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 on: February 12, 2020, 02:40:14 pm 
Started by RuudNL - Last post by klaus
A defective capsule would have been my first guess from your descriptions, and 18pF somewhat points to it, despite your calculations. I do remember that nominal capacitance for CK28 is around 40pf, but will check my records to confirm.

 on: February 12, 2020, 02:39:09 pm 
Started by RuudNL - Last post by uwe ret
Check C5 (nom. 1F). Hope that's it. Otherwise I fear it might be the output transformer, or the Nuvistor.

 on: February 12, 2020, 02:37:03 pm 
Started by fffaustino - Last post by klaus
SK73 suffixes are correct: 'a' = Stabylit, 't' = transistor (solid state) replacement module

 on: February 12, 2020, 01:02:27 pm 
Started by RuudNL - Last post by RuudNL
I am having a problem with an AKG C28c microphone.
It sounds 'thin'.
I have checked the C28 capsule on a C451 body and in that situation the capsule sounds and measures good.
Next, I measured the frequency response of the C28 body, by injecting a signal through a 1000 pF capacitor to the nuvistor input.
This is what I get:

100 Hz : -1 dB
63 Hz : -2 dB
40 Hz : -3 dB
20 Hz : -6.5 dB (Everything referred to 1 KHz.)

So, maybe not spectacular, but certainly not too bad.
It seems the combination of capsule and C28 body causes a problem.
Now the grid resistor(s) and the resistor(s) for the polarisation voltage (2 x 200 M.ohm each) are inside a metal part between the impedance converter and the microphone capsule. And as far as I can see it isn't easy to get into that part...
Any suggestions? (I could be the 1000 pF capacitor inside that 'in between part'.)
The frequency response of the combination (C28 capsule and C28 body) starts falling below 1 KHz and is about 10 dB down at 40 Hz.
Also the self noise seems higher than usual, this also gives me the impression that the capsule coupling capacitor might be the problem.

EDIT: I just checked the 1000 pF capacitor and the value seems in the right order.
The capsule measures about 18 pF, which I think is a rather low value. (I would have expected something like 30 pF.)
But... 18 pF and 200 M.ohm (2 x 400 M.ohm in parallel) gives a low -3 dB point of ~44 Hz.)

 on: February 11, 2020, 11:34:19 pm 
Started by fffaustino - Last post by uwe ret
BTW, SK73a is the designation of the Stabilyt module in the N52a as indicated in the previously posted schematic diagram.

 on: February 10, 2020, 10:26:07 pm 
Started by fffaustino - Last post by klaus
Uwe, you are the best, and continue to be a treasure to the recording community.
Keep them coming! 

 on: February 10, 2020, 10:08:14 pm 
Started by fffaustino - Last post by uwe ret
Pulling a few strings led me to this schematic for the N52t and SK72t. I am stll trying to ferret the one for SK73a.

 on: February 08, 2020, 04:33:05 pm 
Started by Eddie Eagle - Last post by mbrebes
It can easily be seen from the picture that the lower spring has broken off at both ends.  Accutronics was bought by another company.  I think the best available springs now are made by MOD.

 on: February 08, 2020, 07:16:34 am 
Started by klaus - Last post by Derek Reese Music
I did a quick vocal with my DIY pop filter and without and I could not hear an audible difference:-)
Very happy.

 on: February 07, 2020, 06:34:46 pm 
Started by everettmoran - Last post by klaus
A knurled, threaded ring nut tightens the connector to the p.s.'s wall. from the inside. It is usually sealed with lacquer to prevent loosening.

The proper schematic will probably be uploaded by Uwe. If not, I'll dig one up eventually. Make sure to specify whether your NKM still has Stabilyt cells or the solid-state regulation fo the heater voltage.

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