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[Prive]

Hi, i'm Marcelo from Argentina, so any contact MTG for repair is not the correct answer  we are in another planet almost.
The thing is, it started working ok (i don't remember if the sound was clear and open at first) but just in the cardioid position, when i switched to another position the sound was muted. At first i comeback to cardioid and the sound comeback too. Later it changed to actual situation, you have some seconds of sound when you plug it but 3 or 4 seconds after the sound fades and don't comeback.
I've read lots of info here, i have downloaded the schematics, i'm an electronic tech and used to work with audio but NO EXPERIENCE WITH HI END MICS, sorry i'm not screaming, just to be clear. I understand the schems but i'd like some advice before to play with the circuit.
I have the mic disassembled, the pattern selector section seems ok to me, without the capsule i have 51.7V at one wire and -51.5V at the other.
AAAAHHH !!! the entire body of the mic came microphonic, i mean, i can hear the movements i make when the selector is at cardioid and omni, at 8 figure it's muted at all.

I hope somebody experienced can give me a hand to fix this, thanks a lot in advance!!!

Marcelo, LU4ENP

[klaus]

Hello Marcello,
Argentina is not that remote. It's always touted as the "Italy of South America", with modest climate and a long history of musical culture.

Troubleshooting malfunctioning condenser mics should always proceed along the following flow chart:

Can the trouble event be triggered or worsened by exciting the capsule (breathing onto the mic at close proximity, as if to fog a mirror)?

Yes > capsule problems (in your case, either contamination or irreversible deterioration of the PVC membrane material) = De-contaminate or replace capsule

No > amplifier problems.  In your case, the first order of business is confirmation that the correct supply voltage is reaching the mic, which you seem to confirm with the measurements of  (+) and (-)  ca. 51 VDC.

After that, the next most likely candidate of failure would be the FET. There should be a distinct voltage differential between the FET's Drain and Source. This should be a few DC volts at least. And there should be no DC voltage measurable on the Gate.

If > insufficient voltage differential = FET  is defective. Replace with same type.

I would start there, and let's see what you find.

Best of luck,



P.S.: The company is named after and located in Gefell, not Geffel.

[Kai]

It's even possible to feed audio into the amplifier to check if it's working.

Use a small series capacitor of about 200pF (exact value doesn't matter).
Remove the capsule and feed in where the capsule was connected to the FET gate.

This way you can check function of the whole amplifier by one test.
If it doesn't deliver sound it's easy to trace the signal to the point where it's lost.

Regards
Kai

[Prive]

Thanks a lot for the superfast answers!!!!
Gefell jajajaja writing fast is a problem when you write unknown words and maybe my english isn't too good
Ok, now i have some important tests to do this weekend.
I'll do it and post the results as soon as i can!!!!

Thanks for the info!!!

[Prive]

I've test and found this:

1-reading before the 4m7 i have 71v and -71v, if i connect the capsule this tension ramains so, no shortcircuit at the capsule, a little more happy now.

2-i've done the audio insertion test, attention here!!! If i insert audio at the 10uf bypass cap i hear a strong but treble sound, i see it ok, maybe the cap value i use is small; if i insert audio at pin 9 of the IS i have almost nothing but a 11.29DCV (without the fet).

3-i've replaced 3 caps i see and don't like but at dmm it seems ok, the output two and other 1uf wich are together at pcb.

4-of course i've test it with the pcb into the case for the switchs contacts but i find that pushing the pcb a little more mi finger the sound changes, i don't know.

I hope the IS is not damaged but to me is the first suspect. Anybody knows what it is? Replacements availables? Internal schem?

I'll try to replace the fet wit another one at least to try.

Thanks for your experienced help and guidance.

[klaus]

It would help if you first do the FET voltage test I recommended, before continuing this discussion.

[Prive]

I have installed a 2N5852, the one that i find here in my bunch. I have audio but when i installed the capsule, nothing.
Again, when i select cardioid or omni the transformer gets microphony but at 8 figure the audio is totally muted.

At fets pins:
G=0V
D=10.79V
S=2.23V

Thanks!!!

[klaus]

From all you report, it seems that audio is being amplified from the FET source on downstream, but high impedance (capsule) signal seems elusive.

Have you tried inserting a sine wave signal directly into the FET, bypassing the lead from the capsule? If that also yields full output, then it's either the polarization voltage or a short in the capsule itself that prevents audio.

[Prive]

Well!!!

Good news!! Not the best but i have no more time for this today.
The mic have sound!!!
At the 8 figure just the back capsule is working.
The sound is good, the hi-pass works ok but no -10db pad, maybe i have to reassemble the body and check the highness of the screws.
At first, when the mic arrived the capsule working was the front one so i have to re check the switch.
One strange thing is: at 8 figure i have sound but if i speak strong at the mic the sound dissapear for some seconds and come back, maybe the Vgsoff of the fet i'm using, i could measure a buch to have the right one.
Any advice will be welcome!!!

Thanks and saludos from Argentina!!!

[klaus]

...but if i speak strong at the mic the sound dissapear for some seconds and come back, maybe the Vgsoff of the fet i'm using...

No. Capsule contamination or deterioration is more likely the problem. Did you do the breath test I recommended earlier?

[Kai]

i have downloaded the schematics,

Could you please post the schematic or the download link?
I can't find it.

Regards
Kai

[Prive]

It's here at the forum, i can't remember where but search for MV692, that's the model of the preamp section.

[John Willett]

Hi, i'm Marcelo from Argentina, so any contact MTG for repair is not the correct answer  we are in another planet almost.
The thing is, it started working ok (i don't remember if the sound was clear and open at first) but just in the cardioid position, when i switched to another position the sound was muted. At first i comeback to cardioid and the sound comeback too. Later it changed to actual situation, you have some seconds of sound when you plug it but 3 or 4 seconds after the sound fades and don't comeback.
I've read lots of info here, i have downloaded the schematics, i'm an electronic tech and used to work with audio but NO EXPERIENCE WITH HI END MICS, sorry i'm not screaming, just to be clear. I understand the schems but i'd like some advice before to play with the circuit.
I have the mic disassembled, the pattern selector section seems ok to me, without the capsule i have 51.7V at one wire and -51.5V at the other.
AAAAHHH !!! the entire body of the mic came microphonic, i mean, i can hear the movements i make when the selector is at cardioid and omni, at 8 figure it's muted at all.

I hope somebody experienced can give me a hand to fix this, thanks a lot in advance!!!

Marcelo, LU4ENP

I know this is not the answer you want - but I would *only* have a Gefell mic. repaired by Gefell at the factory in Germany.

They will not send the M7 capsule out to anyone - so a capsule can only be replaced at the factory.

Also - they will put the mic. back in the anechoic chamber after repair and test it so that it will be within spec. like when it was new.

I would not do anything else.

I am the Gefell distributor for the UK and all Gefell mics go straight back to the factory - it is the only way to be sure.

John Willett
Sound-Link ProAudio Ltd.

[klaus]

Thanks, John, for giving a recommendation about a vendor, AND disclosing your connection to that vendor, Microtech Gefell. That way, readers can judge for themselves whether they want to trust your advice or may regard it as biased.

In this case, I happen to agree with you: MG is the first address for repair, parts, or service of their mics.

But I also need to caution owners of  Gefell mics with PVC M7 capsules: my experience and that of others in the business of condenser microphone service  has been that the current PVC capsules MG manufactures are hardly recognizable as the same thing they produced up to 7-8 years ago: PVC material, and core manufacturing/assembly personnel have changed, with the audible consequence that current M7 PVC capsules sound markedly slimmer in the bass and honkier in the mids, as consequence of the considerable curtailment of low frequencies.

This change is perceived by some as unpleasant, with the consequence that an increasing number of MG owners who need PVC capsule repair use Siegfried Thiersch to have their capsules re-diaphragmed. While by no means perfect, or indentical in sound to older MG/Gefell M7, at least the low end comes back after his reskins.

[John Willett]

Well, I am always open and honest when I post.

I used to work for Sennheiser/Neumann and I know most of the European microphone manufacturers personally, so I am always open and fair when I post.

And - declaring an interest also denotes knowledge.

Bias really only comes into it when you start heavily recommending stuff - and I don't actually do that.