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Author Topic: Neumann M269c heater trimmer voltage question  (Read 176 times)

Mark Lemaire

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Neumann M269c heater trimmer voltage question
« on: July 04, 2019, 01:16:18 pm »

I have owned a pair of M269Cís since the early 90s. I occasionally check the heater trimmer voltage to see if it needs adjustment, and the voltages have been remarkably stable over the years. I am wondering, though, if they need adjusting at this point, and if so how to go about doing it with the very difficult to reach screws that adjust the voltage.

Number 129 measured 4.05 to 4.07 V at various times from the year 2000 until today. I have notes from Klaus (from back in 2000) Stating that the voltage should be 4 V plus or -.05 V. I figure I will leave that one as it is?

Number 233 has measured from 3.91 V to 3.95 v at various times from the year 2000 until now. However, this year, after a 12 hour burn in where it hovered around 3.95 V, itís settled at around 3.53 V this morning. This, like #129, is outside the suggested range, but still pretty close.

Considering how expensive replacing the tube on these microphones is, Iím asking here if I should adjust the voltage even though itís a PIA? While adjustment is easy to do on my U67 pairís power supply (The adjustment screw is easily accessible), the N52T power supply that goes with the M269ís is another matter. If I have my information right, the screws that I need to adjust are shown in the accompanying photographs (in the attached, the screwhead is covered by a resistor, making it even harder to access). Looks like I can only post one photo per post, so the other will be in a minute...

It looks like, in order to turn the screw, I have to remove the circuitboard from the power supply assembly, or I could try to turn the screw from the side, using a pair of needle nose pliers, without removing the board.

Your suggestions? Should I just leave it alone?

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Mark Lemaire

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Re: Neumann M269c heater trimmer voltage question
« Reply #1 on: July 04, 2019, 01:18:54 pm »

here is the other N52T. No resistor covering the screw head!
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klaus

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Re: Neumann M269c heater trimmer voltage question
« Reply #2 on: July 04, 2019, 04:10:59 pm »

The screw in the photo is a circuit mounting screw, not a voltage adjustment screw, The N52t adjusts the heater voltage by paralleling a selectable resistor with R6. Use Ohm's law and the calculation for resistors in parallel to find the correct value.

More importantly, before you do that: You need to have an experienced electronics tech analyze WHY your voltage dropped so much. This could be a failed component in the supply or on the mic amp. The latter is easily tested by connecting the other M269 and its cable to the supply and observing the heater voltage. If it is still low, the supply needs to be analyzed and repaired.
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Klaus Heyne
German Masterworksģ
www.GermanMasterworks.com
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