Gefell conversion confusion

[chriss]

I have a schematic like PRR's

with D1 = 13V Zener
but R3 = 10k
for Q1 I used BC848

and I get 12,5V wich is exact what the MV692 is made for. But I haven't tried it inside of the mic yet. I will give it a try tomorrow.

 

[Gus]

The 12V zener and 1N4148 (type) give about 12.6V 12.7V at the base then you have the about a .6Vvolt drop base to emitter for about 12V out.

My 692s were modded before I bought them. The machine work at the xlr end of the body looks like it was done by a pro. Big threads cut down and a matching metal ring glued on. I will open one up again and take pictures. Peter's and PRR's drawings are the same except for 6.8ks in PRR's. My microphones still use the center tap of the transformer.


FWIW I modded another 12V 692 with a 12K resistor for D1 C12 works fine. I machined a switchcraft XLR housing down in my lathe then machined the stock end housing I then glued the parts together. I have ground wires attached to both metal parts and connected together inside the microphone because I did not have conductive epoxy.

 

[chriss]

The machine work at the xlr end of the body looks like it was done by a pro. Big threads cut down and a matching metal ring glued on.

That sound exactly like Siegfried Thiersch did it. He is cheap, friendly, perfectional but slow...

 

[guavatone]

I actually haave a mv691 that was modded for 48v before I got it. I was trying to see if some components went bad and got schematics from MG.

Here are a bunch of shots to compare. I think the 48v mod is by TR1. There is a component underneath that may have been added but I am not sure. Also i have a shot of a schematic that seems similar to yours.

Oh, If anyone has any idea of rplacement components please chime in. I was thinking about changing that thin red (cap?) by the capsule that says 200h. Is that a paper cap?

There is a trace cut between the 2 solder points of the small black part(transistor?)

I can take a detailed look at any components if you want.
Hope this helps!

 

[Gus]

Charlie

Thanks I think I might have traced mine wrong. The picture of the microphone circuit looks a lot like mine maybe mine were done by gefell?

 

[peter purpose]

Charlie... may all your children make you smile like a very smiley man at a smiling competition... :thumb:

 

[PRR]

> Peter's and PRR's drawings are the same except for 6.8ks in PRR's.

Those are the Phantom resistors inside the mix-board/mike preamp. I just put them there so we could think about what is really happening.

> My microphones still use the center tap of the transformer.

That's what I intended to imply, but I didn't have a CT winding symbol handy, and it isn't strictly needed for the DC thinking.

Charlie's blueprint, closeup:

Ah, I see Peter was right, except he left out the most important part........

 

[peter purpose]

I was right, except I was wrong.... that sounds about right..!!

 

[guavatone]

Thanks for the blessing Peter. Geffel was very friendly with getting me a few schematics. I can't find my original pdf's so i took a pic of one, since i have no scanner.

 

[toffifee]

Cooool! I got a MV692 today (with M94), and now I have the schematics to convert it to 48V :thumb:.
But how do I open it up? The switch caps are in the way!? :roll:
Is there a source for the 7pin female-connector? It´s not necessary for me to have it converted to XLR...
thanx! Toff

 

[peter purpose]

[quote author="toffifee"]
But how do I open it up? [/quote]

Get some small pliers and pull the little black switch tops out, or stick a small screwdriver underneath them and lever them out.

peter

 

[chriss]

Toff,

I can sell You a connector. Send me an E-Mail.

Chris

 

[toffifee]

@Peter: Thanx, done :sam:. So no hidden secret this time. I tried to lift them before I asked, but they were very tight - did not wanna destroy it.

@Chris: Sent u a PM (cannot see your email adress).
Thanx to Chris; Chris(-Toff) :grin:

 

[ToobieSnack]

i used to have many of these and the 691's as well.
i contacted geffell and they said these would take 48v without any mods.
and i used them with 48v with no problems.
I have 2 (690,691,692) power supplies here (for 4 mics .. 2 i/o's each) if any one needs them let me know. 3 cables too.

the 691'a will NOT accept the 48v phantom power ..
i no longer own any of these mics but i would get one again.
the UM70 heads with these are AWESOME!..

this thread has lots of cool pics and data here..
geffell said these would be easy to DIY too! (pretty nice of then to mention)

hope this helps
later
ts

 

[rodabod]

I'd quite like to make an in-line 48V to 12V adapter for my Gefells (I have seen these for sale), but was wondering how I would go about doing so.

I've looked at the circuit mod here, but can't see how I could incorporate this into an in-line adapter without affecting the audio....

I was thinking of just using a series resistor on the 48V lines to "soak up" 36V, but one of my preamps only gives 40V phantom... :sad:

Any ideas? There must be a simple solution since you can buy them (shame they are so expensive!)

Thanks,

Roddy

 

[rodabod]

Hmmm. I think I can just use the 48V to 12V section and just decouple with some big caps...

 

[DerEber]

I just got my PM750 7-Pin/12V Version and was searching for Information now for some hours.

I found peter´s shematics of the MV692 12V and 48V Versions in this threat very usefull:

http://www.groupdiy.com/index.php?topic=7500&highlight=gefell

Is seems to be true that the electronic of the PM750 and the MV692 is nearly equal. (dispite the filter/pad)
I traced the first few parts on the PCB and it was exactly like the 692 schematic.

If you look on the MV692 / P48 you will see a simple series resistence instead of D1/C12
Anyway I have 12V Phantompower box and will hook the Mic up tonight.

Again thanks for everyone this forum. You realy helped me a LOT LOT LOT!
:thumb: :thumb:

 

[rodabod]

[quote author="DerEber"]
Is seems to be true that the electronic of the PM750 and the MV692 is nearly equal. (dispite the filter/pad)
I traced the first few parts on the PCB and it was exactly like the 692 schematic.

If you look on the MV692 / P48 you will see a simple series resistence instead of D1/C12
Anyway I have 12V Phantompower box and will hook the Mic up tonight.
[/quote]

Yeah, there is a version of the PM750 with pad and roll-off called a PM860 and it even says MV692 inside!!

I have just ordered two Schoeps 12V power supplies, but I'd like to convert them for 48V...

 

[Gus]

If you use a phantom that is a good 48V one you can mod it with adding one resistor IIRC I ended up with a 12K and removing a part or two. I also machined the ends for a XLR. I will look for my notes.

 

[rodabod]

[quote author="Gus"]If you use a phantom that is a good 48V one you can mod it with adding one resistor IIRC I ended up with a 12K and removing a part or two. I also machined the ends for a XLR. I will look for my notes.[/quote]

Thanks for that, Gus.

I remember CJ suggesting doing this a while back as an easy fix.

The only slight problem is that I measured one of my (crap) preamps and it only gives 40V on-loaded.... Hopefully this would only cause less SPL handling and poorer S/N ratio though.