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Author Topic: AKG C28 LF Loss Problem, or: How to Measure Capacitors in Circuit  (Read 960 times)

RuudNL

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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.)
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uwe ret

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Re: AKG C28 LF loss problem
« Reply #1 on: February 12, 2020, 02:39:09 pm »

Check C5 (nom. 1µF). Hope that's it. Otherwise I fear it might be the output transformer, or the Nuvistor.
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klaus

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Re: AKG C28 LF loss problem
« Reply #2 on: February 12, 2020, 02:40:14 pm »

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.
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Klaus Heyne
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RuudNL

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Re: AKG C28 LF loss problem
« Reply #3 on: February 12, 2020, 03:42:05 pm »

Thank you for your reaction(s).
The strange thing is that the capsule performs excellent on a C451 body. (See the red line in the graph in the attachment.)
Also the microphone body doesn't show very much LF roll-off. (-2 dB @ 63 Hz)
But for some strange reason the combination of the two gives problems. (Green line in the graph.)
When the capsule is exchanged with another C28 capsule, the result is the same: poor LF response...

I will have a further look at the (not original) power supply.
Because if the +HT would have a significant output impedance, there is a chance that this would cause LF feedback from the anode to the polarisation voltage. (The reactance of the 5 nF (c1/C3) and 10 nF (C2) capacitors will get higher at lower frequencies, causing LF feedback.)

In the meantime I have ordered a couple of NOS nuvistors. Let's hope that this is the solution!
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RuudNL

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Re: AKG C28 LF loss problem
« Reply #4 on: February 14, 2020, 02:51:44 pm »

In the meantime I have done some more tests with the AKG C28.

- Although the capsule capacity (~18 pF) might be a bit low, the capsule performs normal on an AKG C451 body, with a full low end.
- When I inject a signal to the capsule input (the center connection) of the C28 body, through a 15 pF capacitor, I measure a frequency response that is only 1 dB down at 40 Hz.

But...

When I checked the +HT of the (not official) power supply, I measured a dangerously high voltage of 167 Volts!
This means that the polarisation voltage will be in the order of 83.5 Volts, instead of the desired 60 Volts.
So: the first thing I am going to do is to reduce the +HT voltage to +120 Volts.
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klaus

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Re: AKG C28 LF loss problem
« Reply #5 on: February 14, 2020, 03:06:20 pm »

You indeed forgot to mention that small little detail: bad voltage from power supply!
So much for a rigorous troubleshooting protocol.

Best of luck, and please keep us posted.
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Klaus Heyne
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afterlifestudios

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Re: AKG C28 LF loss problem
« Reply #6 on: February 14, 2020, 05:38:11 pm »


When I checked the +HT of the (not official) power supply, I measured a dangerously high voltage of 167 Volts!
This means that the polarisation voltage will be in the order of 83.5 Volts, instead of the desired 60 Volts.
So: the first thing I am going to do is to reduce the +HT voltage to +120 Volts.

Measured under load (microphone connected), right? 
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RuudNL

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Re: AKG C28 LF loss problem
« Reply #7 on: February 15, 2020, 05:25:11 am »

Yes, of course I measured the voltage under load!

Problem is solved now. Because I didn't trust some components, I replaced R1, R2, R3 and C2 (10nF).
After this, during a quick test, my voice sounded much more natural than before, no more 'telephone sound'.
A measurement showed a tremendous improvement: level at 40 Hz had increased by 10 dB...
It is still not clear what has caused this problem, because the 10 nF capacitor that I removed measured the correct value...
The only thing to replace now is the nuvistor, because the microphone seems noisier than it should be.
But that should be no problem, because a couple of NOS nuvistors are already ordered.
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klaus

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Re: AKG C28 LF loss problem
« Reply #8 on: February 15, 2020, 07:43:53 am »

Evaluating the health of capacitors under working conditions has never been easy (at least no for me):
Simulating work i.e. what happens when a stressor is applied, does not compute under static capacitance test conditions (with a meter).

I'd gladly take some lessons from someone who knows how to improve capacitor testing under real-world conditions, preferably while they are still installed in the circuit.
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Klaus Heyne
German Masterworks®
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RuudNL

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Re: AKG C28 LF loss problem
« Reply #9 on: February 16, 2020, 04:41:59 am »

About the capacity of the CK28 capsule:
According to BBC Report No. L-048 ("A.K.G. Electrostatic microphones type C26 and C28") dated December 1961: "the type CK28 capsule has a capacitance of about 25 pF".
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Kai

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Re: AKG C28 LF loss problem
« Reply #10 on: February 16, 2020, 08:25:16 pm »

Evaluating the health of capacitors under working conditions has never been easy (at least no for me):
Simulating work i.e. what happens when a stressor is applied, does not compute under static capacitance test conditions (with a meter).

I'd gladly take some lessons from someone who knows how to improve capacitor testing under real-world conditions, preferably while they are still installed in the circuit.
A universal way testing capacitors in-circuit does not exist.
There are too many different purposes a cap can serve inside a circuit.

And there are too many parameters that might be wrong in a broken or half-broken cap:
- loss of capacitance
- loss of isolation, current leakage, shortcut
- loss of voltage capability, temporary or permanent (kind of similar to above, but different symptomatic)
- increase of series resistance
- and the easiest to identify: physical leakage of acid from electrolytics.

Audio coupling caps are relatively easy to examine in an active circuit:
measuring the AC voltage across the cap (best done with an oscilloscope) should result in very low to zero values, except for very low frequencies.
DC leakage shows up by residual DC on the output side, e.g. at the input of the Xformer of a mic it should simply be 0mV.
In general, if a DC voltage inside a circuit is off, caps are usually under the first suspects.

Similar applies to power supply filter caps, you should be able to observe the usual rounded ripple wave form, else they are dried out.
When into this, it's time to check if the frequency of the ripple is consistent with the type of rectifier used, often one of the rectifier diodes is broken which shows up here.

Oscilloscope (non grounded like the Fluke handhelds) + signal generator are the best tools to trace and measure signals in an audio circuit, and with a bit of thinking it's mostly possible to find the primary suspects.


Having a vacuum desolderer makes the next step easier, external component testing:
https://smile.amazon.de/Regelbare-digitale-Entlötstation-ZD-915-ESD/dp/B00T6FYP1A/ref=sr_1_5?__mk_de_DE=ÅMŎÕÑ&keywords=Entlötstation&qid=1581899463&sr=8-5

For component testing I acquired this, and it's astonishingly good to work with at that price:
https://smile.amazon.de/gp/product/B07C5LC486/ref=ppx_yo_dt_b_asin_title_o03_s00ie=UTF8&psc=1


For testing of components that need higher voltage I have a very, very old Bruel&Kjaer 2423 Mega-Ohm-Meter that can apply up to 100V (up to 1000V with external feed) to a cap and exactly shows the leakage/isolation value up to 10 Tera-Ohm (!), even on foil caps, capsules, cables, everything.
If a foil-cap passes this test, it's clearly not broken.


If I want to go one step further, I can measure the harmonic distortion of a cap down to -155dB or 0,00000002%.
There is no cap (except some special WIMA types) that does not show distortions in this test, -120dB to -140dB is the common range.
The measurement is at the edge of physics and time consuming, not an every day task.


Final thought:
Of course it's often easier just to replace a suspect part, but if you work on historic originals that's not always an option.
Having the capability to thoroughly test a component is mandatory in such cases I think.
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klaus

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Re: AKG C28 LF loss problem
« Reply #11 on: February 16, 2020, 09:48:20 pm »

Brilliant summary.
Thank you!
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

boz6906

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Re: AKG C28 LF Loss Problem, or: How to Measure Capacitors in Circuit
« Reply #12 on: February 24, 2020, 04:07:54 pm »

What about the Dick Smith ESR meters?
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Kai

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Re: AKG C28 LF Loss Problem, or: How to Measure Capacitors in Circuit
« Reply #13 on: February 25, 2020, 05:18:54 am »

What about the Dick Smith ESR meters?
If you have it, use it within its limitations.

If not, get the one above.
Despite it's ridiculously low price it seems to measure quite exact.
I did some simple tests with resistors in series and parallel to caps to verify this.

And it tests everything: capacitors, resistors, transistors, FETs, TRIACs, diodes, even zener.
Just the protection is not very good, I shot one when I forgot to discharge a cap before measuring.
But hey, this was just like a 25€ expensive fuse, lesson learned  ;D
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