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Author Topic: Geffel UM70S needs repair.  (Read 9747 times)

Prive

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Geffel UM70S needs repair.
« on: July 12, 2012, 11:53:49 PM »

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
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klaus

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Re: Geffel UM70S needs repair.
« Reply #1 on: July 13, 2012, 12:43:23 AM »

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.

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Klaus Heyne
German Masterworks®
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Kai

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Re: Geffel UM70S needs repair.
« Reply #2 on: July 13, 2012, 03:35:59 AM »

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
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Prive

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Re: Geffel UM70S needs repair.
« Reply #3 on: July 13, 2012, 07:35:02 PM »

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!!!
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Prive

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Re: Geffel UM70S needs repair.
« Reply #4 on: July 15, 2012, 07:05:54 PM »

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.
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klaus

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Re: Geffel UM70S needs repair.
« Reply #5 on: July 15, 2012, 07:54:12 PM »

It would help if you first do the FET voltage test I recommended, before continuing this discussion.
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Klaus Heyne
German Masterworks®
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Prive

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Re: Geffel UM70S needs repair.
« Reply #6 on: July 15, 2012, 08:35:07 PM »

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!!!
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klaus

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Re: Geffel UM70S needs repair.
« Reply #7 on: July 15, 2012, 10:40:04 PM »

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.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Prive

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Re: Geffel UM70S needs repair.
« Reply #8 on: July 15, 2012, 10:58:07 PM »

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!!!
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klaus

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Re: Geffel UM70S needs repair.
« Reply #9 on: July 16, 2012, 03:18:49 AM »

...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?
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Kai

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Re: Geffel UM70S needs repair.
« Reply #10 on: July 16, 2012, 05:56:56 AM »

i have downloaded the schematics,
Could you please post the schematic or the download link?
I can't find it.

Regards
Kai
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Prive

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Re: Geffel UM70S needs repair.
« Reply #11 on: July 17, 2012, 04:23:07 PM »

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

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Re: Gefell UM70S needs repair.
« Reply #12 on: July 19, 2012, 08:31:46 AM »

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

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Re: Geffel UM70S needs repair.
« Reply #13 on: July 19, 2012, 02:18:15 PM »

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.

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German Masterworks®
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John Willett

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Re: Geffel UM70S needs repair.
« Reply #14 on: July 19, 2012, 04:51:15 PM »

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.

John Willett

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Re: Geffel UM70S needs repair.
« Reply #15 on: July 20, 2012, 07:40:36 AM »

Gefell do vigorously deny this, of course, and say that nothing has changed at all in the manufacture of the M7 capsule and it's the same as it has always been.

Kai

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Re: Geffel UM70S needs repair.
« Reply #16 on: July 20, 2012, 10:20:44 AM »

... the current PVC capsules MG manufactures are hardly recognizable as the same thing they produced up to 7-8 years ago...
Is this still the case, did you have the chance to compare M7s currently manufatured with older ones?

Might be that MG reacted on this lately?

Something like lack of bass should be easy to measure if you had both types for comparison.

You could even send me files recorded from two capsules placed side by side (or one after the other, exactly placed at the same point), at two different distances 30cm and 100cm from the source.
No need for an unechoic chamber, and any broadband sound will do as source.
I can do an objective comparison (measurment) here and post the result.

Regards
Kai
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klaus

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Re: Geffel UM70S needs repair.
« Reply #17 on: July 20, 2012, 11:30:17 AM »

A few years back I was making a prototype for a new microphone to be issued as flagship model for Microtech Gefell. The bottleneck in the development was the then-current M7 PVC capsule, which, compared to the PVC capsules made until about 10 years ago, had changed dramatically, I thought. 

After I complained about the, in my opinion, considerable degradation of the current capsules' frequency balance and low end, I provided running production samples to MG, about seven years ago. Objective measurements were then made by MG in its anechoic chamber by the head of capsule development, for the purpose of proving or disproving my subjective experience.

While an average loss was measured between 2 and 4dB @50Hz in the capsules I sent, the drop-off was deemed insignificant and within specs by the MG employee, in an email to me. That effectively stopped any further development of the microphone, as I was not willing to put my name on a product with what I regarded as an unsatisfactory sounding capsule.

I have no indication that either more of the original, now obsolete, PVC material which MG used for decades, has been found since the measurements were done, or that the original specialist in charge of M7 PVC manufacture has been un-retired, and his extensive skills revived by the current staff.

I am always eager to alert the public if things have changed, and welcome any information you may have that would point to an improvement of MG's PVC capsules.

Postscript: This morning I received an email from MG threatening me with a lawsuit unless I immediately retract my opinion. (I will start a new thread on the subject of manufacturers' intimidation of free speech, and the corrosive effect this has on the exchange of information and improvement of microphones.)

I feel honored that my opinions are taken that seriously.
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David Satz

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Re: Geffel UM70S needs repair.
« Reply #18 on: July 21, 2012, 12:35:49 PM »

> While an average loss was measured between 2 and 4dB @50Hz in the capsules I sent, the drop-off was deemed insignificant and within specs by the MG employee, in an email to me.

Klaus, just to clarify: Do you mean that the 50 Hz sensitivity of these capsules was 2 to 4 dB less than that of some reference M 7 capsule(s) that you'd agreed upon? Or do you mean that these capsules were 2 to 4 dB less sensitive at 50 Hz than they were at 1 kHz?

The former might be news, but the latter would not.

--best regards
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Oliver Archut

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Re: Geffel UM70S needs repair.
« Reply #19 on: July 21, 2012, 12:41:00 PM »

Quote
but I would *only* have a Gefell mic. repaired by Gefell at the factory in Germany

Hey John,
That is wishful thinking here in the US and any other countries outside the EU. And, even for the EU your statement might be right for current productions, but for everything before MTG (the "peoples owned" times) there are several techs that will do a better job than Gefell can do today. (That point has been covered numerous times in the past in the old forum.)

Regarding Gefell M7 PVC.
Gefell knows about those problems, Klaus points out from several others techs including myself. You are right that they deny it. I bought several new Gefell capsules and they re-worked several original Neumann capsules for me, and at first everything seems fine but at one point about 5 years ago, the quality changed. All my inquiries were returned by Gefell that nothing had changed, but after some digging and talking with former/retired Gefell workers, the picture was quite different. It appears that some personal changes happened around the time the issue started.
All my dealing with Gefell did not come to a point that was acceptable, I had been stranded with 5 capsules that had been delivered and died within 8 months, and all showed the same problems, the other ones that did not rapture were not useable at all. After their unwillingness to even take my point seriously, I wrote off the costs and moved on. In the last three years my company bought over 100 PVC capsules from Mr. Thiersch, and only one had some problems, Mr. Thiersch exchanged it with no questions asked.

Regarding Gefell's statement that the M7 PVC has not changed: The original PVC substrate developed and manufactured by IG Farben (before WWII) called "Igelit" was outlawed in Western Germany in 1958, because of its toxicity. In the Eastern German VEB Bitterfeld company (formally IG Farben) the same "Igelit" material was produced to about 1990, but with the German reunion, it was discontinued. Gefell changed diaphragm material after they ran out, [and since use] an alternative PVC material (source: Gefell). If they deny that today, that is probably due to marketing.

Best regards,

Oliver
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We are so advanced, that we can develop technology that can determine how much damage the earth has taken from the development of that technology.

klaus

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Re: Geffel UM70S needs repair.
« Reply #20 on: July 21, 2012, 06:43:50 PM »


Klaus, just to clarify: Do you mean that the 50 Hz sensitivity of these capsules was 2 to 4 dB less than that of some reference M 7 capsule(s) that you'd agreed upon? Or do you mean that these capsules were 2 to 4 dB less sensitive at 50 Hz than they were at 1 kHz?

The former might be news, but the latter would not.

--best regards

The former. And in direct comparison with MG capsules from previous generations that I own.

There may be more than just bass response discrepancies that account for the distinctly different sound produced by the two MG capsule periods. I have recently seen 3-D imaging of moving capsules that, for the first time, seem to correlate timbre to diaphragm material, whereas even similar frequency responses of differing diaphragm materials produce audibly different oscillations (and, anecdotally, audible variations.)
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Klaus Heyne
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