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Author Topic: M49 lower output?  (Read 5226 times)

Koen

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M49 lower output?
« on: November 18, 2016, 02:56:38 am »

Hello again,
another troubleshooting question for you guys: I have a M49b that has a considerable lower output then my earlier M49.
I swapped capsules and power supplies to no avail, the earlier M49 has at least 5-8db more output... (And yes, compared to the 'b' in fixed cardioid position...).
Could it be the valve that is falling apart? Or some cap? Or worse, the transformer? (They're both strapped for 200ohms)
The only quirk of the M49b is that it puts out quite some noise when powering off (a long decreasing pppfffffffffff- like something deflating), the other doesn't do that.
Any ideas?
Thanks,

Koen.
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klaus

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Re: M49 lower output?
« Reply #1 on: November 18, 2016, 03:07:00 pm »

To start the troubleshooting, you will have to make a couple of voltage tests, right on the amplifier board:

Anode (plate) voltage should be in the > 40VDC range, cathode voltage around 1.6 to 1.8 VDC, heater 3.8 - 4.0VDC.
Also measure B+ at the 100K resistor; it should be around 120VDC ±2VDC.

If these voltages are right, the polarized electrolytic cathode cap (22mdf/6V) is suspect, and should be replaced.

That you experience noise at power-down points me to the supply, not the mic. Is it an original Neumann NN48 or N52?
But you already excluded the supply and the capsule as the culprit of the low output.

I also do not think a bad tube is the issue, as you do not report high noise with the low output. Likewise, the symptoms you describe do not point to the transformer.
It could certainly be a bad capacitor somewhere in the circuit, but the only true test is substitution, one after another.

If the above tips don't help, you will need to surrender the mic to an experienced tube mic technician to get a definitive answer.
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Klaus Heyne
German Masterworks®
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Koen

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Re: M49 lower output?
« Reply #2 on: November 18, 2016, 04:08:03 pm »

Many thanks! I get to work tomorrow and report back!
(ps. I tried a good working original N52t from my M269 and a good working aftermarket psu from H&E)
Best regards,
Koen.
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Koen

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Re: M49 lower output?
« Reply #3 on: November 18, 2016, 04:37:27 pm »

Concerning the 22mfd cap: Do you mean C9 on the schematic?
(I only found a m49c schematic)
Thanks
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klaus

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Re: M49 lower output?
« Reply #4 on: November 18, 2016, 05:06:12 pm »

C9 is correct.
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Klaus Heyne
German Masterworks®
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Koen

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Re: M49 lower output?
« Reply #5 on: November 18, 2016, 05:18:05 pm »

To be clear: this microphone is a M49b - is there a C9 cap at all? I don't find it on the board...
Sorry for my insistance here...
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Koen

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Re: M49 lower output?
« Reply #6 on: November 18, 2016, 05:55:34 pm »

Heater = 3,95V / Anode 33,65V /B+ 119,1V
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klaus

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Re: M49 lower output?
« Reply #7 on: November 18, 2016, 08:34:20 pm »

Anode voltage is way too low for an M49c's circuit.
Measure cathode voltage, and if that is ok, lift one leg of the coupling capacitor. If the anode voltage goes up to proper range, replace the capacitor. If not, replace the tube (my suspicion, despite your claim that noise is not an issue) and see where that leads.
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Klaus Heyne
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usattler

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Re: M49 lower output?
« Reply #8 on: November 18, 2016, 08:46:33 pm »

Biasing the M49b version is the major difference to the later M49c and does NOT use a cathode resistor bypass capacitor, = no C9!. The voltages reported are indicative of a slightly higher than normal current through the AC701 tube. Aside from regulating the current the -c type biasing reduces the microphone's self noise, and you may contemplate to have this modification (negative DC-feedback) performed by a qualified microphone technician. If you want to stay with the -b version, check the values of the two resistors between filament and ground, R6 (200kΩ) and R7 (300kΩ). The voltage at their junction is fixed at 2.4V, providing a fixed bias of -1.6V, but does not allow for stabilizing the plate current with the tube's inevitable aging.
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Uwe Sattler
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klaus

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Re: M49 lower output?
« Reply #9 on: November 18, 2016, 08:54:29 pm »

Uwe,
It's been my experience that 90% of all still existing M(2)49b have the 'c' bias, and the fixed-bias M(2)49b are now a small minority:

1. Most were 'b' labeled models but with a 'c' stamp from the factory: 'M(2)49b serial number plates were embossed with a 'c' after the 'b' or after the serial number.

2. All M(2)49b sent in for service to Neumann/Berlin after 1961 were converted to 'c'.

3. Many other 'b' were modified to 'c' by third parties, even when the serial number plate does not contain the embossed 'c'.

I'll let the mic owner confirm whether his 'b has the 'c' biasing and whether C9 is present, and my suggestions for troubleshooting the cathode section apply.
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Klaus Heyne
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Koen

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Re: M49 lower output?
« Reply #10 on: November 19, 2016, 04:07:25 am »

Hello again, there is no C9 present in this particular microphone...
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Koen

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Re: M49 lower output?
« Reply #11 on: November 19, 2016, 09:20:57 am »

To Uwe:
do you mean R2 (200K) and R7 (300K)
(because R6 = 100Mohm in the regular schematic I have here)

To make sure we're on the same page here I attached some pictures of the microphones board.
Thanks again for your input.
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Koen

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Re: M49 lower output?
« Reply #12 on: November 19, 2016, 09:21:37 am »

Another angle...
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Koen

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Re: M49 lower output?
« Reply #13 on: November 19, 2016, 09:22:22 am »

And a third angle...
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Koen

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Re: M49 lower output?
« Reply #14 on: November 19, 2016, 12:06:17 pm »

More measurements and findings:
- Cathode to ground is now 1,5 VDC
- R7 was broken, replaced by new 300K resistor
- Anode voltage is even a little less now, around 30 VDC (measured to ground)
- I tried 2 different ac701's, all read more or less the same (or would I be that unlucky that I acquired two broken spares? They were boxed, leads unbent and unsoldered)
- Noisefloor is indeed rather high, but same level with the three tubes.

Puzzled...
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klaus

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Re: M49 lower output?
« Reply #15 on: November 19, 2016, 12:20:45 pm »

Replace the brown ceramic disk capacitor C1 (10.000pf/0.01mfd) viewable here:
https://groupdiy.com/index.php?topic=44667.40

If that does not work, your work might be done, and a real brain surgeon may need to take over.
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Klaus Heyne
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Koen

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Re: M49 lower output?
« Reply #16 on: November 19, 2016, 01:10:04 pm »

Will do.
Replace it with the same type ceramic disc or another type?
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klaus

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Re: M49 lower output?
« Reply #17 on: November 19, 2016, 01:31:57 pm »

Any non-polarized type that fits is fine. It's a low voltage application, so any type with a 50VDC rating will do.
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Klaus Heyne
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usattler

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Re: M49 lower output?
« Reply #18 on: November 19, 2016, 09:24:29 pm »

Aside from the NDWR prototype, to the best of my knowledge, there were only three original versions of the Neumann model M49:
the original M49 (without any suffix), the M49b - like the initial release this version had its bias derived by a simple voltage divider (R6/R7) from the filament voltage, and the M49c with the negative DC feedback via the cathode resistor (R7) and the AC feedback canceled by the bridging capacitor (C9).
The photos do apparently depict an unmodified M49b.

Uwe Sattler,
Technical Director (retired) - Neumann|USA
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Uwe Sattler
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usattler

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Re: M49 lower output?
« Reply #19 on: November 19, 2016, 09:26:21 pm »

Schematic for the first generation M49 attached.
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Uwe Sattler
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usattler

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Re: M49 lower output?
« Reply #20 on: November 19, 2016, 09:27:35 pm »

Schematic for the M49b and M49b3 attached.
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Uwe Sattler
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usattler

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Re: M49 lower output?
« Reply #21 on: November 19, 2016, 09:28:35 pm »

Schematic for the M49c attached.
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Uwe Sattler
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klaus

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Re: M49 lower output?
« Reply #22 on: November 20, 2016, 01:25:58 am »

Excellent resource.
Thanks, Uwe!
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Klaus Heyne
German Masterworks®
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Koen

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Re: M49 lower output?
« Reply #23 on: November 20, 2016, 07:31:10 am »

Wow, yes, many thanks!

Last point of confusion:
Klaus, the original schematics show that C1 calls for 1000pF (1nF) instead of the 10000pF (10nF) you mentioned earlier in the thread.
Is there a particular reason to alter this value to the one you suggested?
Kindest regards,

Koen.
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usattler

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Re: M49 lower output?
« Reply #24 on: November 20, 2016, 11:14:56 am »

 Capacitor C1 bridges both halves of the 80pF capsules, and the difference in value between 1000pF and 10,000pF (1nF and 10nF) in this circuit does not make any difference for the function. Since this is the most sensitive section of the circuit though, it is important for this capacitor to be of the highest quality available, and Styroflex types have been used in the originals.
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Uwe Sattler
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klaus

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Re: M49 lower output?
« Reply #25 on: November 20, 2016, 01:56:11 pm »

Last point of confusion:
Klaus, the original schematics show that C1 calls for 1000pF (1nF) instead of the 10000pF (10nF) you mentioned earlier in the thread.
Is there a particular reason to alter this value to the one you suggested?
The confusion came from you mixing up the designation of "C1" in the photo I referenced, with "C1" in the Neumann schematic. They are not the same part! C1 in the photo is labeled as the ceramic brown disk cap (10.000pf). I mentioned this part as possibly being suspect, and to replace it.

 "C1" of the schematic Uwe referred to serves an entirely different function. For space reasons, I would retain the 1000pf styrene cap currently installed.
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Koen

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Re: M49 lower output?
« Reply #26 on: November 20, 2016, 01:58:00 pm »

Hi all,
After replacing the defective 300Kohm resistor and the 0,01 mfd ceramic disc, Measured from anode (at the valve pin) to ground I still only have 29V, but measured at the actual turret on the board, I see it climbing up to 35V (???). Is this at all possible?
Also: a few db's more output. The noisefloor however is the same (like a distant highway kind of noise).

Concerning the actual M49'B' schematic: I carefully followed the leads on the microphone's board and think the ceramic disc cap is referred to as C8 on this schematic - maybe Klaus or Uwe could confirm this?

Overall, compared to my other M49, it the output is still considerably lower and I'm experiencing some bass rolloff as well.
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klaus

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Re: M49 lower output?
« Reply #27 on: November 20, 2016, 02:32:51 pm »

C8 is correctly identified as being the ceramic disc capacitor.
I suggest to send the mic to Neumann or a qualified tube mic specialist for repair;  tele-diagnosis beyond this point seems fruitless.
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Klaus Heyne
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Koen

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Re: M49 lower output?
« Reply #28 on: November 20, 2016, 02:48:49 pm »

 :'(
Ok, thanks for the help.
I'll inquire with Andreas Grosser and Neumann Berlin on what their turnaround is.
Kindest regards,

Koen.
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