AKG C414EB sucking my phantom power down to 10 volts !

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Kid Squid

Well-known member
Joined
Jun 4, 2004
Messages
1,011
Location
Port Toilet, South Wales
Hello Chaps,

Just wondering has anyone come across the same problem with their 414eb's.

Plug in 414, pulls whole phantom voltage down to 10 v.d.c.

opened up 414, split the two boards, found that polarising voltage card is at fault, But nothing seems burnt, or untoward.

But I haven't got no drawing / schematic.

been over to Jakob's site, and the 414 schematic he has, must be for the comb type 414.

so, any help would be much appreciated.

Best Regards

Steve :thumb:
 
Steve,

Is the mic functioning okay? Where were you measuring the phantom "sag"? The two 6k8's are effectively in parallel, so the drop is across 3k4 (which would be a 38V drop meaning ~11mA draw) What is your non-loaded phantom?

Mark
 
Hiya Mark,

with regard to your question about the mic - No, all of a sudden, the mic stopped functioning, and immediatly, all other phantom powered mics stopped working.

I started unplugging mics on the quick, and found, that the 414 was at fault. - all other mics came back to life !

Checked phantom volts on Trident TSM ( Global phantom), 47 volts -ish,

Tried mic back on my home digi 001 (phantom volts 47.5 volts), and immediatly pulled phantom down to 10 volts.
the mic is working, but at a very low output - unusable level.

so, I split the two boards, powered up the polarising card, with phantom, and found that the problem seems to be connected to this board.
I will start to draw the circuit out this afternoon.

thanks for the help

Steve :thumb:
 
Mark you are a star !

Cheers Mate.

I started going through and drawing out the circuit, have found some little discrepancies between the drawings and the mic on the power side of things, like the three 68nf caps on your drawing, i got a 240pf, and two 5.1pf, also where the +60v get onto the capsule- there's a 330 ohm resistor, which is not on the mic. everything else seems to tie up.
i wonder if this is anything to do with serial numbers. this one's number is S497.
The BZX zener seems to hold the mic at 9.1v, but i cant understand why it should pull everything else Other Mics) down to this voltage. i've taken the 22nf cap and the 10uf tantalum, out of circuit and tested them, and they seem fine.-although this seems to be the most obvious explanation.
I'll change them in the morning and see whats the score with a fresh head, I bet it's the Tantalum !

Well, changed the tantalum, -no change !

here are some of my voltages,

top of zener = 9.1 v

point L = -58.2 v,
point M = +58.2v
point N = -20.75v

after the 4.7 meg resistors, voltages are :-

(-60v) - I've got -40v,
(-20v) - I've got - 10v,
(+60v) = I've got +37.5v

any ideas ?
I've also checked out the 15pf across the output of coil - seems o.k.
to be honest Mark, I'm not too sure about the operation around the coil/ transformer circuit.
how does the transformer 'step up' D.C voltages, I'm only used to A.C step Ups.( I know some trannies can handle a bit of DC current (A-25 etc),
but how does this work on DC Volts ?
Sorry to sound a little green regarding this,

Cheers again Mark

Best Regards,
Steve :thumb:
 
Are there any other electrolytics? When things stop working from one day to the next, it's often old 'lytics gone bad. Often enough they don't look bad but when you measure them capacitance is next to zero.
 
Hi Steve,

The capsule polarisation voltage is derived from a "DC to DC Converter" which is the BC183 and surrounding circuitry. This is effectively a high frequency oscillator driving the primary of the transformer U26a. The oscillator is a Hartley oscillator, and the tap on the primary supplies the feedback to keep the oscillator "alive". The frequency is determined by the 470pF cap and the inductance of the primary. The frequency will be well above the AF range- probably 50 to 100kHz?

The AC voltage developed at the secondary is rectified to form three separate rails- +60V, -60V and -20V. The smoothing capacitors are very small values because the frequency of the AC is high and the current demand is minimal.

Your voltages actually sound okay- your DMM will be loading down the +60/-60/-20V rails because you're looking at a voltage across a 4M7 resistor and the current supply here is very low- it normally looks into >1G or so of the capsule, and your DMM will have 10M or so input impedance. That explains the "droop" in your post-4M7 readings.

Have you removed the "12V phantom" jumper across the 8k2 resistor? If you haven't, and are giving it the full 48V, the 330R resistor before the 9V1 zener will be working quite hard!

Could you measure the voltage across the 8k2 resistor? Ohms law will tell you how much current is being consumed by the whole mic circuit. Measuring the voltage across the 330R resistor on the capsule head amp board will tell you how much current is being used by the head amp and is worth checking. The zener voltage sounds spot on, but check the voltage across the 330R resistor feeding the zener and the 100R resistor at the anode of the zener.

For the DC-DC converter:

current through 330R - current through 100R = current consumed by oscillator

Measuring these voltages will help figure out if there are any untoward levels of current flow in the circuit.

:thumb:

Mark
 
Here's the schems for extra discussion and perusal:

414eb_audio.jpg


414eb_dc.jpg


Let perusal commence...

Mark
 
[quote author="Samuel Groner"]This is normal, any condenser mic reduces the voltage as it draws some current.

Samuel[/quote]

naargh..that I would not take as normal...Not if the Phantompower circuit is proberly regulated & designed and not a 37 Volts drop..;-(

kind regards

Peter
 

Latest posts

Back
Top