wave

AKG C414 (early version) issue
« on: August 10, 2018, 01:02:59 PM »
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

Re: AKG C414 (early version) issue
« Reply #1 on: August 10, 2018, 05:17:56 PM »
I'm having an issue with an AKG C414.
Could you describe it in detail?
Quote
I'm getting +38, -38 and -11.
Where did you measured this values? Directly on the diodes or after resistors?
Quote
I'm wondering: can I use a standard 1N4148 in those positions?
Yes.
Quote
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

Re: AKG C414 (early version) issue
« Reply #2 on: August 10, 2018, 05:28:17 PM »
> 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

Re: AKG C414 (early version) issue
« Reply #3 on: August 10, 2018, 07:21:36 PM »
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

Re: AKG C414 (early version) issue
« Reply #4 on: August 10, 2018, 08:10:32 PM »
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?
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

abbey road d enfer

Re: AKG C414 (early version) issue
« Reply #5 on: August 10, 2018, 08:22:26 PM »
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.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

chops

Re: AKG C414 (early version) issue
« Reply #6 on: August 11, 2018, 11:09:38 AM »
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

Re: AKG C414 (early version) issue
« Reply #7 on: August 11, 2018, 11:41:34 AM »
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
« Last Edit: August 11, 2018, 11:50:57 AM by Gus »

wave

Re: AKG C414 (early version) issue
« Reply #8 on: August 11, 2018, 03:08:26 PM »
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


 

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