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Neumann/Gefell GNM14 mic

Discussion in 'Electronic Repair Modifications DIY' started by Jon Hildenstein, Dec 24, 2014.

  1. Jon Hildenstein

    Jon Hildenstein Active Member

    Hi all,

    My first post here at recording.org. Nice to meet you!

    I was given a broken vintage Neumann mic a while back and fixed it recently. It is an M14: a gooseneck mic complete with PS and impedance converter. The sound is heaven, and it's very quiet. I'm hoping anyone out there has some experience with this rare bird. I'll post some pics.

    Thanks for reading.

    -Hildy
     

    Attached Files:

  2. rmburrow

    rmburrow Active Member

    Did that mic use an AC701 or a 6AB4 type tube?
     
  3. DonnyThompson

    DonnyThompson Distinguished Member

    What was wrong with it? Mic or PS?

    I'm pretty sure the M49/50 came with an AC701... and they ain't cheap to replace, either. ;)
     
  4. rmburrow

    rmburrow Active Member

    M49/M50 used the AC701. Did the M14 also use the AC701? The case shows it is a Neumann Gefell mic...Neumann Gefell supposedly operated independently of Georg Neumann GmbH in those days....
     
  5. Jon Hildenstein

    Jon Hildenstein Active Member

    Wow, I hadn't realized you guys had responded.. To answer your question:

    The tube in the mic is an RV12P2000. I did a little research and learned it was a very flexible tube used in almost all of the German military equipment back in the day. You could drive a speaker with it, or modulate an AM transmitter. Or in this case, amplify a mic capsule. It looks older than the hills, but it is immensely quiet. Goes to show that German workmanship really was on point.
     
    audiokid likes this.
  6. Jon Hildenstein

    Jon Hildenstein Active Member

    A couple simple things. Initially, when I popped open the power supply, I noticed a tube rattling around in there along with a broken tube socket. I learned it was the 100V regulator tube. I just repaired the socket, plugged in the tube and powered it up on a variac with a fuse, just in case. It worked! The wiring inside the mic was pretty old, and I suspected was creating some noise, so I replaced it with new.

    Then, later on the mic just quit. I traced the issue to a dead resistor between the grid/capsule and ground. I believe it was 50M or so. That solved that issue.

    Everything was cruising along nicely until a little while ago when the PS emitted a really acrid smell. I turned it off immediately and consulted a tech friend who knows tube gear pretty well. He suggested checking the power transformer, saying the smell reminded him of burnt tranny wire. The rest of the components do not look burned, so I'm going to follow his lead and test the coils. Only thing is, I don't know what the secondary voltage should be if I should need to replace it. They say glow regulators like to see an unregulated V 15-20% higher than their regulated voltage. So I'm guessing 170AC?
     
  7. Jon Hildenstein

    Jon Hildenstein Active Member

    Here's the guts.
     

    Attached Files:

  8. rmburrow

    rmburrow Active Member

    Jon... Disconnect everything from the transformer secondaries, apply power, and see if you still have the burning. (Be prepared to remove power quickly!) Make sure the fuse is the correct value. (How often has the wrong fuse been installed in equipment like this?) If the transformer doesn't get hot or smokes, then the problem is down stream....Test the AC voltage on the secondary windings with everything disconnected. One secondary may put out around 120 vac and the other is the low voltage for the heater. If the transformer turns out OK, an ohmmeter will tell the tale on the rectifiers and most of the other components. You could also have a bad electrolytic cap in there.

    Thanks for the info on the tube. I would have to look that one up.
     
  9. Jon Hildenstein

    Jon Hildenstein Active Member

    done. While I was at it, I tested DC resistance on the secondaries. The plate supply was a couple hundred ohms, the heater supply was really low, like 8 ohms or so. I hooked the tranny up to a variac and some meters. When the AC hit about 60VAC, the current spiked and the fuse blew. So it looks like the transformer is bad. The rest of the components seem to test OK. There is a glow-regulator which I will have to test once I get a new transformer in it.

    Maybe the heater supply is the source of the short? I wonder what the resistance is supposed to be on that sucker? Also, anybody know of a place that sells a transformer for something like this? I can find entire kits, but looking for just a transformer, I found only huge ones for guitar amps and high plate voltages. This one needs only 120VAC and 10-12VAC.
     
  10. rmburrow

    rmburrow Active Member

    I assume you had EVERYTHING disconnected from the power transformer secondaries for the variac test. You should have (had) bare terminals on the power transformer except for the primary AC. Something was apparently still connected if you observed a sudden current spike.

    Two more tests? Did you take the ohmmeter and check for a short from each secondary to core? If you have a short with everything disconnected, this is a problem. Did you lift the primary terminals completely and check using the ohmmeter for a primary short to core?

    There are some "winders" who periodically search these sites. You may be able to find someone who can rewind your power transformer for reasonable cost and keep the unit genuine, and not hacked with a third party transformer....
     
  11. Jon Hildenstein

    Jon Hildenstein Active Member

    Yes, the secondaries were nude. I even removed the transformer from the housing. I am guessing that wire in one of the windings may have ever so slightly burned through its enamel and will "act" normal up until the point where voltage is high enough to arc to... Another coil? The core?

    Once the transformer was out of the box, it was clearly the culprit of the "burnt" smell. I tested all contacts for DC resistance. No shorts anywhere.
     
  12. rmburrow

    rmburrow Active Member

    OK on lifting all outside wires from the transformer and the burnt smell. Looks like you have done all you can to isolate the transformer as the problem. There are some "winders" on the DIY Audio site that may be able to get this transformer rewound. They may also lurk on here. There is one with username "CJ" in California who appears more than qualified to do this.
     
  13. rmburrow

    rmburrow Active Member

    Another note: The RV12P2000 tube has a 12.6 v heater. The power transformer secondary for the heater is probably in the 20 volt range. The power supply probably used a bridge rectifier (back then, generally selenium; visible square plates) and an adjustable resistor to set the heater voltage to 12.6 volts (with the mic connected).

    Sometimes, the selenium rectifier will short out. The short across the power transformer secondary can ruin a power transformer. "Smoke" from a defective selenium rectifier can be toxic.

    The HT secondary is probably in the 120 v ac range. Bridge rectified with low load, the HT at the rectifier output can be around 180 volts or so. A dropping resistor into a 150B2 or equivalent (OA2 in USA) gives regulated 150 volts dc. More filtering R/C legs and the HT is ready for the mic. I didn't see any "stabilytes" in the picture of the supply.
     

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