Schoeps M221A - PSU issue

[saint gillis]

Hi,
I ve got a pair of Schoeps M221, this version was made for french radio (ortf), with din plugs, during the mid-50s

It goes with a Neumann PSU modified a long time ago by a microphone specialist in Switzerland ("Playback Record Geneva", I ve made a research on the net and found nothing), it delivers 5V for the heater, the AC701 tube works with 4V, there are lots of infos on the net conserning AC701 delicacy.
  There was a kind of 50Hz hum, so I ve changed all the old caps of the PSU, and also I lowered the heater voltage down to 4V...
  The hum disappeared but a considerable level of noise appeared, like if it was ok with 5V, before going back to 5V I prefer ask here for some advice,
here's a schematic of the mics' body, look like there is an inversion of heater voltage and ground, but in the mic body I can confirm that the anode is linked with the heater side that is not connected to ground... (strange isn't it?)
Here is the schematic :

Here are also some schematics a friend of mine found on the net :
http://gypsyfarm.free.fr/echange/Schoeps_221B.pdf
http://gypsyfarm.free.fr/echange/Schoeps_N20B_Power_Supply.pdf

 

[moamps]

The heater voltage for AC701 should be between 3,8 and 4,2 V.
Did you measured the heater voltage with connected microphone?
 

 

[saint gillis]

You are right, I didn't mesure it with the connected mic

 

[mjrippe]

Yup, it's a common mistake.  Voltage may drop considerably under load.

 

[pasdesignal]

I presume you mean the cathode is connected to the heater side that is not grounded?
Also, there must be a missing conductor on your schematic from the top side of transformer primary to the resistor network?

It looks like the signal ground is lifted to +5v, and the grid is at 2.5.... if thats the correct way of describing it?

 

[saint gillis]

I presume you mean the cathode is connected to the heater side that is not grounded?
Also, there must be a missing conductor on your schematic from the top side of transformer primary to the resistor network?

It looks like the signal ground is lifted to +5v, and the grid is at 2.5.... if thats the correct way of describing it?

Yes of course the cathode sorry... You are right there should be a missing conductor, just like in the original schematic, the info comes from here indeed I think http://www.filmsoundsweden.se/backspegel/schoeps.html thus I didn't find it...

 

[saint gillis]

Yup, it's a common mistake.  Voltage may drop considerably under load.

Thank you now it works great!

 

[rmburrow]

The Schoeps M221 series can be run off the Neumann NKM or ELAM931 power supply...be CERTAIN the heater voltage is  3.8 to 4.0 volts UNDER LOAD with the mic connected.  Higher heater voltage will destroy a very expensive tube.  It's not difficult to clone a NKM supply..  Peter Drefahl in Germany makes a nice aftermarket supply (DNS221)..

 

[saint gillis]

Yes I ve made a kind of NKM calibrated a bit less than 4V for loaded heater, I've also added a kind of bypass like in guitar amps to let the tube warm a bit before applying anode voltage and before switching off..

 

[saint gillis]

  I got one of them which is a bit noisy.. hope it's not the tube.. I'd like to try to change the electrolitic cap and some resistors, does it sound like a good idea?
  If I put 187M resistors instead of 180M will it be ok?

 

[panman]

  I got one of them which is a bit noisy.. hope it's not the tube.. I'd like to try to change the electrolitic cap and some resistors, does it sound like a good idea?
  If I put 187M resistors instead of 180M will it be ok?

It would not make much of a difference, but why would you want to change it? I have never seen those resistors fail. If the mic is noisy, I would suspect the capsule first.  But I guess you have swapped the capsules?

 

[saint gillis]

Yes, it's not the capsule.
  I've already seen some very old high value resistors being more conductive...

 

[panman]

The first 400 or so(just my estimation) were Telefunken badged M221(no"A").  I have one M221(no"A") in my possession with sn. 318. Your pair seems to be M221A:s with "Klein Tuchels", so they seem to be otherwise regular production and not specially made for ortf, because to my knowledge the ones made for ortf had special "Sogie"-connectors and were marked M221F. Someone correct me if I am wrong!
Anyway, I never was able to find any info about the M221 and M221A schematics, just the M221B. It is very difficult to trace the schematics from my M221, because the body is shorter than  M221B and M155. M155 is only omni specially made for Telefunken in limited  amounts. Never found schematics for M155 either. All these mics(M221, M221B, M155) have differend schematics. The schematics you posted from your M221A:s seem to have flaws as was pointed out like connection missing from the trafo-primary and three resistor values(R3, R4, R5) do  not seem to match  with my M221. Also one 500k resistor is missing. Otherwise there seem to be enough similarities to assume tjhe schematics are not much differend from mine. I believe some changes might have been made when changing from M221 to M221A. Could you please check out those resistor values again if they really are like the schematics say!
Here is my adapted M221 schematics with parts list:

 

[saint gillis]

Hi!
Indeed it's not a soogie connector nor a tuchel, but a din !
And they are M221B..
  You're right looks like I forgot a connection at the primary,I will check it again

 

[panman]

Indeed it's not a soogie connector nor a tuchel, but a din !
And they are M221B..

Firstly, Klein Tuchel is the same as DIN. Secondly, if your mics are M221B:, then not too much matches to the M221B schematics.
There is too much confusion here now. Does it say M221B on the mics? What about the resistor values I pointed out?

 

[panman]

M221B schematics with parts list:

 

[saint gillis]

You're right M221B isn't mentionned anywhere.. there's only a ref n° which is : Nr 855 RTF
  Im sorry but I don't have the time to check the resistors values right now, but I'll do it as soon as I m not too busy

 

[panman]

No hurry! But it you check out the link you provided: http://www.filmsoundsweden.se/backspegel/schoeps.html, take a look at pics about the capsule from M221A, which is M934a and compare your capsules, you are able to tell, if your mics are M221B or M221A. Easy, because the threads are inwards on the B-capsule and outwards on the A-capsule.

 

[saint gillis]

No hurry! But it you check out the link you provided: http://www.filmsoundsweden.se/backspegel/schoeps.html, take a look at pics about the capsule from M221A, which is M934a and compare your capsules, you are able to tell, if your mics are M221B or M221A. Easy, because the threads are inwards on the B-capsule and outwards on the A-capsule.

  Hello, you have to turn the capsule clockwise to put it in the mic body, but there are no ferrite like in the M221B pics.

  I checked the circuit and I can tell that the schematic is correct, however  R4 is 100K and R3 is 150k, and filament voltage is 4V of course and not 5V...

  Seems weird but ... taking the AC potential difference between tubes's anode and High Voltage, why not?

 

[saint gillis]

And also the ceramic cap is 2n5