AKG C414 (early version) issue

[wave]

Hi all,
I'm having an issue with an AKG C414. It's the 1st generation version with the screw on Canon connector.
I have isolated the issue to the Dc board. When fed the normal 48V phantom, the board loads the supply down and only outputs about half the voltage it should. I'm getting +38, -38 and -11. I have replaced the caps and the transistor on the board and now I'm looking to replace the diodes.
The schematic for the early version does not have any component values but since it seems similar to an EB, I'm assuming that the diodes are BAY 45. I can't find much info about those and I'm wondering: can I use a standard 1N4148 in those positions?
Also, is it possible to bower the board from a bench supply for testing?

Thanks!
-Dave

 

[moamps]

I'm having an issue with an AKG C414.

Could you describe it in detail?

I'm getting +38, -38 and -11.

Where did you measured this values? Directly on the diodes or after resistors?

I'm wondering: can I use a standard 1N4148 in those positions?

Yes.

Also, is it possible to bower the board from a bench supply for testing?

Yes. The simplest way is by applying 48V via 3k3 resistor to R3.
Have  you checked C3 and zener C1?

 

[PRR]

> fed the normal 48V phantom, the board loads the supply down and only outputs about half the voltage it should. I'm getting +38, -38 and -11.

Phantom is 48V *through 3.4K of resistance*. If the mike draws power it MUST be less than 48V on the terminals. 38V suggests the mike sucks 3mA which is entirely reasonable.

What is the actual problem with the microphone? Dead? Splatts? Hissssy? Picks-up Peruvian radio?

 

[wave]

The original symptoms were the mic sounded very hollow and the capsule would collapse with a plosive.
The DC board was putting off the low values and I have swapped out C1, C3, C4, C5, C6 and all the diodes.
I also removed and tested all the other parts. I now am getting the correct voltages (installed and uninstalled) but the capsule is still collapsing.

EDIT: I measured the voltages at the capsule initially then at the outputs of the DC board. There was no change until I swapped components.

 

[abbey road d enfer]

I'm getting +38, -38 and -11. I have replaced the caps and the transistor on the board and now I'm looking to replace the diodes.

I suppose you measured the points that are marked as +60,-60 and -20V. These are actually connected via 4.7Meg resistors. Unless you have an unusual voltmeter, yours has probably the typical 10Megohm input impedance, so in fact the actual voltages are 1.5 times the readings.
Capsule collapse has to probable causes:
Too high bias voltage or loose diaphragm.
Your description seems to point out the latter.
Just to make sure, are you positive it's not the typical problem of diaphragm contamination, where humidity and spit short the bias voltage?

 

[abbey road d enfer]

If the mike draws power it MUST be less than 48V on the terminals. 38V suggests the mike sucks 3mA which is entirely reasonable.

Bias voltage is from a DC/DC converter that's powered by a 9V zener-regulated voltage. The voltages are measured at the DC/DC converter outputs after an RC filter.

 

[chops]

I had e 414eb with similar symptoms. Do you have another capsule to try?  Is it the original brass ring or nylon ring?
Against all odds I  VERY gently cleaned one of my brass ring ck12's and it's been problem free for years...

 

[Gus]

As abbey road d enfer posted there is a zener diode to regulate the DC to DC converter power supply.
Did you check the zener voltage?

If is is higher then spec the DC to DC voltages will be higher than what is was designed for and the higher voltages might have degraded the original parts in the output side.

To work on a older more valuable microphone a person or shop should buy or build the measuring tools needed.
For instance you could buy high value 1% or better resistors say 100 meg(this can be made a higher value to a point of where you will not get a high enough voltage for a good reading the DMM) and 1meg  or and make a voltage divider that will not load down the supply as much.
Look up the input resistance of the DMM  being used calculate the combined resistance value of the "bottom" of the voltage divider(DMM and 1meg) then calculate the real voltage division and measure the capsule voltage add in R5 and R4 and figure out the voltage.

Have you measured the phantom supply you are using?
http://www.shure.com/americas/support/find-an-answer/how-to-test-phantom-power-voltage-and-current

 

[wave]

Hey guys,
Thanks for all the help and advice. To answer some of the questions asked...

Gus: i have a high impedance meter that I built for working on mics specifically. It does the trick! I do appreciate your advice on the divider method and doing the math!

Chops: The capsule is an original brass CK-12 which was reskinned just before the work on the DC board. It has been tested in 2 other mics and is working and sounding great!

Abbey: The 9.1V zener and the 10uF cap were the culprits.  After replacing, I checked the voltages with my hiz meter and they are correct. i also checked with a standard 10M input meter (Fluke) and got the lower reading voltages which multiplied by 1.5 gave the correct values. Thanks for that! There was a point where I had accidentally put in a 15V zener instead of a 9.1 and that was causing the collapse post initial component swap...

I haven't rebuilt the mic amp section aside from replacing the 33uF output cap. I'm still getting a gain issue. The mic needs about 5dB more gain from the preamp so I think I'm going to rebuild the amp section now.

Does anyone have a recommended replacement part for the BC413C transistor?

Thanks everyone! The GDIY community has once again proven how great you all are!

-Dave

 

[mutterd]

Attached is a copy of the schematic with the component values...

I have 3 of these mics, 2 with original Brass Ck12's and one Teflon CK12 - I use them every session and love them - they are fantastic all around mics. In my opinion, far superior to all the models that followed.

Biggest issue I have is sourcing decent replacement gaskets for the mount tightening mechanism to keep them in place...

In all of them I have had to replace C3, the 10uF tantalum across the zener, i hear this is a common thing. The symptom was either increased signal to noise or intermittent crackles and pops...

T3 should be easy enough to verify, with the capsule removed and signal applied across a like value cap (or even just leave the capsule in place for now, it may corrupt critical measurements but for this test should not matter) if you have approximately the same signal at the top of R6/base of T3 and R7/Emitter of T3 then the follower is working...

I hate to say it, but if your voltages are correct and your still having issues it sounds like a capsule issue, either way - please let us know how it all pans out.

Timothy

 

[Whoops]

I haven't rebuilt the mic amp section aside from replacing the 33uF output cap. I'm still getting a gain issue. The mic needs about 5dB more gain from the preamp so I think I'm going to rebuild the amp section now.

Why are you re-building the microphone?
Shouldn't you just fix the mic?

Are you just shotgunning all the components?

 

[wave]

I want to bring this back up as it's still giving me issues. I have replaced everything in the amp section except for the transformer and the mic is still about 8-10dB quiet.
I have checked with mulitple known CK-12 capsules and I'm reading the correct polarization voltages with my electrometer.
Anyone have any more advice as to what could be the issue?

Also, I'm measuring 39R of resistance in the primary winding (Blue/Black) and 32R in the secondary (red, white) with the blue lead disconnected from the board. Seems like there should be more difference there, right?

Thanks,
Dave

 

[abbey road d enfer]

Also, I'm measuring 39R of resistance in the primary winding (Blue/Black) and 32R in the secondary (red, white) with the blue lead disconnected from the board. Seems like there should be more difference there, right?

It looks very much like a partially shorted primary. You need to test it for inductance.

 

[wave]

It looks very much like a partially shorted primary. You need to test it for inductance.

Any easy way to test that without an LCR meter?

 

[abbey road d enfer]

Any easy way to test that without an LCR meter?

The method has been described here a number of times. You need a sinewave gen (your DAW does it well), some resistors or even a pot in series with the inductor to be measured, a meter across the resistor (it can also be your DAW).
Typically, the impedance of the meter must be >10x the resistor, so if using your DAW, the resistor should be < than 1k.
TBC

 

[abbey road d enfer]

Then you draw a graph of teh voltage across the resistor vs. frequency.
You should arrive at something similar to the attached.

 

[abbey road d enfer]

Then choose a suitable point on the graph, e.g. 100Hz, where the attenuation is 36dB, or a ratio of 66:1. That means the impedance of the inductor is 66 times 1kohm
Using Z=L.2pi.F => L=66 000/628 or about 100H
You're not actually interesting in knowing the exact inductance value, for several reasons:
A) this is usually a non published parameter
B) the actual inductance varies with level and frequency
C) you just want to know if the winding is not shorted; if it is, the calculation will give a much lower value than expected, probably less than 10H, and the graph will look different.

 

[wave]

Then choose a suitable point on the graph, e.g. 100Hz, where the attenuation is 36dB, or a ratio of 66:1. That means the impedance of the inductor is 66 times 1kohm
Using Z=L.2pi.F => L=66 000/628 or about 100H
You're not actually interesting in knowing the exact inductance value, for several reasons:
A) this is usually a non published parameter
B) the actual inductance varies with level and frequency
C) you just want to know if the winding is not shorted; if it is, the calculation will give a much lower value than expected, probably less than 10H, and the graph will look different.

Abbey, I hooked up the transformer primary to an LCR meter and it reads at 3.61H. Seems like it's too low?
I'm trying to get the winding data... but it's a long shot

-Dave

 

[abbey road d enfer]

Abbey, I hooked up the transformer primary to an LCR meter and it reads at 3.61H. Seems like it's too low?

It doesn't seem unreasonable, since it is meant to be driven by the low impedance of the emitter-follower.
You need to pass signal into the primary and measure the output. I would think the ratio to be about 2:1, so you should observe a 6 dB loss. In order to better assess the xfmr's performance, you should measure it in reverse, so as to observe 6dB gain. You need to make sure you don't use excessive level when doing it. About 100mV (-8dBu) should be fine.