Microphone broken as a result of phantom power shorting in patchbay

[ubxf]

Hello,
What parts are most likely damaged as the phantom power get shorted in a tt patchbay. The mic is completely silent now the only sound i can hear is a faint hiss that vanishes in a sec when i change pattern on the mic.

 

[JohnRoberts]

A common fault is the active input circuitry in solid state mic preamps, when fully charged DC blocking caps get shorted to ground driving the mic preamp circuitry hard to -50V until those caps discharge.

I am not familiar with reports of damage to mics.

JR

 

[ricardo]

What parts are most likely damaged as the phantom power get shorted in a tt patchbay. The mic is completely silent now the only sound i can hear is a faint hiss that vanishes in a sec when i change pattern on the mic.

You need to be more specific.

What is the mike.  Got a link to the schematic?  Is the preamp itself OK?

 

[ubxf]

The mic is a u87ai and the preamp an Avalon M5 survived with no damage

 

[pucho812]

check the capsule.  You may have the membrane sticking due to getting pushed in one direction due to shorting one side to ground when patching into the patchbay

 

[ricardo]

The mic is a u87ai

Got a link to the schematic?

 

[gyraf]

http://recordist.com/ampex/schematics/neumann/u87asch.jpg

The only thing I can see that could possibly be damaged from any external wrong-power-error is T2 (BC182B). But somehow i doubt this is the case - never seen this happen irl.

Jakob E.

 

[Bo Hansen]

ubxf,

If you have maximum bad luck, it can become disruption on secondary winding in the output transformer.
It happens sometimes on small transformers in small condenser microphones.

Have never had this error on a U87 transforms, but has had such errors on lots of small microphones and even some AKG C414-EB/P48.

--Bo

 

[ubxf]

Thank you guys , i see on the schematic that there is an input for calibration. Could i connect the mic with phantom power, then inject a small
test tone at the cal input and follow the signal to see where it stops ? Or what is the best way to see if the problem is T2 or the secondary of the transformer?
Many Thanks
francois

 

[ubxf]

In this microphone there is a little voltage booster that brings the 48V to 60V to polarize the capsule could that be damaged somehow ?

 

[Bo Hansen]

Francois,

If you have approx. 33 volt over the D107 Zener diod, the T2 is ok.
If you have signal after the C110 before the transformer, (between transformer leads red and black) you have fault in the transformer.

To measure voltage after the voltage step-up converter, you must do it with very high ohm input voltmeter as a FET or Tube voltmeter.

--Bo

 

[ubxf]

Thanks Bo

 

[ricardo]

Thank you guys , i see on the schematic that there is an input for calibration.

Got a link to this?

 

[ubxf]

here is the link that Jakob posted
http://recordist.com/ampex/schematics/neumann/u87asch.jpg

 

[Pip]

Phantom power on patch bay bad idea as a general rule. But not uncommon. The problems start when you break the patch with the mic hooked up you loose the balanced voltage  thus its potential changes and then you have it full on across both the secondary of the mics transformer and the electronics. It is not a short as such it is a loss of power on one leg and the other referencing ground. As others have stated start checking components from the pins 1 and 2 back.

http://en.wikipedia.org/wiki/Phantom_power

http://www.sounddevices.com/notes/general/phantom-power-and-unbalanced-outputs/

 

[ubxf]

I was able to mesure 32.5 V across the Zener diode , but i think i misunderstood how to test the transformer . I'm looking for a high impedance vtvm to test the 60 V

 

[joaquins]

I don't know where you are looking to measure 60V, but for measuring a high impedance source you can use the opposition method, where you arrange a controlled voltage supply and a current meter, you put the current meter between the measuring point and your well known voltage source, which is low impedance, look for zero current, then you measure the voltage of your low impedance voltage source, which may be a 70V PS and a 50k pot, which is low enough for a DMM to measure directly, that way you get rid of the insertion error by loading your high impedance source, since you are measuring at zero current.

For the transformer I would start measuring continuity, if there is some you are good, but it still could be shorted, if you have a L meter could be useful to know the value outside the circuit, in which case, if it's roughly close to expected no shorted windings, if you have shorted windings inductance will be really low, some mH or less, depending on the windings, someone who knows the transformer we are talking about may give you some better approximation... If you don't have an L meter you could shot some signal to it and see what happens at the other side, or measure inductance with a signal generator and a DMM with a series resistor.

JS

 

[Gus]

First set divide up the components to test

Have you tried the microphone with a known good cable and preamp?  Microphone , XLR cable then preamp? I read the thread but did not see that

Can you check the first preamp with another known good microphone , xlr cable, preamp? 

I saw "Avalon M5 survived with no damage" how did you test it?

 

[ubxf]

This problem happened a few years ago, the preamp has been in use since then without any obvious fault (it sounds ok to our ears). The microphone was tried multiple times with different cables and preamps and it doesn't work. Recently i just got a hold of the schematic and i'm trying to investigate what might have gone wrong. If i had another U87 i would have done a capsule swap as a quick test but i wasn't able to do that yet.

 

[Gus]

ubxf,

If you have maximum bad luck, it can become disruption on secondary winding in the output transformer.
It happens sometimes on small transformers in small condenser microphones.

Have never had this error on a U87 transforms, but has had such errors on lots of small microphones and even some AKG C414-EB/P48.

--Bo

I would check the  transformer winding like Bo posted you need to be mindful of the two 2.2K resistors when taking measurements  that supply power from pin 2 and 3 they are in parallel with the transformer secondary, R115 and I can't read the other number.  You might want to disconnect one lead at the primary and secondary of the transformer if you check it with an inductance meter or DMM set to ohms

The good thing is the regulator is a Zener shunt and it still measures 32VDC

What is the voltage at the drain of the JFET or do you have the small PCB board instead of a single JFET?

GUESSES in order of what might be wrong
The transformer secondary opened
The jfet got damaged from a voltage transient, note from secondary to primary is a step up. DC voltage readings from the source and drain will help tell if the JFET is damaged