you layed out (i.e. someone has already messed with the original circuitry, which on top of it all, is a Nuvistor type, where four different official Neumann set ups alone exist!) is frustratingly unproductive, judging from past similar experiences.
Again, judging from your comments you do not seem familiar enough with the workings of a tube condenser mic (even a fairly simple one, as in this case) that I have confidence I can give you meaningful input.
Regarding your concrete questions: Can you blow up the capsule if too much voltage gets to it? In theory, yes, but in practice, most unlikely.
If you measure 110V at the mic's input, which must be your starting premise, the dropping resistors for the polarization voltage will divide it down to around 66VCD- and this is what the capsule on a Nuvistor U47 wants to see.
Meaure this voltage where the 2 Meg ohm and 3 Meg ohm resistors meet against chassis ground.
If you have the original VF14 tube socket still in the mic that plugs into female tube socket contacts (some Nuvistor models used such a plug-in module with the Nuvistor sitting on top of a sawed-off , modified VF14 8-pin bottom) you may have contact noise at the tube contacts, especially for the three plate pins of the tube sockets. If so, clean the contacts very carefully and thoroughly with Caig Pro Gold, and hope for the best.
Beyond the above hints, I am honestly telling you that long distance cures of sick condenser mics have their limits.
Best of luck, again,
Klaus Heyne
German Masterworks
www.GermanMasterworks.com
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| Re: nuvistor u47 popping [message #73946 is a reply to message #73610 ] |
Fri, 24 June 2005 11:25 |
sparechessroom
Messages: 5
Registered: June 2005 |
Member |
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Klaus,
thanks a lot for the info. i will look into what you said about the socket, which is a sawed off vf14 socket. with the knowledge that the capsule will not as easily as i was fearing, get enough voltage to damage it, the voltage is 115vdc, and the capsule was getting 65vdc. it started the on/off thing while i was taking readings, then i measured for ac across the caps in case they were leaking- there was none, and it stopped as soon as i touched the probes to the .5 mfd cap leads, and has not come back. the popping is gone for now too. i will leave it on and see. there is a little tinkle sound whenever you move the mic, i have another nuvistor coming. are there contacts inside the nuvistor that short if their is a failure inside it like some tubes?
is there any reading you would recommend for the type of knowledge you are talking about me needing to know for your input to be more meaningful? i think honestly that i represent i know less than i do because i know i need to know more than i do, but i think i understand the basics of what should be going on inside this mic. the main issue i have had is having been trained to be overly cautious, and believing too much that i do not know more than i do. many of the things one is taught these days do not come along with a lot of background. like knowing what a capsule is, but not knowing at all the tolerance it has for voltage and therefor not wanting to take certain steps out of fear for damaging old and precious equipment without the answers i need to feel safe. i am no expert of anything yet, but i have done extensive work on my own mics, and tape machine like re-ribboning and retensioning my rca 74's, dropping the vf14 tube from my 47 and cleaning the pins after being told that the tube was dead by someone more qualified (it has been great for 4 years since then) rebuilding an entire mci 636 console with multiple blown power supplies and internal shorts, repairs and modifications on various neve modules, putting direct outs on my own neve 5455 sidecar, and lots of tube amp repairs. i am very serious about learning, but again have a hard time finding much background on the concepts i am taught from those that i have apprenticed with. i have thought of more school, but most of the top working techs in the ny area do not profess to have learned a lot in school, if they went at all. it seems like this background would be useful.
thanks for your time,
kevin
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| Re: nuvistor u47 popping [message #73972 is a reply to message #73946 ] |
Fri, 24 June 2005 14:55 |
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Klaus Heyne
Messages: 2974
Registered: April 2004 |
Platinum Member |
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| Quote: |
i am very serious about learning, but again have a hard time finding much background on the concepts i am taught from those that i have apprenticed with. i have thought of more school, but most of the top working techs in the ny area do not profess to have learned a lot in school, if they went at all. it seems like this background would be useful.
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Getting experience for repairing recording mics, especially tube models, is a lonely job:
Most who know anything will not tell- or they are to busy to tell.
And what little you can read about the subject (mostly all theoretical treatises and overviews) is useless, by and large, for acquiring empirical knowledge of the kind you want.
Leaves you with the Internet:
On time I read posts upon posts on specialty forums about how to replace radius arms of 911's and, equipped with all that helpfulness, with hints, warnings, bad and good experiences from others who have done it before me, I dared one day to do the job alone, all by myself , and successfully, I might add.
You too will need to use a lot of searches on the few forums available for practical micophone hints (this one, two Neumann forums, the AKG forum, and I am sure, alert readers will give you a couple more that are technically helpful in your advancement.)
Once again, best of luck,
Klaus Heyne
German Masterworks
www.GermanMasterworks.com
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| Re: nuvistor u47 popping [message #75654 is a reply to message #73610 ] |
Mon, 04 July 2005 20:41 |
sparechessroom
Messages: 5
Registered: June 2005 |
Member |
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hi,
i am happy to report that with some resoldering of sloppy circuitry, replacing the old dropping (bias?) resistors, and some cleaning of the interconnects of the mic, especially where the capsule joins the body, and the sawed off vf14 socket pins that it has been operating nicely for two days. i am glad i did not give up.
thanks for your input.
kevin
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