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Author Topic: M49 lower output?  (Read 5396 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®
www.GermanMasterworks.com

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®
www.GermanMasterworks.com

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
German Masterworks®
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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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