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Author Topic: Gefell UM70S broken amplifier module  (Read 7724 times)

vmsa

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Gefell UM70S broken amplifier module
« on: April 07, 2011, 03:03:10 pm »

Hi,
This is my first post, and I hope you can assist me with my trouble.
I'm an electronic engineer, so I have some electronics skils
A friend gift me an old Gefell UM70S microphone (non-modular, one body) because It don't work.
From the reading of your forum (old posts 2004-2008 ) I learned how to dissasemble it, and I found with a little board cover with a white rubber labeled "IS 29-25" broken. I know that it is an amplifier.
I removed gently the rubber in the cracked zone.
It is glass, and have one TR in it and some resistances impressed on the glass substrate.
I want to try repair it, then may be a much help yours opinions, or the real diagram for this part.
I hope you can help me,
Victor
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klaus

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Re: Gefell UM70S broken amplifier module
« Reply #1 on: April 07, 2011, 03:12:37 pm »

Hello Victor,
Because MG is such a unique small-scale manufacturer which still prides itself of attention to detail in service, and because there is no authorized service center outside of East Germany for the brand, I strongly recommend to ship the mic to its birthplace and have MG replace the broken module.

Let us know how it went!
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

vmsa

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Re: Gefell UM70S broken amplifier module
« Reply #2 on: April 07, 2011, 04:23:55 pm »

Hello Victor,
... I strongly recommend to ship the mic to its birthplace and have MG replace the broken module.

Oh! Dear Klaus, I can't stay even !!
Really, if I can't repair it, I will use it as decoration.
Then, your knowledge is very welcome.
Victor
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Sredna

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Re: Gefell UM70S broken amplifier module
« Reply #3 on: April 08, 2011, 08:22:52 am »

Hey Viktor!

PM me your address, I think a have a IS29-25 somewhere.

(Or sell this piece of decoration to me!  ;))

Best,

Anders
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Anders Grop

Jim Williams

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Re: Gefell UM70S broken amplifier module
« Reply #4 on: April 08, 2011, 11:01:27 am »

Those thin film resistors should be ok, I suspect it's the BJT. Since you have it stripped of rubber, I would put it on the bench and try to replace it. If it's a SOT23 part, there are plenty of parts you can replace it with. You will need to determine NPN/PNP, but that shouldn't be too hard to figure out as well as the pinout. Most likely it's a NPN transistor, a BC550 will work in a pinch. If you want a low noise part, the Japanese 2SC3329 or 2SC2545 are great choices.

After all, your an EE! You have nothing to lose in attempting to repair it yourself.
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vmsa

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Re: Gefell UM70S broken amplifier module
« Reply #5 on: April 10, 2011, 11:39:23 pm »

Hi,
I began with the repair work.
Firstly, I inspected the hybrid board and took out its circuit diagram. (inverse engineering!!) The MV691 diagram helped me very much.
Fortunately the breaking didn't touch any resistors, already is in the middle between pins 4 and 5. I glued it on a back side support and is just fine.

I found that the transistor was burned, it is a PNP one, and I replaced it with a 2N3906.
(Yes Jim, you are correct, I could do it!)

I found that the FET also was burned-out and I replaced it with a 2N3819. With these two transistors changed, the microphone came back to life.
But it does not work in cardioid and omni modes. I suspected that the problem is in the selector switch.
Then I proceeded to take apart the head, and I found a "strange and twisted" switch below the unit of the membranes. When I disarmed the top head part, I found the diaphragms in good condition, they are full of little points of dry splashings, but don't have dust. By now, I decided leave them in that way, because I don't know how to clean the membranes.
Next, I will be going to connect the capsule directly with cables to test if both membranes work, changing manually the voltage combinations. If so I will go to check the shaky switch.

By my forensic analysis I suspect that a "crazy monkey" serviceman trying to repair it, opened it and he bursted everything, maybe short-circuiting the FET input with the high voltage pins!!
And he forced with a tool the hybrid board up, to break it just in the middle. Could it have been done purposely?

Anyway, now it is closer to be operational again.
It is a costly microphone, and for me is a challenge.

By now, Anders I don't need your services, but thank you, maybe in the future.

More news soon...
Victor
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Sredna

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Re: Gefell UM70S broken amplifier module
« Reply #6 on: April 11, 2011, 09:02:03 am »

Glad to hear that you make progress.

I attatched the circuit for MV692, should be closer to the UM70s than MV691.

On the MV692/UM70 combo (dont know about UM70s) the pattern circuit works a little different than
most other condenser mics (Neumann, AKG etc.).
The backplane of the capsule is connected to ground through a 200MOhm resistor and the front membrane gets
about + 40 Volt and the back membrane gets - 40 Volt.

The switch inside the UM70 capsule has some very small parts, if a "crazy monkey" were in there, there's a large possibility that some parts are missing. MTG should still have them though...

Best,

Anders
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Anders Grop

Jim Williams

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Re: Gefell UM70S broken amplifier module
« Reply #7 on: April 11, 2011, 10:26:34 am »

A low noise PNP transistor like a 2SA1084 or 2SA1316 will be quite a bit lower in noise than that switching transistor. Even a BC560 is better.
Also, the load resistor is a bit low for a jfet input. R3 is 200 meg ohms. I would fudge a 1 gig ohm metal oxide resistor in there, you can get them from Digi-key for about 5 bucks. That will improve the low end.
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panman

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Re: Gefell UM70S broken amplifier module
« Reply #8 on: April 11, 2011, 12:35:32 pm »

The backplane of the capsule is connected to ground through a 200MOhm resistor and the front membrane gets
about + 40 Volt and the back membrane gets - 40 Volt.

To my knowledge normally the MV692 body gives +/-60V as polarisation voltage. 40V would not be enough for a M7 to work properly, the info from Gefell and also my experience is that it should be 50-80V, but usually the older ones used 60V and later 80V(e.g.CMV563,M582). If I remember right, the "S" was indicating the higher 80V in UM70S, but plain UM70 mic was MV692+UM70(capsule) with 60V polarisation.
The confusion about the 40V may be due to the use of SMS-series capsules with MV692. M94(M62, older version)needs 40V but cannot take 60V, so a 10M resistor is integrated in the capsule-head to adjust the voltage.
Regards,
           Esa   
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Esa Tervala

klaus

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Re: Gefell UM70S broken amplifier module
« Reply #9 on: April 11, 2011, 02:53:42 pm »

It should be mentioned that "older" or "newer" capsule polarisation voltages in these mics where selected by a simple logic, and not any changes in actual capsule design: higher polarization voltages improve the signal-to-noise ratio in a mic.

As the folks in Gefell knew: properly tensioned, you can zap a 10µ diaphragm with up to 80 volts, and it will not collapse. So they did just that, to improve the comparatively poor S/N of most of their models.
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Klaus Heyne
German Masterworks®
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Sredna

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Re: Gefell UM70S broken amplifier module
« Reply #10 on: April 11, 2011, 07:06:51 pm »

If I remember right, the "S" was indicating the higher 80V in UM70S, but plain UM70 mic was MV692+UM70(capsule) with 60V polarisation.
The confusion about the 40V may be due to the use of SMS-series capsules with MV692. M94(M62, older version)needs 40V but cannot take 60V, so a 10M resistor is integrated in the capsule-head to adjust the voltage.
Regards,
           Esa   

Esa, you're absolutely right! I made a mistake and measured after the 4,7 MOhm resistors, my DMM doesn't cope with such high impendance. Directly after the rectification I get about 57-58 Volts.

Sorry for the confusion.

BTW, does anyone got the schematic for UM70s?



Anders
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Anders Grop

vmsa

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Re: Gefell UM70S broken amplifier module
« Reply #11 on: April 24, 2011, 07:27:01 pm »

Hi!
Sorry for the delay in my reply, but I had a very busy weeks.
Thanks boys for your info.
I have tested the microphone without the switch and actually it has + / - 40V of bias voltage.
I found that one of the membranes is in "shortcircuit", because the bias voltage (and the +12V) go to practicaly 0V when I connected it. Then only one membrane is working.
Now the question is, can anything be done to remove this shortcircuit?
Sincerely,
Victor
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