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Author Topic: M269c Schematic & Voltage Question  (Read 348 times)

rdraudio

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M269c Schematic & Voltage Question
« on: January 05, 2021, 12:45:44 pm »

This is more of a question of curiosity than anything else. 
I was just going over my M269c, making sure all was well, and was looking at the schematic and had 2 questions:

The St1 connector, as shown in the schematic shows C15 to pin 4, where the 4V should be.  In a continuity test, when looking at the markings on the mic, C15 seems to be Pin 5...Am I reading this correctly?  Are the markings in the mic truly the C markings on the schematic?  I attached a picture to show where I'm looking.  Also, as there is no marking between C13 and C15, but the pin with the green wire going to Pin 6...and the pin in the center behind,  with the Yellow wire going to St1 Pin 4. 

Also, I'm sure this has been discussed, but when checking voltages at the mic, where is the best place to test? 

Thanks, and I hope this isn't too elementary a question.
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klaus

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Re: M269c Schematic & Voltage Question
« Reply #1 on: January 05, 2021, 02:25:38 pm »

Questions too elementary? Yeah, sure, we all start the day with quantum-physical analysis of electron flow swirls at the backplate!

Addressing #1:
Neumann changed the pinouts of U67/M269 heater and B+  connections between connector and stand-off capacitors a few times back and forth, without changing the schematics. So always measure the corresponding pins/wires to establish which scheme was used in your mic.

#2: You can conveniently measure B+ and the polarization voltage (the high voltages) anywhere you like, from power supply output to anywhere in the mic. As there is little current flowing in the cable conductors there is barely any voltage drop from power supply to mic. I prefer measuring B+ voltage at the supply, because it's awfully cramped at the mic's measuring points you show in your photo: a momentary slip of the probe, and you short out things.

Heater voltage should aways be measured at the tube or close to it, due to its current consumption. Achieving correct heater voltage is especially critical in AC701-powered mics (see acceptable and nominal ranges below).

U67 B+ = 210VDC ±2V
U67 Heater = 6.3VDC + 0/-0.2 V
M269 B+ = 120VDC ±2V
M269 Heater = 4.0VDC ± 0.1V
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Klaus Heyne
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rdraudio

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Re: M269c Schematic & Voltage Question
« Reply #2 on: January 06, 2021, 11:17:33 am »

Klaus,
As always, thank you for your knowledge and reply. 

I measured the voltages, and have 4.001VDC filament voltage at the tube, 4.17VDC at the power supply (Neumann N52T). 
For the B+, I have 121VDC at the supply, but at the tube, with the red marking, 46.55V (which from the schematic seems correct...is it?) 
Also, when measuring the cathode, I get 1.505V.  I am guessing this is the bias voltage? 

Lastly, I am rewiring the custom-made box for the power supply, as it was very poorly done, using better wire, and making a new cable for the mic with Gotham cable.  The box that the supply is housed in has a step-up transformer. I measured the incoming AC voltage from the wall at 120VAC, and then where the transformer output is tapped, it's 201VAC. There is another winding that gives me 220VAC. Is there a reason that 200V would be chosen over 220V?

Lastly, I have two M269s.  One is a 'c' version, one is not.  I was told by a former mic repair specialist that will remain nameless that I couldn't interchange the supplies. The 269c has the N52T, and the original has a Neumann NN48 hu.  Is there any reason that one mic would be specific to one supply? Trying to learn every day :)

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klaus

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Re: M269c Schematic & Voltage Question
« Reply #3 on: January 07, 2021, 10:40:02 am »

Let me start with your last point, as it is most applicable to readers of your post. Except in an emergency, you should never switch or mix power supplies and mics. Each supply's voltages should be correctly calibrated to a specific mic and the supply designated and labeled for that specific mic.
The reason is straight forward: only the correct supply voltages guarantee a long, trouble-free life for the tube and capsule. That is even more relevant since obsolete tubes and capsules have shot up in price. Nothing extends the life of these precious components more than periodic supply voltage checks and adjustments.

The voltages you measured are all perfect. If you could get these DC supply voltages even with a 200VAC B+ output from the supply's step-up transformer, so be it. (N52 were originally designed for 220VAC).
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rdraudio

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Re: M269c Schematic & Voltage Question
« Reply #4 on: January 12, 2021, 09:17:31 am »

Thank you again Klaus. 
I have all of my tube mics assigned and labeled to their specific supply, and understand and agree with the reasoning. The only reason I was asking was that on this particular setup, I was told it could only be used with this supply, which I found a bit odd, but as the tech in question was not responsive to questions at that point, I never got a straight answer. I just can't figure why it would be specific.

Anyhow, a couple more questions on this, if I may.
I would like to replace the step up transformer, as I find it odd the way the current one is performing. I can get 221V from the 240V tap, and like I said, 198-201V from the 220V tap.  I would really prefer to get a correctly operating transformer, and send the supply the actual 220V it was designed to use.  I wired the transformer to the 240V tap, read the voltage to be 221, and then did some tests of the supply without the mic attached, and then with the mic.  The voltages with the M269c, supply running at 220V were as follows :
B+ 122.8 ( too high as per your suggestions ),
Filament 4.048
Bias 1.528

So this brings me to my first follow up question.  If I want to run this at 220V, I believe it's a resistor change to get the B+ to 120V.  Is this correct?  Which one and at what spec?

Second, when removing the board from the N52T chasis for inspection, I found it all original....and some caps that definitely need attention.  Should I re-cap the entire board?  Suggestions on what caps to use?  I am thinking that the one that is definitely open might want to be a higher V rating.  As always, I appreciate the help.

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klaus

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Re: M269c Schematic & Voltage Question
« Reply #5 on: January 13, 2021, 05:28:10 pm »

1. The voltages you measured could not be more perfect. Therefore:

2. I seen no need to replace the step-up transformer when the AC output voltage you measured is within 1 volt from nominal 220VAC, whatever tap got you there. That's probably closer than you will get starting fresh, considering that step-ups are rarely accurate in their claimed output voltage.

3. The N52t's B+ voltage is adjusted via a pair of shut resistors in positions R10 and R11. Play with these to get to 120VDC.

4. Many filter caps in N52a and N52t have deteriorated over the decades, particularly the ones filtering the heater supply - C5, 6, 7, 8.
To check, lift only one leg and measure capacitance. If close or under nominal, replace with a good Nichicon, Panasonic or similar polarized electrolytic of appropriate value and voltage rating. Do that one after the next, or spend an afternoon replacing them all preventatively. Watch the form factor when ordering replacements: there is limited space!

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rdraudio

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Re: M269c Schematic & Voltage Question
« Reply #6 on: January 13, 2021, 10:51:06 pm »

Klaus,
Thanks again.  I'm going to do a full recap...got some time on my hands.  Most of the axial I can find are Sprague, some matching the originals.  I assume those would be ok.  I do usually use Panasonic or Nichicon for most of my work.  And now, am I missing something....can not find
R10 / R11 on this board...I see them on the schematic I have...I don't like being dumb.

Mark
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klaus

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Re: M269c Schematic & Voltage Question
« Reply #7 on: Yesterday at 04:49:06 am »

Follow the (+) of C3 and look for a buffer resistor between it and the B+ output. Might be there under a different parts number. If there is none, install one.
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rdraudio

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Re: M269c Schematic & Voltage Question
« Reply #8 on: Yesterday at 10:40:52 am »

I followed C3 +, and there is none.  As there is no R5, R10, or R11, would I need to build this little network and install all 3?  Doesn't seem to be an accurate schematic for this rev of the board.
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klaus

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Re: M269c Schematic & Voltage Question
« Reply #9 on: Yesterday at 12:55:30 pm »

You don't need to design and install a whole new circuit section just to reduce B+ by a small amount.
Just add either a shunt after C3 or series resistor. As the voltage reduction in your case is small, I would opt for an inline resistor.

(Unless someone has another simple and better idea)
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Klaus Heyne
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