AKG C414 Problem

[colonel_sanders]

Hello!

I'm hoping some one can help point me in the right direction.  I left my C414 BULS plugged in over the weekend from a session on Friday, we tried to record vocals on Monday morning and the signal was very, very faint.  Tried a different pre and cable, no luck.

I opened her up to check for loose wires, bad solder joints but couldn't find anything.  However, I did notice that the PAD switch doesn't seem to affect the output level of the mic at all (0 dB is the same output as -20dB).

I took some measurements at the PAD switch and found (http://www.akg.com/mediendatenbank2/psfile/datei/51/c414B_ULS_412c87df992e6.pdf) at point 6 I'm getting around 16VDC, and at point 5 I'm getting 5.3V.  Significantly lower than the schematic!

I measure phantom power correctly at pins 2 + 3 of the XLR. 

Does anyone have any advice on how to proceed?  It would be appreciated very much!

Thanks.
Arie

 

[PRR]

.

 

[colonel_sanders]

Thank you PRR!!  I will report my findings tomorrow morning...

 

[colonel_sanders]

Hi PRR,

I have taken measurements.  Red indicates when both PCB's of the mic are connected, blue indicates only the PCB on which the XLR directly connects (and the one with the transformer mounted), with the other disconnected.

It seems the fault lies in the PCB controlling the PAD and Low cut?

Any advice on what to test/replace now is greatly appreciated!

Thanks so much!

 

[pucho812]

have you rested any of the  parts at all?

 

[colonel_sanders]

Hi Pucho,

I'm not sure I understand the term 'resting' parts?

Thanks,
Arie

 

[pucho812]

Hi Pucho,

I'm not sure I understand the term 'resting' parts?

Thanks,
Arie

I ment to say tested any parts for failure

 

[colonel_sanders]

Hi Pucho,

Thanks for your reply.  No I have not.  I assume I will have to take them out of the circuit to test them?  I will do a search on how to test transistors and capacitors.

Thanks,
Arie

 

[pucho812]

Hi Pucho,

Thanks for your reply.  No I have not.  I assume I will have to take them out of the circuit to test them?  I will do a search on how to test transistors and capacitors.

Thanks,
Arie

I am a big fan of first guess being a cap usually an eletrolytic, but then again that's if I do not have a schematic  and voltage measurements

 

[gyraf]

Could very well be a fault on the other PCB "print 1" in the pdf - loading down the 10-14V regulated by T105.

Try lifting only the 10-14V power line to print1 to isolate where fault is

C8, C11 electrolytics/tantalums hangs off this line, maybe replace those first just in case?

Try lifting one leg (where it connects to 10-14V line) of T4-5-9 -R5 in turn, to find where excessive drain takes place..

Jakob E.

 

[Rossi]

I had a similar problem with an older C414 EB P48. It turned out to be a defective transformer. Since the mic is powered through the trannsformer secondary tap, this also affects the voltages.

My advice therfore is, check the transformer resistances; you may find one of the windings to be open.

 

[pucho812]

transformers are less likely to fail but anything is possible.

 

[gyraf]

Ah yes - but I've actually seen a few of the AKG mic transformers failing (more than any other type)

Jakob E.

 

[Rossi]

It may have to do with the power sourcing through the transformer secondary tap. Most other mics (e.g. Neumann) use resistors.

 

[colonel_sanders]

Thank you guys so much for the replies!!

I am very busy in the studio but hope to have a moment this weekend to work on it...  I will post my findings.

Thanks again!! 

 

[colonel_sanders]

I had a second so I measured the resistance between the lugs of the transformers (hope I'm doing that correctly...).  They all measured around 40 ohms.  Next I will replace the electrolytic capacitor and then try Gryrafs idea!

Cheers guys!  8)

 

[pucho812]

I had a second so I measured the resistance between the lugs of the transformers (hope I'm doing that correctly...).  They all measured around 40 ohms.  Next I will replace the electrolytic capacitor and then try Gryrafs idea!

Cheers guys!  8)

which lugs did you measure?

 

[colonel_sanders]

There seems to be two lugs on either side of the transformers.  I measured between those two lugs...  I will take a picture, maybe it will explain it better.

I replaced the electrolytic and I think its working again!  Problem is I broke a few wires when assembling the chassis... :-[.  Will post soon!

 

[colonel_sanders]

If anyone has issues with this mic there are good reference pictures here: http://www.saturn-sound.com/images%20-%20microphones/akg%20c414b-uls%20-%20inside.jpg

I replaced the electrolytic capacitor.  The PAD seems to be functioning now, but the second PCB is still dragging down the voltages...  Next I will try Gyrafs ideas.

I am getting more level now, but the level is still very low.

Regarding the transformers, I am getting correct voltages with one PCB unplugged, therefore, I would think the transformers are OK?  I could of course be completely wrong  :-\.

 

[colonel_sanders]

Transformer measurements: