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Klaus Heyne's Mic Lab / Re: U67 Voltages And Other Questions
« Last post by uwe ret on August 22, 2018, 10:57:08 pm »
Before changing either of the two series resistors (6.8k∩ and 8.2k∩) please check the value of the shunt resistor, as together with the tube's condition is most responsible for the accurate high voltage value. According to my calculations I predict it is close to 205k∩ instead of the desired 68k∩. I further predict that you must have the early version of NU67, because with the later version the high voltage would not be able to rise to 239V, even with the shunt resistor open. When replacing R4 (the shunt resistor, KH) make sure it has a powerr dissipation rating of 1W or 2W!
BTW all calculations use simple Ohm's and Kirchhoff's laws.
Klaus Heyne's Mic Lab / Re: N52a question
« Last post by klaus on August 22, 2018, 10:26:42 pm »
Its located exactly midpoint between the two big silver transistors, sitting upright to be adjusted from the sides rather than head on.
Klaus Heyne's Mic Lab / Re: U67 Voltages And Other Questions
« Last post by klaus on August 22, 2018, 10:23:25 pm »
With that high B+ go to 10k Ohms on R3 or reduce shunt resistor (May be a 150k or higher value, depending on original calibration) by the next major value increment. For example from 150k to 100k.

As you mentioned, bend up the S2 wire against the tube socket and stabilize to hold in place  with a dab of E6000 or silicon glue.
Thanks for all the thought and effort you put into this.  One quick question: Is your reference vintage u67 strapped for 50ohm?  And is that your preference?

Klaus Heyne's Mic Lab / Re: U67 Voltages And Other Questions
« Last post by afterlifestudios on August 22, 2018, 12:54:18 am »
Thank you, gentlemen. Both of my nu67 PSU's have the trim pots for heater voltage and are now both outputting 6.20VDC measure at the mic.

On PSU number 2, I measured R3 resistor out of circuit and it was 8.05k instead of 8.2 so I popped in a fresh 8.2k resistor  (measured 8.18k beforehand) with no change to B+ voltage.  (Still 229VDC)

Any math wizards out there feel like guiding me on how to calculate what R3 should be to bring B+ from 229 down to 210VDC?

Regarding S2, when you snip at one of the right angles, do you slightly bend the resulting horizontal length of wire to prevent it from making contact with it's severed nub?

And my main PSU must be from the European market, because it has no pad network.  But on my spare PSU I see R11, R12 etc labelled on the board, but the resistors have been removed.

Thanks in advance,
Klaus Heyne's Mic Lab / Re: N52a question
« Last post by J.J. Blair on August 21, 2018, 11:56:15 pm »
Wow, Uwe!  I've never seen that replacement schematic!  Thank you so much.  I only have the N52a schematic, which is of course useless without your schematic.  So, I guess I just have to locate R5, and that's my heater voltage pot. 
Klaus Heyne's Mic Lab / Re: N52a question
« Last post by uwe ret on August 21, 2018, 04:06:27 pm »
The original N52a did uses two banks of Stabilyt cells for filament regulation, followed by a dual (adjustable) series resistor to provide the desired 4 V filament supply. Stabilyt cell have a life expectancy of up to 2 years and sure have expired decades ago. I assume that section has been modified? When Stabilyt cells were discontinued in the mid 1970s Neumann briefly offered a modification kit SK 33d with Zener diode regulation.  Modern voltage regulator ICs allow for much more effective circuit substitution with precise regulation and tube saving soft filament supply ramp-up. The later version N52t used solid state (transistor) circuits which used selected resistor values to compensate for component tolerances and adjust for accurate filament and plate/bias supply voltages.
Klaus Heyne's Mic Lab / Re: U67 Voltages And Other Questions
« Last post by uwe ret on August 21, 2018, 03:38:06 pm »
Only the first generation (mid 1960s to early 1970s) featured a trim pot for adjusting the filament voltage. Later versions rely on pre-regulation to 15 V with a Zener diode and 3 series resistors to drop the excess 8.7 V when the filament of 200 mA is drawn. If desired, resistors of marginally different values may be substituted to get close to the desired 6.2 V to 6.35 V at the tube socket. The exact high voltage  measured at the microphone connector (careful, avoid risk of rather unhealthy shock!) does depend on the condition of the tube, and should be within 5% of the nominal 210 V. It can be brought close by tinkering with either the series and/or shunt resistor values, or both. Observe the power ratings for these resistors!
Klaus Heyne's Mic Lab / N52a question
« Last post by J.J. Blair on August 21, 2018, 03:20:52 pm »
Had some random clicking in one of my M49s, so I replaced the tube, and everything is better after a couple days of burn in.  However, my heater voltage is 4.3 VDC .  I don't know if the extra .1 is going to be a problem or not, but I'm wondering if there's a way to adjust the heather voltage in those, without having to solder a resistor into the circuit. 
Klaus Heyne's Mic Lab / Re: u67 voltages and other questions
« Last post by klaus on August 21, 2018, 12:39:40 pm »
Readjust both power supply heater voltages to measure ca. 6.2VDC at the tube. There are trimpots for this, accessible from the underside (printed side) of the power supply circuit board.

Reduce B+* on both supplies to 210VDC 2 volts, by replacing and increasing one or both of the bridging resistors R1 and R3 by one value up, or reduce dropping resistor (at least a 1/2W rating!) by the next value down.

RE S2: Neumann made this bass choke (and a similar one in most M49) removable, at users' discretion, but I find the associated phase shift and sizable low end chocking in the U67 objectionable to high fidelity, and routinely snip the wire at one of its 90 angles (but keep it installed, just in case you want to resolder it).

P.S.: And make sure that the audio-pad network, standard for U.S. delivery U67, is removed.

* B+ is the incoming high voltage that is split up inside the mic to provide polarization voltage for the capsule, and, depending on the health of the tube, will determine a plate voltage of ca. 75VDC, usually a bit more.
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