R/E/P Community

I'm fairly certain the hole has been enlarged by the wrong size screwdriver. Any idea how to handle that?

I will definitely order a set of these screwdrivers and try that first though. Thank you!

This is a typical happening with AKG’s CK1 capsules.

It’s not a big deal.
Get yourself a really fitting screwdriver (they are sold in sets for a few bucks in every hardware store), something like in the attached picture.
Very nice is the bits-set in the 2nd picture.
Doesn’t need to be very special.

The screwhead is 1.3 mm diameter, 1.5 mm driver does NOT fit.
The slot is on the narrow side.
Typically a 1.2 mm driver does fit and work.

Turn all screws counterclockwise until they fit, don’t overdo it.
Remember they are small, not much force needed.
Don’t force anything here!

The screws make the ground connection to the diaphragm, so don’t use any securing lacquer, as it might isolate the screws.

Important:
Stop to wiggle around the head, the diaphragm will come loose and the capsule becomes unusable.
There’s only a small brass ring between the housing and the capsule edge where the diaphragm is glued on, and the glue is old now!
For the same reason, do not open it.

Hi Klaus,
Here's an image of the screw I'm referring to:

https://imgur.com/a/0BmE3lm


And a video that (hopefully) shows that the capsule is moving ever so slightly.

https://youtu.be/RNG-RrF50h0


My other 451s don't have this issue.

Thanks!

Chris

Close-up picture, please, to idenitify which screw you are talking about.
Thanks.

Hi All,
I bought an AKG C451 EB that has a slightly loose screw on the CK1 capsule. I don't have any screwdrivers that fit. Does anyone know what size this screw is?

Thanks!

Chris

Neumann specs the mic's heater to be operable from 5.8VDC upwards.
But I agree, neither is the polarization voltage in its sweet spot at that low voltage nor is the noise floor.

I aim for somewhere between 6.1VDC and 6.2VDC, measured at the mic, not the power supply.

Thanks. I will try to do this. The microphone works, but 5.8V is way too low IMHO.

Ruud,
That series of NU67 is indeed a pain to adjust.
The correct value increment of that last dropping resistor is impossible to hit just right, as values in increments smaller than whole ohms are rare.

I would choose a 3Ω resistor in position R8, and a shut resistor after it to ground. Find the right value of the shunt by first installing a trimmer, then replace the trimmer with a fixed 1/2W resistor of equivalent value.

P.S.: An audio pad was only used in NU67 exported to the U.S., as per request form the North American importer.

In a description of the (reissue) U67 power supply (NU67V) I read:
"Redesigned to meet stringent safety standards and accommodate higher filament current of newer premium-grade tubes".
I tried to find the differences in filament current for various EF86 tubes, but all specifications say: "filament current 200 mA".
I am asking this, because an older (recapped) NU67U power supply doesn't put out more than 5.8 V with a 'newer' EF86.
Unfortunately there is nothing to adjust inside the power supply: series resistors are 22 ohm - Zener 15 V - 18 ohm - 18 ohm - 5.1 ohm.
I suppose the best way to get a higher filament voltage, is to lower the value of the 5.1 ohm resistor? (to 2.7 ohm?)

(The power supply doesn't have the internal 'pad'.)

I snipped both of my u67's S2 jumpers and have never missed the roll off.