AKG C28 no low end

[tonzauber]

One of my C28s just decided to produce no more low frequencies.
I of course checked the "easy stuff" already, so capsule, cable, PSU, tube.
My suspect would be the 1uf output capacitor.

As space is extremely limited in this microphone, what type would you suggest to replace it with?

 

[Khron]

Electrolytics, when they age / dry out, they do indeed reduce in capacity (and increase in ESR).

What's the diameter & length of the original one?

 

[tonzauber]

Diameter 5mm,
length 11,5mm

just a picture in comparison with my preferred c12 output cap

 

[Khron]

I've had a browse through the bigger electronics retailers, and i'm afraid the smallest ones i've found are 6,3x13mm (1uF / 150-160v)

Electrolytics, obviously - film ones are even bigger...

 

[tonzauber]

Well I found one on mouser... (maybe the same) Vishay /Sprague TE1500-E3, which is 6,6x13,6mm.
It might be a tight fit, but it could fit....

But shouldn't electrolytics have become smaller over time?

 

[Khron]

Radials have indeed, but axials are quite the "dying breed", so-to-speak. Apart from some "specialist" application (read: vintage equipment restoration / recreation etc), i doubt there's ANY technical reason to use them nowadays.

The longer the lead spacing, the larger the parasitic inductance, not to mention the board space taken up...

 

[Matt Nolan]

The output cap value on the C28 schematic is 3.2uF, and that is also the value in my C28, IIRC. I can't check as I have loaned it out to a friend. It would be worth looking in your other C28(s) to see what values they have.

I'd also look at the grid resistor value and check that the resistor isn't compromised by grime.

 

[tonzauber]

I just checked my other 6072 tube C28, and it also has the same 1 uf output capacitor...
I am pretty sure the C28-C would have a different one as to the nuvistor and different xformer.

 

[moltenwok]

Yep my 3 C28B are all 1uf -damn that's a tiny cap !

 

[ln76d]

You could fit Wima MKS 1uF radial capacitor.

Also check 20uF cathode capacitor, i believe it is tantalum electrolytic and it may be shorted.

 

[Khron]

That fit is a big "maybe", since you want at least a 160V rated cap in there. 100V might/could work, but it's a bit "too close for comfort", if you ask me.

 

[ln76d]

You don't need "160V rated cap in there". After 100k plate resistor voltage is something about 60V.
Wima MKS 1uF/100V is sufficient.
Schematic for C28a had output cap 3,2uF  rated 70V!