C37A intermittent pop

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ComodoComplex

ComodoComplex

Well-known member
Joined
Feb 5, 2014
Messages
102
Location
Brooklyn, NY
Hi all,

I have a Sony C37A on my bench which is experiencing an intermittent pop. This problem has been present for a while. Previous work done to address it has been replacing of the tube in the mic with a NOS variety and replacing all cabling to the mic and from the PSU (a CP-3). None of these have actually alleviated the problem, and the pop is so unpredictable and rare that it is hard to know if the problem is really fixed after attempting a repair. Unfortunately, it is almost certain to crop up in a recording session, which is unacceptable.

The recording engineer showed me the pop waveform in ProTools, and it resembles a sharp transient followed by a damped oscillation. This leads me to believe it is an issue with the high-voltage caps in the power supply. Perhaps one of them is failing, experiencing a brief short which destabilizes the supply. Then, as is the nature of second order filters, a damped resonance occurs to return the state back to equilibrium. Does this theory hold any water?

I ask because replacing the three 20/20uF 350V can caps with a suitable NOS variety (a nice drop-in replacement which will retain the authenticity of the vintage gear) is going to be a somewhat lengthy and expensive undertaking... I am prepared to do this, but second opinions are greatly appreciated! I've attached the PSU schematic here for reference.

Cheers,
Ben
 

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Have you ever been able to hear the “pop” in person?  I’d leave the mic up overnight recording into a daw and try to catch one myself, just to make sure it’s not something else in the chain at the studio?

Not sure if I’d suspect the PSU filter caps as per your theory,  (Assuming they are electrolytic, they don’t usually short when they fail like tantalum.)

But even if it was, I wouldn’t be too stressed about finding the perfect NOS filter caps for the PSU.  I’d use high quality new ones and perhaps mount them on a little bread board to make the installation neat and tidy.  Keep the old ones in case some “collector” wants to put the bad caps back in there.  But when it comes to vintage mic PSU’s  I’d rather have sable/solid b+ rather than “original filter caps” as long as the work is done neatly.  At least this would test your theory.  Then if you really wanted to hunt for some NOS caps, you’d at least have a working mic in the meantime.

 
Its possible for condenser mikes to have resistance in the capsule due usually to dirt, although the C37 is usually pretty good this way. That can produce occasional pops as the humidity and hence the resistance varies. Also tubes can do this. In this application the grid resistor is unusually high. I would disconnect the capsule, and just run the electronics, so you can determine where the fault is.
Faultfinding on condenser mikes is very difficult without the right tools.
John Peluso can rebuild your capsule for you if that is the problem.
 
ComodoComplex said:
I ask because replacing the three 20/20uF 350V can caps with a suitable NOS variety (a nice drop-in replacement which will retain the authenticity of the vintage gear) is going to be a somewhat lengthy and expensive undertaking...
Click to expand...
I very much doubt it's one of the electrolytics, but if it was, it could only be the 4th one in the bottom; the others would not produce a sharp transient, because of the additional filtering.
Just to make sure, you could temporarily replace it with a current electrolytic.
But I would concur with previous suggestions; the most likely suspects are the capsule and the tube.
It may be interesting, though, to check the caps in the HP filter...
 
Ive experienced many odd pops n crackles from condenser mics over the years , almost always due to being stored for long periods then put into use at the last minute . Tube mics are superior in one respect to transistorised , the normal heat in a tube mic will drive off any moisture as long as its left on for a few hours , in the case of transistorised Ive often recomended putting them in the airing cubbord for a few days . or storing with a packet of silica to draw out any moisture that has accumulated . The 6au6 isnt all that suitable for very high grid resistances found in tube mics ,the sony's are especially high , my guess is you'd need a specially selected tube for good opperation in this mic , the other thing is very high value resistors often have a tendency to drift higher over years and years , normal instruments wont be able to measure Gohm resistors either, Ive seen +20%-+50% or more drift in the value of carbon based resistors of high value , surface contamination of the capsule could be another problem that causes mics to 'burp' and 'fart' . Any contamination will allows moisture to get trapped on the surface of the capsule and could cause disturbances in the capsule bias volts until the moisture evaporates at very least ,if its badly contaminated with spittle and dust it might not function satisfactorily until its cleaned , this is a very delicate job ,so do study up loads before you attempt it , Neumann have a written procedure for cleaning capsules ,but do check that this method is ok for your particular mic also . One thing you could do is ,power the mic ,attach voice activated recorder of some kind ,like a dictaphone for instance ,although it can be done in software too with time logging , now you'll get a better idea of how often the issue is occuring ,and you may also be able to spot an improvement if you change something more easily , you cant just sit there for days and days listening for random noises. Tubes may also settle after hours or days of use ,some might tend to get more noisey , some will get less . probably something like 15 to 40 meg ohms is the maximum recomended grid resistor on these tubes ,using a much much higher value normally means tube selection will be  necessary .
 
Tubetec said:
Ive experienced many odd pops n crackles from condenser mics over the years , almost always due to being stored for long periods then put into use at the last minute . Tube mics are superior in one respect to transistorised , the normal heat in a tube mic will drive off any moisture as long as its left on for a few hours , in the case of transistorised Ive often recomended putting them in the airing cubbord for a few days . or storing with a packet of silica to draw out any moisture that has accumulated . The 6au6 isnt all that suitable for very high grid resistances found in tube mics ,the sony's are especially high , my guess is you'd need a specially selected tube for good opperation in this mic , the other thing is very high value resistors often have a tendency to drift higher over years and years , normal instruments wont be able to measure Gohm resistors either, Ive seen +20%-+50% or more drift in the value of carbon based resistors of high value , surface contamination of the capsule could be another problem that causes mics to 'burp' and 'fart' . Any contamination will allows moisture to get trapped on the surface of the capsule and could cause disturbances in the capsule bias volts until the moisture evaporates at very least ,if its badly contaminated with spittle and dust it might not function satisfactorily until its cleaned , this is a very delicate job ,so do study up loads before you attempt it , Neumann have a written procedure for cleaning capsules ,but do check that this method is ok for your particular mic also . One thing you could do is ,power the mic ,attach voice activated recorder of some kind ,like a dictaphone for instance ,although it can be done in software too with time logging , now you'll get a better idea of how often the issue is occuring ,and you may also be able to spot an improvement if you change something more easily , you cant just sit there for days and days listening for random noises. Tubes may also settle after hours or days of use ,some might tend to get more noisey , some will get less . probably something like 15 to 40 meg ohms is the maximum recomended grid resistor on these tubes ,using a much much higher value normally means tube selection will be  necessary .
Click to expand...

Yes, the schematic specs the grid resistor at 100Mohm, which is quite high... Could drift in this resistor really cause pops though? I think it's quite likely this is the capsule and varying temperature/humidity. After everyone's input, I would bet that the electrolytics are actually fine  ::)

The other possibility is the 6uF capacitor which couples the cathode of the microphone to the output transformer and HPF/LPF section. Does anyone know what type of cap this is and if they're prone to failure in the manner described? Picture attached.
 

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