Rode NT2 Fault

Discussion in 'DIY Pro Audio Forum' started by western plumber, Jan 24, 2011.

  1. western plumber

    western plumber

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    Hello, our well-used RODE NT2 condenser mic has developed a problem.

    When miking at low-levels it's fine in every way.
    But when we use it for vocals or high-pressure stuff it only works for a couple of minutes. Then the audio fades over a few seconds to be replaced with a fading-in loud hiss. If we allow it to sit without any audio for a few seconds it recovers again.

    It doesnt matter what pattern we have selected (omni/cardioid) or what output level is selected.

    I have replaced all the semi's on the pcbs (2 audio preamp transistors, the oscillator and fet) with new ones (they're only pennies) but it hasn't cured it. I've gently baked the capsule over the weekend in a warm closet that's shared with the boiler, this seemed to make it work for a good 10 minutes afterwards but now it's back to its old tricks.

    Any suggestions ppl?
     
  2. MrEase

    MrEase

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    I'm not familiar with the circuit of the NT2 but from the sound of it, it seems likely that it is a faulty capacitor. The fact that it recovers after a few seconds of no audio makes it sound like the DC biasing is going AWOL with large signals. The favourites would be any electrolytic cap's which can go leaky or dry out. This could possibly be in the supplies derived from the phantom power.

    The only other possibility I can think of, off the top of my head would be the mic insert biasing resistor (several gigohms) or the oscillator. I doubt this though as they should not really be affected by the audio level - but stranger things have happened.
     
  3. Boswell

    Boswell Moderator Has Studio Services

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    Is this the NT-2 or the NT-2A?

    To aid diagnosis, I would check the phantom power while the mic is plugged in. You could use something like an XLR splitter cable (not an insert cable), taking the free XLR end to a multimeter. Connect the meter -ve lead to pin 1, and then check first pin 2 then pin 3 with respect to pin 1. The quiet potential should be around 40V, but look to see if it takes a dive on loud sounds. If it does, it could indicate that the plate spacing in the mic capsule has suffered and it's going short circuit on loud sounds.

    If that is the case, I don't know if there is anything you can do other than sending the mic back to the RODE agents for service. I know from experience that Rode has a very good long-term warranty arrangement, but they may be a bit hesitant about your having replaced parts on the circuit boards.
     
  4. western plumber

    western plumber

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    It's the standard NT2 with the pattern and gain switches on opposite sides of the body. There are only 2 electrolytics on the mic, both are new. The other caps are polyester which are generally reliable things.

    The phantom power to the mic itself is fine, I'm using a phantom power adapter dongle for the moment but it does the same in the studio anyhow on a desk.

    I did think the 1G resistor might be drifting, I'll change that I think, I dont know how it would affect the circuit.
     
  5. MrEase

    MrEase

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    If you've already replaced the electrolytics then I would suggest doing the test Boswell suggests. Although you know that the phantom power is normally OK, if the mic insert is shorting you will see the dip he mentions.

    I think you would do better to try and diagnose the problem rather than just changing out parts in the hope you will eventually hit gold. Like I said in my first post, I doubt if the problem is really the 1G resistor. BTW the biasing resistor is used to establish a "constant" charge on the insert. As this is a twin diaphragm variable response mic then there will normally be two resistors to establish the bias voltage on each diaphragm independently.
     
  6. RemyRAD

    RemyRAD

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    Crap, I've had experience with this exact same problem with Newman U67's & 87's. One of the reasons for this is that these particular microphones require full +48 V DC polarization voltage for the capsule as I believe your NT2 also is. This is because they do not have permanently polarized capsules. When these capsules are in high humidity environment and then barraged with tight vocals without a foam or lollipop pop filter, the additional humidity from your voice will cause a capacitor/condenser capsule to short out, from the additional humidity. If that should happen, it would provide exactly the same problem you're describing. So this may be nothing more than a humidity issue.

    I don't always use my 67, 87 nor my 414's with any kind of pop filters and when people are really tight & breathy, the microphone will sometimes have a tendency to fade right out and then you'll get this rush of noise as the capacitance of the capsule reforms, as soon as the additional humidity evaporates. This is not as prevalent with back electret microphones, made today, but were actually quite a problem, with a similar problem, for the Japanese Zero pilots as their microphones were very early back electret. They would just fade out as they were speaking into them. This was also a problem with early electres, including the fact that they were always considered lower in quality than the non-polarized true capacitor/condenser microphones. Modern back electrets have improved to the point where they have virtually no problems anymore, with humidity or anything else and can be powered from as little as 15 V of phantom, to simply power the microphone's internal electronics. And this is where dynamic microphones excel as they are largely impervious to a much wider range of saturated humidity than condenser microphones without any of the afformationed problems.

    Conversely, some audio consoles along with some outboard microphone preamps do not provide a full +48 V Phantom supply but lower voltages, typically 15-25 V. There some that produce no more than +25 V. Some +48 V required condensers will work for a very short period of time, generally be lower in output level, etc.It's only those particular supplies that can be used with the permanently polarized back electrets & not non-polarized microphone since +48 is absolutely required and 100% necessary. Nothing lower works reliably and could be producing your stated problems?

    Power to the people. Power to the microphones.
    Mx. Remy Ann David
     
  7. Link555

    Link555

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    Do you have any photos? It might help to see it. I have the same mic and have not seen that issue yet. How did boswell's test work out?
     
  8. western plumber

    western plumber

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    I can't really diagnose anything on the mic with live tests because as soon as you take the body off the whole thing just goes haywire and oscillates and buzzes.

    I have concluded it's the capsule, the RODE doesnt really have a complicated circuit, theres a pcb marked 'oscillator' and a pcb marked 'power supply' and only a handful of discretes on both.

    A duff capsule more or less writes it off as uneconomic. Maybe time to go shopping!
     
  9. Link555

    Link555

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    well if you don't want the mic send it my way please :)
     
  10. Boswell

    Boswell Moderator Has Studio Services

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    The capsule is the most likely, although it would be the first Rode capsule I have heard of go short in this way.

    I would still be interested in the results of the multimeter tests using a splitter cable, obviously done with the mic fully assembled. Around 36V is the expected potential at the XLR connector when the NT2 is loading a 48V phantom power circuit, since the current for powering the circuitry in the mic flows from the pre-amp's 48V source though an effective resistance of 3.4K Ohm to the XLR socket.

    The other possibility is that the oscillator is stopping under high sound level conditions, but this would be more difficult to diagnose without proper microphone test facilities.
     
  11. RemyRAD

    RemyRAD

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    Boswell, I was also thinking there are some mixer/consoles whose 48 V supply really doesn't deliver for microphones like U87's, since those of higher current requirements. Many of the manufacturers that I have discussed this with have indicated that although they offer 48 V Phantom supplies, they're really more geared toward the back electret, permanently polarized phantom powered microphones, whose current requirements aren't bad of U87's. So I think that there may be this possibility in this particular instance and not necessarily a failure of the capsule. This might just possibly be the Phantom supply not designed for a microphone such as this? It might just need more oooomph?

    Remember not to oooomph too hard otherwise you might mess yourself.
    Mx. Remy Ann David
     
  12. TheJackAttack

    TheJackAttack Moderator

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    AT still makes a six channel phantom power unit that might work if it is a phantom issue. Only the Shadow knows!